PDP-11/45: Some parts from eBay

I've been keeping an eye on eBay and have collected a few more goodies: a DD11-D nine-slot expansion backplane, a fully populated (128K x 18-bit) MS11-L MOS memory board, and a couple of replacement BC08-R cables for connecting the console to the CPU cards. The MS11-L is a bit of a luxury; I am figuring it will be easier to deal with during bring-up than the core memory systems I have on hand. It will also be nice to run with a full address space.

Below is an updated shot of the CPU chassis with the expansion backplane installed, populated with bus jumpers, terminators, grant continuity cards, the MS11-L memory, a DL11 serial interface, and an M792-YB bootstrap ROM. I've gone ahead and slotted in the FPU in the first four slots, since the cleaned and refurbished CPU cabinet is probably the safest place to store them now. Console cables are also installed:

The M792 ROM is an interesting bit, and probably worth a comment. This is a 32-word diode-matrix ROM card. The bits are physically laid out on the card (see pictures below); where there is a diode, there is a logical 1 bit and where there is the absence of a diode there is a logical 0 bit. Typically the matrix would be loaded with a bootstrap program, to save the operator from having to toggle it in from the console on each boot.

The program could be modified by physically adding or removing diodes in the matrix. My card has had such a mod; there is a handwritten note attached from some engineer describing this. The mod customizes the bootstrap to always load from an RK disk unit, to avoid having to toggle in the device address at boot. I will probably revert this mod because I like having things in fairly stock/usual condition.

For fun, here's the source listing of the stock bootstrap. You can match the octal digits of the machine code against the diodes in the ROM above (low word addresses at the top of the matrix, and least-significant-bits on the left).

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173100 013701         MOV     @#177570,R1     ;READ SWITCH REG FOR ....
       177570
173104 000005 BEGIN:  RESET                   ;FORCE CLEAR IF RETRY
173106 010100         MOV     R1,R0           ;....DEVICE WC ADDRESS
173110 012710         MOV     #-256,@R0       ;SET TO READ 256 WORDS
       177400
173114 020027         CMP     R0,#177344      ;IS IT DECTAPE?
       177344
173120 001007         BNE     START           ;NO. GO TO START
173122 012740         MOV     #4002,-(R0)     ;YES. MOVE TAPE TO FRONT
       004002
173126 005710         TST     @R0             ;WAIT FOR ERROR!
173130 100376         BPL     .-2
173132 005740         TST     -(R0)           ;IS IT ENDZONE?
173134 100363         BPL     BEGIN           ;NO. TRY AGAIN
173136 022020         CMP     (R0)+,(R0)+     ;ADJUST POINTER
173140 012740 START:  MOV     #5,-(R0)        ;NOW START ACTUAL READ
       000005
173144 105710         TSTB    @R0             ;WAIT FOR DONE
173146 100376         BPL     .-2
173150 005710         TST     @R0             ;ERROR ENCOUNTERED?
173152 100754         BMI     BEGIN           ;IF SO START OVER
173154 105010         CLRB    @R0             ;FOR DECTAPE,STOP TRANSPORT
173156 000137         JMP     @#0             ;GO TO ROUTINE LOADED

Hmmm, the Pygments syntax highlighting package used by my blog generator doesn't seem to grok MACRO-11; may have to do something about that...