Reverse Engineering 15 yr old electronics

[Tony Duell]

> 
> 
> While I, like a lot in this group, tinker with old computers, I have 
> started another project that involves more than old computers.  Hope this 
> is close to being on topic, some of you may find it interesting.
> 
> I purchased parts of a Nicolet Fourier Transform Spectrometer with the goal 
> of putting together a working Michaelson Interferometer.  What I have is 

What fun!

[...]

> Any tips on how to efficiently reconstruct circuit diagrams from inspection 
> of the electronics?  There are two cards, with standard TTL and analog 
> parts.  The printed circuit has a design date of 1985, while the IC's have 
> 88-89 dates.

Good luck! I've traced out a fair few schematics over the years, so can 
possibly give you a few hints.

Firslty, difficulty of reverse-engineering depends on the complexity of 
the components. Easiest first, it goes like this : 

Large LSI chips that are known (e.g. microprocessors, LSI I/O devices). 
These can essentially only be used in one way, so yu can identify a lot 
of signals from one chip (like address and data buses)

TTL gates and flip-dlops. Harder. A NAND gate could be doing a million 
different things (it could be half of an SR, too...). 

Disvretes. Even harder. A transistor might not even be a digital function 
(it might be part of a power regulator even on an essentially digital 
board, for example). Do not pick the HP9100 as your first 
reverse-engineering project :-)

Unknown/custom LSI parts. Painful. You don't know what the pins are. If 
you can power the thing up you can make some good guesses as to some of 
them by examining the signals (I did this for some of the HP calculator 
chipsets, I did know roughly what signals to expect, I just had to find 
out which was on each pin).

Assuming you know what all the ICs are, start by indentifying the power 
and ground rails (shouldn't be too hard on a board of TTL). Do watch out 
for chips not powered from the main rails, though.

Make a list of all the ICs and the sections in them. It will look 
something like this : 

U1 a b '74
U2 a b c d e f '04
U3 '163
U4 a b c '10

Then, as you draw out each gate/chip, cross it off the list. That way 
you'll know what you've not done, and conversely you'll not draw out the 
same stuff twace. 

If the ICs are house-coded and you know the equivalents, you put the 
standard number on the list.

If necessary, list the transistors (and even R's and C's) too. 

If there are no 'U1' etc numbers on the board, you have to assign them. 
Either sketch out the layout of the board and write a number for each 
chip, or stick labels on the actual ICs. I normally do the latter -- the 
local stationary shop sells sticky lables numbered 1-160 (3 times over in 
different colours) about 6mm in diameter. They sitck nicely onto most ICs.

Make sure you have data on all the chips. A TTL databook is essential, 
often asking here will get pointers to info (it's worked for me in the 
past :-)).

Use any info you have about external connections to identify signals. Not 
a lot of help in your case, but if, say, you're doing an ISA card, 
obviously use the slot pinout.

This way you'll get a schematic of the board. How much help that is in 
linking up to the mechanical/optical side is another matter.

-tony