I've reverse-engineered a few devices over the years. My method is somewhat slow, but it does work. I don;'t use computers, CAD tools ,or anything like that. Nor do I take photos of the board (I really can't see the point of doing tht if you have the board in front of you). What I use is : Pen and Paper (lots of the latter). Draw the scheamtic freehad, don't waste time usign a ruler to draw thwe wires. You can always redraw ity ot enter it iinto CAD system later if you really want to A _good_ continutiy tester. This must not be 'fooled' by diode junctiuons or by most resistors. A good Ohmmeter, readign down to 1 Ohm or less is useful for checkign that what appears to be a connection is just a connection and doesn't go via a 10 Ohm series trermiantion resistor Data sheets on all the ICs, or aty least as many of them as yu can find. If there are custom chips, you have a lot more work to do working out what is going on,, but you've said this doesn't apply. You want to be able ot refer to several at once, I find having them on paper is the only way that works for me. A good knowledtge of circuitry (yes, my old favourite the 'brain'). The point is that while a schematic 'just' shows the conenctions between the components, it is useful if it's easy to fiogure otu waht's goign on. And there are conventional ways to draw some circuits. A trivial example is a pair of NDAN gatges cross-coupled ot make an SR latch. It's easy to recognise if they;'re drawin one above the other with the cross conenctions shown. It's not hard to see if they're drawn in-lien witha feedabck loop. It's almsot impossible to see if the 2 gates are on differnt sheets with named signals linking them. This, IMHO is where a person does a lot better than any CAD tools OK... Make a list of all the 'significant compoents' on the board. Both 'significant# and 'component' need explanation. It's probably not worth lisiting every last pull-up ressitor, it's certainly not worth listing every decoupling capacitor (it may not even be worth drawing the latter). WHen you come to multi-section ICs, each section si a 'comnponent'. So a 7400 is 4 'componets' -- the 4 NAND gates. The start of such a list might be : U1 a b c d '00 U2 a b c d e F '04 U3 '138 U4 Z80A-CPU U5 6264 Desolder anything form the board that will confuse the continuity tester. In particualr, switches (unless simple on/off types, in which case turn them off arter noting the settings if applicable), configuration wire links (desolder one end), low-value resistors, inductors, transformers, unbuffered delay lines, relays and the like. Now start drawing (!). Start with things you can pin down, certainly find the pwoer supplies (and if there are any on-board derrived lines sort those out now. Find the master clock oscilaltor cirucit and reset circuit if there are on the board. Trace the connectiosn with the continuity tester and/or ohmmeter. It's a lot more accurate to do that than to try to do it visually, although follwign a trace will give you some idea where it goes so you have some idea of a set of pins to test with the continuity tester. If you're doing an expansion board, you have a bus connector that will identify addres and data lines for you. If you have a large chip (like a microprocessor), again it will indetify signals -- but see if any are buffered. When yuou are drawing, don't be afraid to splid the circuit up into little blocks and anem the input and output signals -- but give them sensible names. That takes knoweldge of the circuit design. Often I have to fg oabck and rename a few signals as I get more of it worked out, or I leave signals un-named (but make a note of where they go) until I've traced out another section As you draw out each component, cross it off the list. That way you don't miss anything, you don't find you're drawing the same bit twice. The easiest boards to do are ones with large, known, ICs like microprocesors of complex-ish peripheral ICs because things liek address and data buses are known. Boards of TTL with no other knowledge are worse. Boards of discrete transitors with no knowledge of the design are even worse (I've doen all types). Tou govive you some idea, I find I average 5 ICs pere A4 sheet of paper (this is a remarkably constant value for many types of circuitry, obviously it fails if there's a lot of discrete transistor stuff) and each page takes around 1 hour to trace, draw and document. -tony

On Tue, Dec 6, 2011 at 10:52 AM, Richard <legalize at xmission.com> wrote: I don't think this is exactly what you want, and you probably need to start with a bare unpopulated printed circuit board, but Grant Stockly has had good things to say about this service in the past: http://www.mhtest.com/scanning.shtml That looks to be more for recreating a circuit board than for documenting the circuit.

Why "need"? Why not wait until the appropriate docs pop out of the woodwork? ND made a fair number of those 812 minis, so there is probably a decent chance that eventually you can use the real deal. Of course, it is a gamble, and may take years, but the time spent doing all the reverse engineering could perhaps be better utilized elsewhere. -- Will