Does a schematic exist? (I suspect I know what the answer is going to be....)
Another less intrusive option for diagnosing machines as valuable / unique as this is to use a 16-bit logic analyzer to capture a trace of the data bus and control signals and then attempt to decode it back to instructions.
Do you have access to a logic analyzer?
For the 6502, 65816 and Z80, I have developed trace decoding tools:
- 6502:
github forum thread
- 65816:
github forum thread
- Z80:
github forum thread
These give you and instruction level view of what's happening, for example:
Code: Select all
$ decode6502 --phi2= -sh test/beeb/reset.tmp | head -20
???? : : RESET !! : A=?? X=?? Y=?? SP=?? N=? V=? D=? I=1 Z=? C=?
D9CD : A9 40 : LDA #40 : A=40 X=?? Y=?? SP=?? N=0 V=? D=? I=1 Z=0 C=?
D9CF : 8D 00 0D : STA 0D00 : A=40 X=?? Y=?? SP=?? N=0 V=? D=? I=1 Z=0 C=?
D9D2 : 78 : SEI : A=40 X=?? Y=?? SP=?? N=0 V=? D=? I=1 Z=0 C=?
D9D3 : D8 : CLD : A=40 X=?? Y=?? SP=?? N=0 V=? D=0 I=1 Z=0 C=?
D9D4 : A2 FF : LDX #FF : A=40 X=FF Y=?? SP=?? N=1 V=? D=0 I=1 Z=0 C=?
D9D6 : 9A : TXS : A=40 X=FF Y=?? SP=FF N=1 V=? D=0 I=1 Z=0 C=?
D9D7 : AD 4E FE : LDA FE4E : A=80 X=FF Y=?? SP=FF N=1 V=? D=0 I=1 Z=0 C=?
D9DA : 0A : ASL A : A=00 X=FF Y=?? SP=FF N=0 V=? D=0 I=1 Z=1 C=1
D9DB : 48 : PHA : A=00 X=FF Y=?? SP=FE N=0 V=? D=0 I=1 Z=1 C=1
D9DC : F0 09 : BEQ D9E7 : A=00 X=FF Y=?? SP=FE N=0 V=? D=0 I=1 Z=1 C=1
D9E7 : A2 04 : LDX #04 : A=00 X=04 Y=?? SP=FE N=0 V=? D=0 I=1 Z=0 C=1
D9E9 : 86 01 : STX 01 : A=00 X=04 Y=?? SP=FE N=0 V=? D=0 I=1 Z=0 C=1
D9EB : 85 00 : STA 00 : A=00 X=04 Y=?? SP=FE N=0 V=? D=0 I=1 Z=0 C=1
D9ED : A8 : TAY : A=00 X=04 Y=00 SP=FE N=0 V=? D=0 I=1 Z=1 C=1
D9EE : 91 00 : STA (00),Y : A=00 X=04 Y=00 SP=FE N=0 V=? D=0 I=1 Z=1 C=1
D9F0 : C5 01 : CMP 01 : A=00 X=04 Y=00 SP=FE N=1 V=? D=0 I=1 Z=0 C=0
D9F2 : F0 09 : BEQ D9FD : A=00 X=04 Y=00 SP=FE N=1 V=? D=0 I=1 Z=0 C=0
D9F4 : C8 : INY : A=00 X=04 Y=01 SP=FE N=0 V=? D=0 I=1 Z=0 C=0
D9F5 : D0 F7 : BNE D9EE : A=00 X=04 Y=01 SP=FE N=0 V=? D=0 I=1 Z=0 C=0
They also model memory, and warn if the value read differs from the original value.
The most important tool in facilitating this is a high quality 40-pin DIP test clip.
If you were able to capture a long trace of the machine after reset, then this technique is applicable. I would be happy to start work on a decoder for the 6809.
For valuable old machines, this technique is I think a better first step than the ICE, because it is completely non-intrusive, and much less likely to cause any damage than trying to use an ICE.
Edit: I've just noticed this wonderful picture on your website:
https://binarydinosaurs.co.uk/Museum/Po ... ld-18.jpeg
So it does seem like you should be setup to capture a trace from the 6809. The only issue with the HP 1660A is the low memory depth (4K/8K samples). But it would be a start if triggered properly on reset.
Dave