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PDP-11/05 - S/N 09742

PDP-11/05 original condition
This PDP-11/05 was once upon a time the Control equipment for an Charmilles DCNC system and it arrived from Tommy in Säter. It had been used as spare parts for two other machines and then finally we took care of it. The PDP-11/05 was the OEM version of the PDP-11/10 and was shipped in two main variants. Either in the low 5.25 inch high chassis which was proprietary to the 11/05 or the bigger, 10 1/2 inch high. BA11-D chassis.

When received it dirty and several parts were missing or broken. A first inspection gave that both the top and bottom cover was missing. The front decorative panel beneath the switch front panel has a broken fastening thread. The cable to the switch front panel is damaged. The power supply is lose and not secured in the chassis and the six screws are missing. Further one connector in the PSU has become alarmingly hot. Then also the copper trace to one transistor has became very hot, as much as it has lifted from the PCB. Then also the fans for the card cage are missing. The top filler panel is replaced with an aluminium panel. 

H750 PSU


Work on the H750 power supply gave that the 5V H744 supply (to the left in the picture above) module was in good shape but the main supply (to the right) providing all other supplies had a big filtering capacitor which was in very bad shape. It stayed charged for a few hours while others stayed charged for several weeks. Replacing this with a modern one was a little bit difficult since finding a direct replacement was hard. Putting a modern electrolytic capacitor inside was possible, but it was fairly difficult to get the innards of the old one out off the shell. Then also two small caps were leaking and had to be replaced. Now the PSU was running fine when loaded. 

serial number

The serial number for this machine appears to be 09742 if I read the plate correctly. It is stamped PR which means that it has been produced at the Puerto Rico facilities.


First power up

The control board was marked "Faulty" but the Data Path board was marked "OK". With the console connector repaired we powered it up with only two bus terminators installed in the bus. No memory. At first it was quite responsive but shortly it ended up in a mode where nothing but the START switch made any difference. Pushing down the start switch made the RUN lamp to go dark and the display to increment. All the other switches gave no effect. Time for some error tracing.

Checking the schematics give that the switches enters on the Data Path board but goes directly to the backplane to connect to the control board. On the control board these enter into a register. The Deposit, Continue, Examine and Load address signals is decoded in a PROM to start addresses for the microcode. So it seems that the console handling is done in microcode. Also some parts of the microcode is visible on the backplane which makes it easier to measure those signals.

 Signal  M7261 Backplane Pin
 PROC CLK CN1
 PROC INIT FT2
 MPC07 FM2
 MPC06 FL1
 MPC05 FN1
 MPC04 FJ2
 MPC03 FK1
 MPC02 FM1
 MPC01 FN2
 MPC00 FP2
 ALUS3 EE1
 ALUS2 EF2
 ALUS1 EE2
 ALUS0 ED1
 ALU MODE ED2
 CIN EB1
 SHIFT EA1
 PRE AUX ER1
 START FE2
 LOAD FJ1
 DEP FD2
 EXAM FF2
 CONT FH2
 STOP DH1

Microprogram tracing

With the logic analyzer connected I took two different traces of microprogram flows. Firstly what happened at power up and then what happened after releasing the START switch. The microprogram address is the next address to execute since this is what is present on the back plane.

Since I had another pair of non fully working board I compared the behavior.

At power up

 Failing machine Slightly better working Microcode listing Comment
 000 000 000 
 241 241 241 
 247 347 347 
 010 010  
 245 245  
 246 246  
 247 247  
 206 226  
 110 110 110 Double bus fault
 321 041 041 
 062 302  
 373 302  
 363 302  
 repeat from 010 302 endless  

Why is it going from address 241 to address 247 rather than 347? Could there be a problem with bit 6 in the MPC (Microcode PC) signals from the E102 / A04A2 PROM? The MPC signals are wire ORed with different signals when the machine perform micro code branches. The MPC is active low so address 347 is 00011000 binary which means that it is impossible to wire OR it to get to 247 (01011000). After address 247 it is supposed to get to 226 not 206. This could potentially be a wire-OR problem. It is not a OC driver which has failed permanently since that would have caused problems with all addresses. So instead of probing on the backplane the control board was put on extenders and the logic analyzer clips were attached to various gates. None of the micro branch BUT enable signals were active at the time so nothing should be wire-ORed. Running the machine without the DataPath board gave exactly the same result so there are nothing there that is affecting the MPC bus.

START switch

 Failing machine Slightly better working Microcode listing Comment
 100 100 100 
 322 322 322 
 301 321 321One bit is wrong here!
 054 062  
 377 053  
 010 365  
 245 364  
 246 010  
 247 245  
 206 246  
 110 247  
 321 226  
 046 110  
 003 041  
 004 302  
 016   
 017   
 014   
 100   

Since all address that have problem is in the high nibble of the MPC a micro code PROM is suspect. E102 / A04A2. 

Removing the chip and reading it out show considerable differences from the micro code listing in the schematics. New PROM chips has been ordered. Asking around, there were no one else that has this problem before so we had to I had to resort to use the micro code listing and prepare a Intel HEX file from it. The new PROM was burnt and soldered into place. It worked much better than before. Now it was even possible to execute a small program out of the registers and access memory.

More PROM failures

The next step was to insert a M9301-YF boot card to see if we could make it run the console emulator over serial port so that I could download diagnostics into the machine and test it out further. Unfortunately this was not very successful. It halted at address 165102. The contents of the PROM has been dumped and spliced together. The resulting file was compared with the dump that Noel Chiappa has performed. It matched well. So why did the CPU read some words with the high bit set when it was not set in the PROM dump? This could be a CPU fault or M9301 board fault that has to be researched further.

Testing the M9301 board yet more with a logic analyzer actually showed that there were a PROM content problem in one of the chips. A new dump was made in the PROM programmer and this time there were a few bits most significant PROM that was always set to 1. Burning a new PROM solved this. It was now possible to do examine in the M9301 address space and recover content that matched the listing the Noel did earlier. At least a while. After a short while first one DEC 8881 bus driver failed and then soon another.

BIS instruction misbehaving 

The machine was still not working it just stopped with 165102 showing at the display. This is the code from a disassembly that Noel Chiappa have done.

M9301 code

; Low bank - contains console emulator, CPU diagnostics, and DL11 and PC11
; bootstraps

165000 165000	OPDAT:				; Operation test data
165002 165000
165004 100000
165006 177777
165010 177777
165012 165010
165014 165006
165016 000500
165020 000501

; CPU Test 1 - Single operand tests

165022 005003	DIAGS:	CLR	R3
165024 005203		INC	R3
165026 005103		COM	R3
165030 006203		ASR	R3
165032 006303		ASL	R3
165034 006003		ROR	R3
165036 005703		TST	R3
165040 005403		NEG	R3
165042 005303		DEC	R3
165044 005603		SBC	R3
165046 006103		ROL	R3
165050 005503		ADC	R3
165052 000303		SWAB	R3
165054 001377		BNE	.		; Loop on failure

; CPU Test 2 - Double operand tests

165056 012702		MOV	#OPDAT, R2
165060 165000
165062 011203		MOV	@R2, R3
165064 022203		CMP	(R2)+, R3
165066 001377		BNE	.		; Loop on failure
165070 063203		ADD	@(R2)+, R3
165072 165203		SUB	@-(R2), R3
165074 044203		BIC	-(R2), R3
165076 056203		BIS	12(R2), R3
165100 000012
165102 037203		BIT	@14(R2), R3
165104 000014
165106 001777		BEQ	.		; Loop on failure


Normally the display shows the instruction of the next instruction to execute which means that the halt was at 165100 which is the offset of the BIS indexed addressing operaton. Poking some instruction into core memory and experimenting from the console of the machine revelas that not only BIS has problems, but also MOV and BIC. Could there be some general problem with indexed addressing causing trouble? Testing deferred indexed addressing works just fine.

To understand more I attached the logic analyzer to the microprogram address and did some tracing. Trigger was set on micro address 062 octal which is the first micro step in the fetch phase.

 Non working control board     Working control board  Comment
 062 062 Fetch phase
 053 053 
 365 365 
 364 364 
 061 061 
 215 215 
 025 025 Source mode 6 processing
 026 026 
 027 027 
 030 030 
 004 244 What?

More tracing also gave that there were no micro branching taking place and the failing bits is in the upper next address bits. Was there something wrong with the burnt PROM? Checking the original files gave that unfortunately the location 30 has been missed while typing in the file.

PDP-11/05 - a restoration project







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