25 Oct
2018
25 Oct
'18
1:01 a.m.
On Thu, 25 Oct 2018, Guy Dunphy wrote:
Keep the objective in mind. What you want to end up
with is a schematic,
that is laid out in a way that aids comprehension of how the circuit
works. Typically this means overall left to right functional or power
flow, with separate functional blocks visually separate, visual emphasis
where appropriate, and so on. Something like the original designers
drew, if they were any good.
When you have only a PCB and want to reverse engineer the schematic, the
tasks are:
[...]
This is actually the way how I reverse-engineered the MINCAL 523. Identify
the address and data busses, registers, latches, functional sections (e.g.
ALU, interrupt related, I/O, ...) and put that all together. And yes, it
involves a lot of paper and pencil work, and that is faster and much more
intuitive than doing it with the computer.
To create the schematics I use gschem from the gEDA suite.
Currently, I have started to reverse-engineer the Digico computer. I have
only looked at the CPU board so far, but that leads to a dead-end as I am
not able to unambiguously identify the address and data busses. So I have
to continue with the front panel, start with the display/keypad where you
can select the individual registers for entry/display and go back to the
front panel connector back to the CPU board. There, I hope to find the
instruction register and continue with the instruction decoder section.
Christian