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Vax 11/750 Frequently Asked Questions By James Lothian


This is an updated version of the 11/750 FAQ by James Lothian that used to be online at

Vax-11/750 Frequently Asked Questions
By James Lothian


I haven't in fact actually seen many of these questions asked. However, they're the questions I've found myself asking over four years or so that I've owned my own 750, so I hope they'll be useful to others. This file may be archived if you feel like it and otherwise reproduced, as long as appropriate credit is given.

My own machine currently runs 4.3BSD, so I'm biased toward Unix.

Feel free to mail me with corrections and/or additions.


Paul Roberts at Stirling University gave me my machine. Thanks Paul! No, he hasn't got any more... Various people have helped me out with advice since I got it, including Terry Kennedy, Iain Lindsay at Edinburgh University's Electrical Engineering dept, and Jim Reid at Strathclyde. Martin Reilly gave me a TU80 for spares. I'd also like to thank Bruce Hassall, also of the Edinburgh EE dept, for lending me a spare PCS, a PCS print-set and a DELUA ethernet board.

Another big thank-you to the people who asked for copies of this document after they found out that it wasn't available online any more, and thereby encouraged me to update it a bit.


Anders Magnusson ( Info about cluster interconnect and time-of-year clock.

Mike Umbricht ( 11/751, FPA.

Other places to visit

The Freak's VAX 11/750: pictures of his 750, and links to lots of other places.

The VAX Gallery: Lots of pictures of Vaxes.

The NetBSD/VAX FAQ : If you want an operating system, this is the place to look.

The Retro-Computing Society of Rhode Island: Preservation of old computers, lots of links to other interesting places.

Update Record

28/6/96: Initial version.
1-4/7/96: Tidied up html, put in sections on console commands and console serial line receiver replacement, added EMC memory board info, expanded CI750 information, added brief description of buses, added 'Contents' section and stuff about other places to visit. Then had a very big cup of tea :-).
2-3/8/96: Added RDM details, minor changes to documents list.
9-10/8/96: Power bus description, option installation, expanded RDM description, bit about bus grant jumpers.
11/3/99: Updated contact address, various other minor changes.


What is the 11/750?

What is an 11/751?

What is a Systime 750?

How fast is it?

What options are available for the 750?

What do the boards do?

What is the patchable control store?

Where do the boards go?

How are the options installed?

What are the console commands?

RDM Details

What do the CPU halt codes mean?

What do the console command error codes mean?

What do the microverify error ystime 750, but if they're anything like the Systime PDP-11 kit I've seen, they're probably crap. The PSU is rumoured to be less reliable than the DEC ones.

How fast is it?

Not very! The main processor clock runs at just over 6 MHz.

What options are (were!) available for the 750?

Floating point accelerator (FP750): the basic machine does floating point in microcode, but the FPA speeds up floating point operations considerably. It also gets alarmingly hot.

D & F floating point is done in microcode when the FPA isn't present, and G & H are emulated by the operating system (Mike Umbricht).

Massbus adapter (RH750): allows the 750 to use massbus disks and tapes. Massbus was a fast(-ish) bus used on PDP11s and the early vaxes for disks and tapes.

Second Unibus adapter (DW750): allows the machine to use a second unibus, useful if your unibus bandwidth is topped out.

Remote Diagnosis module (KC750): useful diagnostic tool. Allowed DEC to phone your machine and ask it to open wide and say 'Aaaah' when it was sick. Rarer than hens' teeth, as all were owned by DEC, and were supposed to be returned to them when the machine was decomissioned. I once caught a glimpse of one.

Battery (H7112): a big (3 lead-acid accumulators) battery that kept the memory ticking over during brief power outages. Supported by VMS, but not by Unix, as far as I know.

Writable control store (KU750): a long and complex story! See under 'What is the Patchable Control Store?' below.

What do the boards do?

Some of this taken from a posting by Mike Umbricht ( to the NetBSD port-vax list.

Very roughly, the functionality of the machine is divided among the boards as follows:

L0001 (FPA): Floating point accelerator.

L0002 (DPM): Data path module. Contains general registers, super rotator, main ALU, condition codes chip, microsequencer, microstack, instruction decoding roms, instruction decode chip, interval timer.

L0003 (MIC): Memory Interconnect. Cache, Translation Buffer, Prefetch Buffer, CMI (CPU Memory Interconnect bus -- the main backplane bus) control.

L0003-YA: described as a 'cost reduced' version of the L0003. Anybody know anything about this?

L0004 (UBI): Unibus Interconnect. All the unibus stuff including the unibus map, plus the following ragbag of miscellaneous non-unibus-related bits: time of year (TOY) clock, TOY battery charge circuit, UARTS for console line and TU58, interrupt control chip.

L0005 (CCS): CPU Control Store. You should never see a 750 with one of these, as they were field-upgraded to the L0008 PCS (see below).

L0006 (RDM): Remote Diagnosis Module. Has its own microprocessor (an Intel 8085), 64 words of diagnostic control store, 14k (I think) of memory for the 8085, four roms, four serial line chips, and a huge wodge of TTL. If anybody has the print-set for this, I'd be very interested in knowing what the print-set number is (MPxxxxxx on the first page). I'd also be very interested in getting hold of a photocopy of the print-set.

L0007 (MBA): Massbus adapter.

L0008 (PCS): Patchable Control Store. A major kludge. See description below.

L0009 (CCI): Cluster Interconnect. Supported by VMS and Ultrix. (Info taken from Autumn 85 'Systems & Options Catalogue'.) Consists of CI750 module that installs in CMI add-on slot in CPU backplane, and a free-standing cabinet that contains a box containing three more modules in their own backplane. Requires PCS microcode rev 97 or higher, cpu rev 5, and if you've got a uda50, it has to be rev 7 with rev 5 microcode. So now you know.

L0010 (DW750): Second Unibus Adapter. Never seen one. Presumably has all the bits the L0004 has, without the ragbag.

L0011: Memory controller. All the bits for DRAM refresh, memory configuration and error-correcting code computation. Supports up to a maximum of 8 * 256k arrays. Never seen one. I guess most machines with one of these were upgraded to the L0016.

L0014 (DR750?): CMI parallel interface. Never seen one.

L0016: Memory controller. As L0011, but supports up to 8 * 1meg arrays. Quite common.

L0022: Memory controller. Supports up to 2 * 4meg arrays + 6 * 1meg arrays. I'd love to borrow a set of circuit diagrams for this board.

What is the patchable control store?

This is based partly on information supplied by Terry Kennedy (

This gets complicated...

Unlike the 780, the 750 was designed to have a purely rom-based control store. The basic control store consists of 6k * 80 bit words of rom. However, various minor modifications to the instruction set specification and bug-fixes to the microcode entailed microcode upgrades. This was a pain, because the microcode on the original L0005 CCS module came in the form of 120 piffling little 18-pin proms in sockets, all of which presumably had to be swapped. Apparently, DEC got fed up with this, and produced the L0008. This board contains 30 24-pin proms, and a bank of 1k * 80 bits fast static rams (implemented using 2148s). In addition, there is an 8k * 1 bit static ram (the 'flag' memory, implemented using 2147s). When the machine is fetching the next control word, it checks the flag memory; if the bit corresponding to the word about to be fetched is set, the word is fetched from the rom. If the bit is clear, it is fetched from the corresponding word of the ram. Thus modifications to the microcode can be installed when the machine boots. On BSD, these modifications are contained in a file called pcs750.bin which lives in the root directory and is loaded into the PCS by the boot program.

The L0005 had connectors for an optional UCS (User Control Store) daughter-board, which had 1k * 80 bits of genuine writable control store (as opposed to the patchable control store). With the PCS, this functionality is supported by having several variants of the module, rather than by an optional add-on board. The variants of the PCS are as follows:

L0008-YA: bog-standard PCS, no writable control store. This is the usual variant.

L0008-YB: PCS with 1k * 80 bits of writable control store. Equivalent to L0005 + UCS daughter-board.

L0008-YC: PCS with 2k * 80 bits of writable control store.

Among other things, the patches loaded into the control store are supposed to cure a bug caused by a marginal timing error in the design of the L0003 MIC module, which occasionally causes the machine to take a spurious translation buffer parity error. It's interesting to note that the machine check handler code in the 4.3BSD kernel specifically checks to see if the CPU is a 750 when it gets this error, and ignores it if it is, rather than panicking.

I got hold of a few spare L0008-YAs a while ago, and cannibalised them to build an L0008-YC. Let me know if you want the gory details.

Where do the boards go?

Working from the right-hand side of the CPU backplane: L0001, L0002, L0003, L0004, L0005 or L0008, L0006. The next three slots are for massbus adapters, second unibus adapter, cluster interconnect or other CMI add-ons. The last CPU slot is for the memory controller, L0011, L0016 or L0022. The next eight slots are the memory slots, which should be populated starting from the right. If you've got an L0022, then the two four meg boards should be in the first two slots. The last slot, which is obscured by the two metal rods running from top to bottom of the card-cage, appears to be wired as a unibus SPC (small peripheral controller) slot, though there's no mention of this in any of the manuals. Can anybody confirm this?

How are the options installed?

This is taken from the 750 installation manual, which isn't exactly a model of clarity! If something here doesn't make sense to you, it probably didn't make sense to me, either...

L0001 FPA

Bung the board in slot 1 of the CPU backplane. Power up and check that the green light on the front edge of the FPA board comes on.

L0007 RH750 Massbus adapter

Choose a CMI slot (backplane slots 7, 8, 9). (For first MBA, slot 9 is recommended.)

Note: There are three edge connectors on each L-series module. They are named A, B and C, starting from the top. On each connector, pin 1 is the topmost pin on the component side, pin 2 is the topmost pin on the solder side, and so on.

Several jumpers need to be fitted to the backplane to configure the MBA. For first MBA, jumper A53 to A51 and A54 to A52. For second MBA, jumper A53 to A51. For third MBA, jumper A54 to A52. These jumpers determine where in the CMI address space the MBA appears. Additionally, CMI arbitration levels need to be set. Conventionally, first MBA gets arbitration level 3, second gets 2, and third gets 1. For arb level 3, jumper A64 to A62. For arb level 2, jumper A64 to A63 and A62 to A60. For arb level 3, jumper A64 to A66, A63 to A61 and A62 to A60. As if that weren't enough, the following jumpers need to be removed from the slot: A77 to A78, A73 to A74, A69 to A70, A67 to A68. A jumper between A45 and A43 is normally fitted, but may optionally be removed to reduce the rate at which the MBA initiates DMA transfers on the CMI.

The massbus connector should be installed in place of one of the I/O bulkhead blanking plates. Three ribbon cables connect the backplane connector housings to the I/O panel connector. It seems that the topmost housing is connected to the leftmost (viewed from the rear of the rack) header on the massbus connector.

Inspect the massbus terminator on the massbus drive. If the terminator contains three resistor cards, check that W2 (massbus fail H) on the cable C terminator card has been removed. Connect the terminator to the massbus connector on the drive.

L0010 DW750 Second Unibus adapter

Remove the four grant jumpers from the slot in which you intend to put the board (A77 to A78, A73 to A74, A69 to A70, A67 to A68). Insert the board in the slot. Connect the three ribbon cables to the three backplane connector housings. The housings are labelled A, B and C. The cables are routed out of the CPU box into the expander, onto the M9014 adapter board. Note that nothing is said about which cable goes where (thanks, DEC). The M9014 goes into the Unibus In slot in the DD11 backplane. The expansion box should also contain an M7855 unibus exerciser module, which should be configured thus: Address (E125), switches 1 to 8: all on; Vector (E88), switches 1 to 8: on, on, on, on, off, off, on, off. The slot the M7855 is in should be configured with the NPG jumper removed.

Unibus adapter CMI arbitration level: the L0010 has a fixed arb level of 3. Massbus adapters are normally configured starting at this level, so you'll probably have to shift your MBAs down one level if you add an L0010.

L0006 KC750 RDM

Install L0006 in slot 6 of CPU backplane. Move TU58 signal cable at bottom of section B of the L0006 backplane slot from left to right side of the slot. Move console terminal connector at top of section C of the slot from left to right side. Attach RDM modem port cable to bottom right of section C of the L0006 backplane slot. The connector is labelled to indicate the 'up' end; if the label has dropped off, the two-pin gap in the wires is at the top. The RDM modem port baud rate is programmable; on old models, this is done by W4 to W7, a couple of inches back from the edge connectors at the bottom edge of the board. W4 is the bottom-most jumper, and the others just count upwards. On newer machines, there is a four-position dip switch at the same location. Switch 1 corresponds to W4, switch 2 to W5 and so forth. The rates available are 19200, 9600, 7200, 4800, 3600, 2400, 1200, 600, 400, 300. For 19200, W7..W4 are 0001 (0 is out, 1 is in). The other rates count upwards in binary.

For RDMs installed in Europe, W3 (a jumper near the top edge of the board, I guess about three inches back from the edge connectors) should be removed. It doesn't say why.

H7112 Backup battery

The battery has two cables coming out of it: one has a three-pin mains connector, and should be connected to one of the three-pin sockets on the power-controller. The other is a nine-way connector that connects to a socket mounted on a bracket on the inner left-hand cabinet upright, next to the battery mounting points. Lastly, the three-way plug that floats around the bottom of the rack should be connected to one of the three-way sockets on the back of the battery. Then the battery should be mounted to the mounting brackets on the rack. For older 750s, the battery mounts horizontally, on a pair of brackets in the bottom of the rack; on newer machines, it is attached vertically to a couple of brackets at the front of the rack.

What are the console commands?

Taken from hardware handbook and installation manual:

control-P: Halt processor, enter console mode. Issues '>>>' prompt.
control-D: (If RDM present) Enter RDM console mode, with 'RDM>' prompt.
I: Processor initialisation. Invalidates translation buffer and cache, issues processor init and unibus init.
H: Halt. Redundant, since processor must be halted for console operation anyway, but provided for compatibility with 780.
T: Test. Runs microverify, which is described as a basic sanity test of the DPM and MIC boards.
S nnnn: Start. Initialises CPU, stores specified address in program counter, and starts execution there.
S: Start. Initialises CPU, and starts execution at current program counter.
C: Continue. Starts program at current program counter, without initialising. Useful if you've accidentally bombed into console mode by pressing control-P in emacs, tcsh &c. :-)
N: Next. Single-steps the program (i.e. executes the next instruction, then returns to console mode).
B: Boot. Boots from boot rom selected by front-panel device switch.
B [QUALS][DDCU]: Boot. Boots from device specified by DDCU, using qualifiers QUALS. DD: device; C: adapter; U: unit number. QUALS: /X inhibits microverify, /hexnumber passes boot-flag hexnumber in R5.
X: Binary load/unload. Unlikely to be useful -- used for automated test.
E [QUAL][ADDRESS]: Examine. Examines memory, registers &c.
D [QUAL][ADDRESS][DATA]: Deposit. Modifies memory, registers &c.

		/B Set size to byte
		/W set size to word
		/L Set size to longword
		/P Physical address space
		/V Virtual address space
		/I Internal processor register
		/G General register
		nnnn Hex address
		* last address
		+ next address
		P processor status longword

RDM Details

This section is taken from the KC750 Microdiagnostics and Technical Manual, EK-KC750-TM.002.

The RDM consists of an 8085 microprocessor, some ROM, some RAM, a bank of fast SRAM used for storing diagnostic microcode sequences (the diagnostic control store), a mechanism for generating a scope signal and/or stopping the CPU when a particular control-store address is accessed, UARTs for the console serial line, the diagnostic modem line, the front-panel TU58, and another one marked 'CPU' which relays whatever is typed on the console terminal to the CPU when the RDM is inactive. When the RDM is not being used under Micmon, the diagnostic control store is used to store a trace of the 64 most recently accessed microaddresses.

The RDM has a number of commands built in. Additionally, a monitor program, Micmon, can be loaded from TU58 tape. When loaded, Micmon takes over the RDM and provides its own set of commands. Throughout this description, DRDC stands for 'Digital Remote Diagnosis Centre'.

Console States

The console can be in one of a number of states, and knowing what they are will make the description of the RDM commands a bit easier to understand. I hope.

The Built-in Commands


Purpose: Clear stop-on-micromatch function.

Syntax: CL

Disables stop-on-micromatch but does not change contents of match register. Match pulses will still occur, but the CPU will not be stopped.


Purpose: Enable local copy function.

Syntax: COP

Allows local terminal to get a copy of the dialogue between the RDM and the DRDC. Only DRDC can execute this command.

Copy disable

Purpose: disable local copy.

Syntax: COP/D

Stops local terminal from printing DRDC dialogue.


Purpose: Modify contents of vax memory location.

Syntax: D [/modifier] <addr> <data>

Writes specified data to memory location. Address and data are in hex. Modifiers are W (word), L (long word), B (byte), N (don't specify address, address used is next one, data type is same as last one). Addr field can be replaced with * (use addr specified in last examine or deposit) or + (increment address by default data type).


Purpose: examine vax memory.

Syntax: D [/modifier] [address]

Modifiers W, L, B work as for deposit. If address is not specified, previous address incremented by default or specified data type is used. Address can be specified as *, in which case address used for last examine or deposit is used.

Examine Console

Purpose: examine RDM internal memory.

Syntax: E/C [address]

If address not specified, current default address is incremented and examined. Only addresses 8000 to FFFF are valid.

RDM status register at F820 is mentioned; it encodes the processor state as follows:
bits 1,0: Boot device switch. A = 11, B = 10, C = 01, D = 00.
bits 3,2: Power on action. res/boot = 00, res/halt = 01, boot = 10, halt = 11.
bit 4: Con Halt H
bit 5: CS parity err
bits 7,6: Key switch. remote = 00, rem/secure = 01, local = 10, secure = 11.


Purpose: initialise CPU.

Syntax: INI

Simulates a power-fail sequence to initialise the CPU.


Purpose: Record a list of commands to be executed.

Syntax: LIN

Builds a clist of commands in RDM memory to be executed later with the Perform command. Each command is prompted for with the LNK> prompt. Control-C terminates the list. If the last instruction in the list is a Perform command, the list will loop automatically when it is run until Control-C is typed.


Purpose: Loads a file from TU58 tape into vax main memory.

Syntax: LO <filename.extension> [address]

The RDM locates the file on tape and loads it into sequential addresses in Vax memory. If start address is not specified, it defaults to zero. Filename must have exactly six chars, extension must have exactly three. (I think it really means 'six or less' and 'three or less').


Purpose: Provides local/remote control of RDM.

Syntax: CTL

Causes the RDM to accept commands from both the console terminal and the modem line. Can only be entered from DRDC. Do not enter stuff from both terminals at the same time.

Local Disable

Purpose: Disable Local command function.

Syntax: CTL/D

Cancels effect of Local command. Can only be entered from DRDC.


Purpose: Latches contents of selected microaddress into CPU control store latches.

Syntax: UA <address>

Halts the CPU, loads a new control store address on the CS address bus. When the CPU restarts, microcode at the new address takes control. Cancels any stop-on-micromatch previously set.

Note: microinstruction at 17FD has incorrect control store parity bits, to test parity checking logic.


Purpose: Temporarily loads a specific microaddress on the CS address bus.

Syntax: UA/C <address>

Places selected address on CS address lines until the next master clock tick. At that point, CPU latches data on the CS bus prior to the command. So this command does not change the flow of control in the CPU. Cancels any stop-on-micromatch previously set.

Used for isolation of failures in CS bus, control store, CS latch, CS control signals.


Purpose: Checks vax control store parity.

Syntax: PAR <address>

Runs a parity check of the CPU control store, starting at the selected address and ending at the first parity error. If no error is found, stops at the preset error address 17FD. Displays location of any error found. CPU clock must be stopped before using this command.

WCS may be included in the parity check by specifying addresses 2000 through 23FF.


Purpose: Executes a link program.

Syntax: PER

Executes the program in the link memory until the program stops or control-C is typed.

Repeat Last Command

Purpose: Repeats execution o last RDM command.

Syntax: REP

Continuously executes the preceding command until control-C is typed. Used for generating scope signals off continuous execution of a command. Type control-O to stop terminal output.

Repeat Next Command

Purpose: Repeats execution of specified RDM command.

Syntax: R <command>

Continuously executes the given command until control-C is typed. Type control-O to stop terminal output.


Purpose: Return system to program I/O state.

Syntax: RET

Switches the system from RDM console state to program I/O state. Disables any stop-on-micromatch previously set.


Purpose: Return to control mode of RDM console state.

Syntax: RET/D

Switches the system from command mode to control mode of RDM console state. In control mode, terminal communicates with CPU as normal. Unlike program I/O mode, a control-D returns to RDM command mode, and previously set stop-on-micromatch remains enabled.


Purpose: Enable stop-on-micromatch.

Syntax: SE [address]

Causes the CPU clock to be stopped when the contents of the CS address bus equals the contents of the RDM match register. If the address isn't specified, it defaults to one specified previously. When CPU stops, RDM displays addresses of last instruction executed and next instruction to execute (microinstructions, I guess). Remains set in control mode of RDM console state.


Purpose: Displays current CPU state.

Syntax: SH

Displays current state of CPU: running (executing code), halted (not executing code, but clock running), stopped (cpu clock stopped). Also displays addresses of last and next instructions.

Show Version

Purpose: Display version of RDM firmware.

Syntax: SH/V

Displays current revision number and date of firmware on RDM.


Purpose: Steps CPU through single microinstruction.

Syntax: STE

Causes the CPU to execute a single microinstruction and stop. It then displays the address of the current microinstruction latched in the CPU, and the address of next microinstruction to latch.

Step Tick

Purpose: Advances the CPU in base clock increments.

Syntax: STE/T

There are two base clock cycles for each M clock cycle. During the first half of the M clock cycle, the CS address lines are high-impedance. After stepping the B clock to the second half of the microinstruction, the next CSAD value is visible. By stepping B clock again, the CPU shuld latch the microinstruction at the next CS address. Step tick shows this latching sequence, and identifies microinstructions that take longer than two B clock cycles to execute.


Purpose: Stops CPU clock.

Syntax: STO

Stops the CPU clock, and displays the addresses of the last instruction executed and the next.


Purpose: Changes from RDM console state to talk state.

Syntax: TA

Allows communication between local and remote terminals. Entered from currently controlling terminal. Displays characters typed on either terminal on both terminals. Type control-C to return to RDM console state.


Purpose: Loads and runs microdiagnostics.

Syntax: TE

Loads the microdiagnostic monitor (Micmon), and then runs a series of microdiagnostics. The software resides on TU58 tape. To interrupt diagnostics, type control-C; this takes you to the MIC> prompt. To get out of Micmon to the RDM prompt, type RE to the MIC> prompt.


Purpose: Load microdiagnostic monitor, without running diagnostics.

Syntax: TE/C

Loads the microdiagnostic monitor into RDM memory, and gives it control. Does not run any diagnostics.

Test File

Purpose: Load user program into RDM & run it.

Syntax: TE <filename.extension>

Loads a program other than the microdiagnostic monitor and executes it.


Purpose: Displays 65 most current control store addresses.

Syntax: TR

Displays in reverse order the CS addresses stored in the diagnostic control store. Except in microdiagnostic state, RDM stores the addresses of the 64 most recently executed VAX CS instructions in the DCS. The address of the next instruction to execute is stored in a register and is also displayed. Type control-C or control-D to stop the listing. The CPU clock must be stopped before the Trace command can be used.

Micmon Commands

To be continued...

Error Messages

This is a list of the RDM error codes.

TAP:14		Tape: read length error, not all records fit
TAP:13		Tape: flag received, not command or data 
TAP:12		Tape: directory error 
RDM:11		Invalid operation code in macro 
RDM:10		Operation already in progress 
TRM:0E		Terminal: remote line CRC error 
TRM:0D		Terminal: length of input longer than buffer
TRM:0B		Terminal: command input buffer overloaded
TAP:09		Tape: file not found
TAP:08		Tape: invalid packet received
TAP:07		Tape: no end packet, invalid operation code received
TAP:06		Tape: tape count byte received exceeds maximum
TAP:05		Tape: tape check sum error received
TAP:04		Tape UART overflow received
TAP:03		Tape UART data set ready dropped
TAP:02		Tape UART error received from UART
TAP:01		Tape UART device timed out
CPU:04		CPU UART overflow received
CPU:03		CPU UART data set ready dropped
CPU:02		CPU UART error received from UART
CPU:01		CPU UART device timed out
TRM:04		Terminal UART overflow received
TRM:03		Terminal UART data set ready dropped
TRM:02		Terminal UART error received from UART
TRM:01		Terminal UART device timed out
REM:04		Remote UART overflow received
REM:03		Remote UART data set ready dropped
REM:02		Remote UART error received from UART
REM:01		Remote UART device timed out
TAP:FF		Tape: diagnostic failure
TAP:EE		Tape: partial operation (end of medium)
TAP:F8		Tape: bad unit number
TAP:F7		Tape: no cartridge
TAP:F5		Tape: write protocol
TAP:EF		Tape: data check error
TAP:E0		Tape: see error (block not found)
TAP:DF		Tape: motor stopped
TAP:D0		Tape: bad operation code
TAP:C9		Tape: bad record number
SYNTAX ERROR:	error entering console commands
INVALID COMMAND: RDM does not recognise command
CMI:nn		Error in Vax main memory (nn is error code -- results from
Examine command if area addressed has error)
CMI:00		Non-existent memory
CMI:01		Corrected read data
CMI:02		Read data substitute
ROM		ROM failed RDM power-up self-test
RAM		RAM failed RDM power-up self-test

What do the CPU halt codes mean?

This section, and the next couple, are taken verbatim from the 750 installation guide, EK-SI75F-IN-PRE, with bits also from the hardware handbook.

When the processor halts, it prints a code that tells you why it halted.

00: (hardware handbook only) Halt or single-step from console
01: Test command executed successfully
02: control-P or executed single instruction with N command (installation manual only)
04: interrupt stack not valid
05: double bus write error halt
06: processor halt instruction executed
07: SCB Vector <1:0> = 3, halt at vector
08: SCB Vector <1:0> = 2, WCS disabled or not present
0A: Change mode instruction executed on interrupt stack
0B: Change mode instruction executed, SCB vector <1:0> not = 0
11: Power-up, cannot find RPB, front panel switch at restart/halt
12: Power-up, warm start flag false, front panel switch at restart/halt
13: Power-up, can't find good 64kB of memory
14: Power-up, bad or nonexistent boot rom
15: Power-up, cold start flag set during boot subroutine
16: Power-up halt, front panel switch at halt
FF: Microverify test failure

What do the console command error codes mean?

The table in the installation manual prefixes each of these with '7'. I think that's a typo, because the one in the hardware handbook doesn't. Also, each has some codes that aren't in the other.

10: Illegal GPR register number.
11: Error in accessing IPR or PSL.
19: Reference to next location was preceded by an examine or deposit to an ipr or psl.
20: Memory examine or deposit failed; access violation, translation not valid, machine check, bus error, TB parity error, CS parity error.
30: Checksum error on APT load or unload.
33: Attempt to boot from unrecognised device.
34: Controller not A, B, C or D in Boot command.

What do the microverify error codes mean?

The CPU prints a '%' character when it starts microverify. When it completes successfully, a second '%' is printed. If an error occurs, a different character is printed in place of the second '%', and a halt code program counter is printed. The character printed in place of the '%' can be used with the PC to find the cause of the problem using the following table:
Code	PC+2		Test name/Error message
@			RBus, WBus test
	000		Bad bit in DReg or SUPROT
	001		Bad bit in Rbus or Wbus
C			Mbus test
	031		Bad bit in Qreg
	032		Bad bit in Mbus
E			Scratch pad bit test
	051		Error clearing RTemp
	052		Error filling RTemp with ones
	054		Error clearing GPR
	057		Error filling GPR with ones
	058		Error clearing IPR
	05B		Error filling IPR with ones
	05D		Error clearing MTemp
	05E		Error filling MTemp with ones
F			MTemp explicit addressing test
	061		Error addressing MTemp 0
	062		Error addressing MTemp 1
	064		Error addressing MTemp 2
	067		Error addressing MTemp 4
	068		Error addressing MTemp 8
I			RTemp explicit addressing test
	091		Error addressing RTemp 0
	092		Error addressing RTemp 1
	094		Error addressing RTemp 2
	097		Error addressing RTemp 4
	098		Error addressing RTemp 8
J			IPR explicit address test
	0A1		Error addressing IPR 0
	0A2		Error addressing IPR 1
	0A4		Error addressing IPR 2
	0A7		Error addressing IPR 4
	0A8		Error addressing IPR 8
L			GPR explicit address test
	0C1		Error addressing R0
	0C2		Error addressing R1
	0C4		Error addressing R2
	0C7		Error addressing R4
	0C8		Error addressing R8
	0CE		Error addressing dual port
O			XB/IR/OSR bit test
	0F1		Error in XB<31:0>
	0F2		Error in XB<63:32>
	0F4		Error in IR
	0F7		Error in OSR
Q			Source XB PC increment test
	111		Error sourcing one byte from XB
	112		Error sourcing 2 bytes from XB or incrementing PC by 1
	114		Error sourcing an unaligned longword or incrementing PC
			by 1
	117		Error incrementing PC by 4
	121		Error reading dsize rom operand 1
	122		Error loading/reading rnum
	124		Error reading dsize rom operand 2
	127		Error loading/reading rnum
	128		Error reading dsize rom operand 3
	12B		Error loading/reading rnum
	12D		Error reading dsize rom operand 4
	12E		Error loading/reading rnum
T			Rnum/dsize test continued
	141		Error reading dsize rom operand 5
	142		Error loading/reading rnum
	144		Error reading dsize rom operand 6
X			Cache parity error test
	181		Failed to get cache parity error
	182		Bad machine check error summary register
	184		Bad cache error register
[			TB parity error test
	1B1		Failed to get group 0 TB parity error
	1B2		Bad TB group parity error register
	1B4		Bad machine check error summary register
	1B7		Failed to get group 1 TB parity error
	1B8		Bad TB group parity error register
	1BB		Bad machine check error summary register
]			Control store parity error test
	1D1		Failed to get control store parity error
	1B2		(?1D2? possible misprint) Error in control store 
			parity error 
^			Cache test
	1E1		Error filling cache with ones. Location not initially 
	1E2		Error filling cache with ones. Unable to write ones.

What can be done about a dud TOY clock battery?

The time-of-year clock battery on my machine was totally defunct when I got it. (The TOY battery is mounted on the right-hand rear cabinet frame upright, viewed from the rear). The original battery pack consists of 4 sub-C size nicads attached to a thin metal mounting plate. I used 4 C size nicads and a 4*C holder from Maplin, and a couple of nuts & countersunk bolts to attach it to the original mounting plate (after trimming off the lip along the long edges of the plate with a pair of tin snips). Connect the cable that goes to the connector on the backplane to the terminals on the battery holder (red is positive -- be sure to get it right way round!). The TOY clock battery is rated to last 100 hours, but I find that if the machine has been getting a reasonable amount of use, the batteries will last about a week.

If the batteries die completely, the clock won't count at all. In this case, a stop-gap solution is to disconnect the battery pack from the backplane connector, though you'll have to reset the time every time the machine is powered up. (Anders Magnusson)

What do the front-panel controls do?

Taken from hardware handbook.


'Power': indicates that DC power is OK.
'Run': the CPU is running. If this is on, the console terminal is in program I/O mode (in which it functions as a normal terminal). If off, the CPU is halted and the console terminal is in console I/O mode (in which you use console commands to control the CPU).
'Error': The Big Red Light. If lit, indicates that the CPU has encountered a control-store parity error. This (probably) means you've got a problem with your L0008 module, and is Bad News. Reset by pressing the 'Reset' button, and pray it goes away...

Back-lit Remote Diagnosis indicators:

'Remote D': illuminated by remote diagnosis software.
'RD Carrier': indicates Carrier Detect on RDM modem port.
'RD Test': remote diagnosis tests are in progress.
'RD Fault': fault detected in RDM self-test. Also briefly illuminated while the self-test is running, at power-up.


'Reset': simulates a power-off followed by power-on. State after reset depends on settings of other front-panel switches.

'Power On Action': Determines what the CPU does when it is powered up. Halt: CPU powers up into console mode; Boot: CPU boots whatever device is selected by the boot switch on power up; Halt/restart: tries to do a restart after power outage, halts if it can't; Boot/restart: tries to do a restart, reboots if it can't. (Latter two only make sense if you've got a memory backup battery.)

'Boot Device': selects boot rom to boot from. These roms are in sockets on the memory controller (L0011, L0016 or L0022).

Keyswitch: Controls power and stuff:

Off: All power is off except for TOY clock battery. In any other position, the backup battery (if fitted) will power the memory if the mains supply is cut off. So if you've got a battery fitted, don't power your machine down by just opening the back-panel circuit- breaker, as this will leave the memory still running off the battery.
Local: power on, RDM disabled, CPU can be put in console mode by hitting control-p at the console (do not leave in this position if you use emacs, tcsh &c!).
Secure: power on, RDM disabled, reset button disabled, can't switch to console mode with control-P. This is the normal operating mode.
Remote: power on, console functions can be used from RDM port.
Remote/Secure: RDM port replaces console port. Reset button disabled. RDM can't be used for diagnosis in this position.

Wow, what a BIG power system!

The PSU consists of three main lumps:

875 power controller (glorified on/off switch containing a huge mains filter and a whopping relay that switches the mains to the rest of the power system, controlled by the front-panel keyswitch);
H7104C +2.5V PSU rated 85 Amp (the gate array chips in the CPU require +2.5V);
H7104D +5V PSU rated 135 Amp (just about everything uses this).
Other voltages are produced by plug-in regulators contained in the 2.5 and 5V supplies:

+/-5VB and +12VB (supplies for the memory system, backed up by the battery if fitted) are in the +2.5V PSU;
+/-15V supplies are in the +5V PSU.

Various of the regulators require specific minimum loads before they'll run.

Remove your watch & any rings when working on a live machine! The PSUs are capable of delivering huge currents, and shorting them could present a burn hazard.

What do the controls on the PSU do?


Red neon power indicator: the power controller is connected to the mains.
Overtemp: indicates overtemperature condition in the 2.5V or 5V PSU.
Overvoltage: overvoltage condition in 2.5V or 5V supply.
Overcurrent: overcurrent condition on 2.5 or 5 supply.
+5 fail: fault in +5v PSU.
+2.5 fail: fault in 2.5 v supply.
Power OK: all's well.
Reg fail: failure of plug-in regulator (+/-5VB or +12VB regulators in the +2.5V supply, or +/-15V regulator in the +5V supply).


Circuit breaker: Provides over-current protection and switches power to the rest of the power system (though you're not really supposed to use it as a switch). With the machine switched off from the front panel keyswitch, the power controller still draws a small current. Opening the breaker will prevent this.

Remote/Off/Local rotary switch: Determines how the machine responds to inputs on the DEC Power Bus connectors.
Remote: enables remote power up from the CPU control panel keyswitch.
Local: unconditionally powers the machine up, regardless of the position of the keyswitch.
Off: unconditionally doesn't power the machine up.

Each of the PSUs has a 'margin' switch (small three-position toggle switch) on top. LEAVE THIS IN THE CENTRE POSITION! The +/-5% positions are for testing purposes only. The use of these switches is not described in the CPU or PSU technical manuals.

What are the DEC Power Bus connectors for?

On the back of the power controller, there are two three-pin DEC Power Bus connectors, one labelled 'Normal' and the other labelled 'Delayed'. These are used to allow the front-panel keyswitch on the processor rack to control the mains supply to the other racks in the system. Pin 3 on each of the connectors is grounded. Pin 2 is an emergency shutdown: when grounded, it shuts the power system down. This is normally caused by a temperature sensor in either the processor rack or one of the other racks, or by the airflow sensor in the blower plenum signalling that the blower has failed. Pin 1 on each connector is a power-up request signal; when grounded by the front-panel keyswitch, it causes the power-controller relay to close, supplying mains power to the PSU. On the 'Delayed' connector, the power-up request output is delayed by about half a second; switching the other racks in your system from this output can prevent those embarrassing mains surges that trip the breaker on the mains ring and plunge the house into darkness...

Normally, the power controllers in all the racks of an installation are daisy-chained together, so that the front-panel keyswitch controls power to the whole system.

What's the power consumption like?

Not as bad as you might think. I currently run my 750 system (CPU, CDC sabre disk, BA11-K expansion box, RA81 disk, TU80 tape drive, RX02 8-inch floppies) off one 13 Amp outlet, on a 13 Amp fuse (UK 240 Volt mains). The plug runs a little warm, but no problems otherwise. I guess the overall consumption is about a couple of kilowatts.

What are the buses?

There are various buses within the processor, but the main external ones are as follows:

CMI (CPU-Memory Interconnect): This is the main bus on the CPU backplane. The memory system, massbus adapters, unibus adapter and any other boards plugged into the CPU I/O adapter slots are attached to this bus. It is 32 bits wide and synchronous. The byte address-space is 24 bits wide, so that CMI has a 16Mb address range. The unibus and massbus are interfaced to CMI via adapters.

Unibus: This bus was originally used as the system bus on PDP11 computers. Several early vaxes, including the 750, support it as a bus for attaching peripherals. There are sixteen data bits and eighteen address bits, giving Unibus a 256K address space. The unibus map hardware is used to enable devices on Unibus to access the full address space of the machine for DMA. Unibus is asynchronous, with each transfer being accompanied by a handshaking sequence.

Massbus: A bus for attaching disks and tapes. The bus is 18 bits wide, and transfers are clocked by the peripheral device. The control section is similar to Unibus.

What's a bus grant jumper?

Several Unibus signals, called bus grants, are daisychained through each slot. If a slot is empty, the signals have to be jumpered across the empty slot. This is done using a small, square handle-less board called a bus grant continuity card. (Later versions of the card are apparently full-length and have a handle, but I've never seen this type.) The standard bus grant card is G727A.

The card has no components on it. There are four tracks on the solder side of the board. The board fits into the fourth edge connector of the Unibus slot. Make sure that the side with the four tracks on it faces in the same direction as the solder sides of the other boards.

Unfortunately, it gets worse. There is an additional grant line, the non-processor grant (NPG), which is used only by DMA devices. This signal is normally jumpered by a wire-wrap jumper on the backplane, between the topmost two pins on the component side of the third edge connector of the slot. When a DMA option is installed, the backplane jumper is removed. If the DMA option is subsequently removed, or the slot is re-used for a non-DMA option, the jumper must be refitted.

Alternatively, it is possible to get hold of double-height grant cards, which fit into the third and fourth edge connectors, and jumper both the normal grant signals and the NPG signal. Some non-DMA devices jumper the NPG signal themselves (look for a PCB track linking the topmost two pins on the component side of the third edge connector for a hex-height module, or the top connector for a quad module).

Help! I disconnected the UBI from my machine, and it won't boot!

If (1) you are attempting to boot using the uda driver, and (2) your uda controller is actually an Emulex UD33, then this may be the solution.

The TOY clock is on the UBI module. When you unplug the UBI, the TOY clock loses power. When the clock is reconnected, it will read as zero. The microcode takes this as a sign that the clock has lost power (which it has), and doesn't allow the clock to increment until you write a non-zero value into it. When the UDA driver is probing for a uda (or compatible), it pokes the appropriate values into the controller's registers and waits a specific amount of time for it to interrupt. The time-out is done by taking the current value of the TOY clock (say V) and waiting in a busy loop until the TOY clock is V + some value. The UD33, for some reason, almost never initialises on the first attempt. So, the uda driver waits until either (a) the controller interrupts (which it doesn't) or (b) the TOY value becomes V + some value (which it doesn't, because it's not incrementing). Thus the machine hangs in a busy loop. The solution is to stick a non-zero value in the TOY clock after you've had the UBI disconnected. You can do this by saying this to the console:

D/I 1B 1

(Deposit 1 in internal processor register 1B, which is the TOY). Repeated examination of the register (E/I 1B) should show the clock incrementing normally, and the machine should boot as normal.

What are the memory boards?

The DEC ones were:

M8728: 256k byte memory array;
M8750: 1M byte memory array;
M7199: 4M byte memory array.

The M8728 and M8750 are in fact the same board, with different drams (16k or 64k) and different jumper configurations.

Third Party memory boards:

EMC Corporation EMC VX-1MB-750 (from "VX 730/750 Series Installation Service Guide"): 1M memory board for 750 and 730. Compatible with M8750. Board has an 'on-line/off-line' switch, which should be in the 'up' position for normal operation.

Mike Umbricht reports that he has a couple of EMC 1Mb boards, which have 210 030 600 etched on the board, and a sticker saying 'P/N 240-010-900'.

National Semiconductor Memory Systems: 1Mb memory boards. These boards have
PWB 551109464-002 B
PWA 980109464-001 D
etched on them. They have a push-button switch for disabling the board, and a spare memory chip in a socket on the board. Board has green and yellow LEDS, which should normally both be on. Disabling the board causes the yellow LED to go out, and the red LED on the L0016 to come on, indicating bad memory configuration. (info from Mike Umbricht)

Information about other third-party memory boards is welcome.

The memory backplane should be filled starting from the right-hand side. If you've got an L0022, the 4M arrays should be in the rightmost two slots.

Help! My machine crashes with the 'error' light on when booting BSD!

Note that it is normal for the error light to glow softly; only if it's on bright do you have a problem. If you're not sure whether it's lit or just 'glowing', the lamp test button on the bottom edge of the control panel board (accessible with the door open) lights it full brightness.

Symptoms: the console prints some blurb, ending with 'Updating 11/750 microcode:' and the machine then hangs with the control panel error light lit.

Your PCS is clobbered. This happened to me in the early stages of getting BSD installed on my machine. The BSD boot program is loading and enabling the PCS patches, and a fault in your PCS is then causing the machine to fall over with a control-store parity error.

Possible cures:

(1) fix your PCS; (2) get a spare board; (3) apply the following horrible modification to the PCS to disable the patch mechanism:

a) Locate and remove your PCS (L0008).
b) Hold PCS component-side up, with edge connectors down RHS.
c) The control store roms are the array of 24-pin chips at the top left of the board. Running down the board just to the right of the centre is the column of patch memory chips, which are 2148s. Locate two 2147 memory chips (srams), one above the other, at the bottom centre of the board. These are the flag memory chips. The mod you are about to make will cause the machine to read the flag memory as always 1, causing it always to read microcode from the rom.
d) Locate the only via (through-board hole) between the two srams. Locate the 74S74 directly to the right of the two srams. Leading from the via is a track which goes under the 74S74 (it goes to pin 2, in fact). Cut this track. There is another track directly above it and very close, so take care. This disconnects the output of the flag memory.
e) Locate the 74S74 from the solder side of the board. Add a pull-up resistor between pins 2 and 14 of the chip. I used a value of 10k, which may be a bit big, but did the job. This makes the hardware think the flag is always 1, so that it always reads the rom, not the ram.

There are probably more elegant ways of achieving the same effect, but this mod works and is obvious enough that I was able to figure it out before I got hold of the circuit diagrams for the board.

You should now be back in business. You could rename pcs750.bin to something else or move it out of the root directory, and then undo the foregoing mod -- BSD will just boot up without installing patches if it doesn't find the file. My machine ran perfectly happily without its control store patches for the year or so I had this mod installed. To repair the board properly, you will probably need the print-set for the board -- I recently got hold of a copy, and may be able to help out if you drop me a line. In my case, the problem turned out to be four failed 2148 srams -- very odd!

Help! My machine isn't listening to the console!

Symptoms: machine starts up normally and prints the successful microverify double percent and the console prompt, but ignores console input.

This may be because the RS232 line receiver on the console port has given up the ghost. I've had to replace the one on mine twice. Note that it isn't a good idea to leave the console terminal switched on if the machine is switched off -- the line receiver chip doesn't like this and tends to fail eventually.

To replace the line receiver:

a) Locate and remove the L0004 UBI module.
b) Hold the UBI component-side up with the edge connectors down the RHS.
c) Locate E53, a 1489, at the right-hand side of the board, near the top of the second edge connector. This is the only 1489 on the board.
d) Replace E53. I strongly recommend using a socket for the replacement.

Help! I've got a soft ECC error!

The 750 has error-correcting code (ECC) on its memory system. Each longword is stored as 32 bits of data, plus 7 check bits. Each check bit is formed by a parity computation on a different subset of the data bits. When the 750 reads a longword from memory, it recomputes the check bits. If a bit fails, the ECC logic can tell which data bit it was by looking at the check bits that come out with the wrong values. Knowing that the bit is wrong, it can then correct it simply by inverting it. It then generates an interrupt to report the error. This is called a 'soft' ECC error. If more than one bit is in error, the error cannot be corrected and is described as 'hard'. The check bits seem to be computed such that (1) a single wrong-valued check bit indicates that the check bit itself is in error, (2) any other odd number of wrong-valued check bits indicates that a single data bit is in error, and (3) an even number of wrong-valued check bits indicates that more than one bit is in error. This last case indicates a hard error.

On a soft error, BSD reports the physical page base address of the page which contained the error, and the error 'syndrome', which can be used to identify which bit has failed. (The syndrome appears to be the xor of the check bits read from memory and the computed check bits -- in other words, it indicates which check bits have the wrong values.) Unfortunately, the translation from syndrome to failed bit is not obvious. None of the print-sets and manuals I currently have access to describes the syndrome to failed bit mapping on the 750. However, the pdp-11/44 ECC mos memory boards use a similar system, and the mapping is described in the 11/44 print-set. I've used that mapping to fix my 750 when it had a soft error, and reproduce it here. Bear in mind that it's not guaranteed to be completely correct for the 750:

Syndrome	Failed bit
1		CB (check bit) 0
2		CB 1
4		CB 2
8		CB 3
16		CB 4
32		CB 5
64		CB 6
88		DB (data bit) 0
25		DB 1
26		DB 2
91		3
28		4
93		5
94		6
31		7
104		8
41		9
42		10
107		11
44		12
109		13
110		14
47		15
112		16
49		17
50		18
115		19
52		20
117		21
118		22
55		23
56		24
121		25
122		26
59		27
124		28
61		29
62		30
127		DB 31

I've written the syndromes in decimal and BSD reports them in hex -- sorry! To isolate the failed chip, you will probably need the print-set for the memory board concerned -- if your board is an M8750, you can probably get by with the one for the M8728, which is identical except for the drams and the jumper configuration.

What sort of hardware documentation can I get for the machine?

Quite a lot, in fact, though it's not cheap. As far as I can tell, the 750 print-set is still available from DEC, though the last time I asked, the price was upwards of 700 pounds. I don't know whether this includes print-sets for the optional add-on boards, or whether it's just the basic machine (and neither did the bloke I spoke to at DEC). I'm currently looking for print-sets for the RDM, the L0022 memory controller and the M7199 memory board. Manuals:

(Note that I haven't verified the availability of all of these -- I've only got copies of the ones I've listed prices for.)

VAX-11/750 Central Processor Unit Technical Description: EK-KA750-TD
About 90 pounds. Describes most of the non-optional bits of the processor. Does not describe the memory controllers, PCS or FPA. Very detailed in some areas, maddeningly vague in others. Makes extensive reference to a microcode listing which isn't included, and which I don't think you can get from DEC (certainly no part-number is given). The hardware handbook mentions (but gives no part number for) an 'Introduction to the Vax-11/750 Microarchitecture' (hardware handbook P. 213).

VAX-11/750 Unibus Interface TD: EK-UI750-TD
About 30 pounds. Describes UBI in detail. Much of this is described in less detail in the CPU manual above.

VAX-11/750 Power System TD: EK-PS750-TD
About 30 pounds. Describes the power system in detail.

FP750 Floating point accelerator TD: EK-FP750-TD
Apparently no longer available -- needless to say, I'd love to get hold of a copy.

RH750 Massbus Adapter TD: EK-RH750-TD
About 30 pounds. Describes massbus adapter in detail.

MS750 memory system TD: EK-MS750-TD
About 30 pounds. Describes L0011 memory controller, but not L0016 and L0022.

VAX-11 Diagnostic System User's Guide: EK-VX11D-UG

VAX-11/750 Diagnostic System Overview: EK-VXD75-UG
(Another manual lists this as EK-VXC750-UG)

VAX-11/750 Microdiagnostic Mini Reference guide: EK-KC750-RM
About 30 pounds. Brief reference to RDM commands, Micmon commands, 750 console commands, various error codes, internal processor registers &c.

KC750 Microdiagnostics and Technical Manual: EK-KC750-TM
Describes use of RDM. About 30 pounds.

VAX-11/750 Gate Array Chip Reference Manual: EK-GA750-RM

Installation/Acceptance Manual: EK-SI750-IN
(but my 'Vax 11/750 Installation Guide (preliminary)' is EK-SI75F-IN-PRE)

VAX Hardware Handbook 1980-1981
This book is very useful. I very much doubt that it's still available!

VAX Architecture Handbook 1981
Describes the instruction set in detail, addressing modes and memory management.

VAX Software Handbook 1980-1981
Describes VMS.

How do I get in touch with DEC?

To order spare parts and so forth, you need to get in touch with DEC. In the UK, the first port of call is probably the Digital (Dis-)Service Store, which can be contacted on 01734-201366. They can quote prices and so forth, and can (if they're in a good mood) find part-numbers for you.

Outside the UK, see the contact number listing on the Digital Assisted Services web-page for local phone numbers.

End of Vax 11/750 Frequently Asked Questions By James Lothian.