CC> I'm Bummed :-(

On Wed, 8 Oct 1997, Jeff Kaneko wrote: Hmmm... Now, I've never seen this card, but that suggests one of 2 things to me - either bit-rot in the control EPROM (I hope you've dumped it...) or video memory failure. If this thing uses 2114's for video memory, replace the set. I've had more problems than enough with that excuse for a RAM chip :-). I've had them fail in Tandy M3's, CBM 8050 drives, Vectexes (Vectrices?), etc. Now's the time to learn the gentle art of reverse engineering... Seriously tracing out schematics is not hard (which is why companies who refuse to supply them 'as it makes it harder for our competitors to copy' are being plain stupid). IMHO it is easier, quicker, and plain less hassle to draw out any schematic of < 400 chips than to battle with the manufacturer. How to go about it : 0) Get a good continuity checker. Make sure it responds quickly (HP suggest < 1ms...), and that it's not fooled by diodes, etc. Designs for these have been published in electronics magazines. A beeper type of output is very useful. 1) Look at the board. Make a list of all the IC's and get data on them. Obviously there's no point in looking up all the TTL functions if you have the TI bible on the shelf. But make sure you have all the others. Custom chips are a pain, and I'll cover those later. 2) Now list all the IC's _and sections_ on the board in component-number order. You'll end up with a list like : IC1 a b c d 74LS00 IC2 a b c 74LS10 IC3 6845 IC4 a b 74F74 IC5 Z80A CPU IC6 a b c d e f 74LS04 etc. 3) Start drawing. Start with the largest/most complicated IC - like a CPU. There's essentially only one way to use a CPU, and once you've identified it, you probably have the data and address buses found as well. A thing like a NAND gate. Trace out, finding things like memory arrays, address decoders, buffers, etc. As you draw each section of a chip, cross it off the list. That way, you know what you've drawn, and you don't waste time doing the same bit twice. 4) If there's no CPU, start from some other reference point. A bus connector, perhaps. Or a large video chip. Or _anything_ that identifies a lot of signals. 5) You'll end up with the 'glue' logic. You basically have to work through this, which is a pain, but often you can identify well-known sections, particularly if you've seen a lot of schematics. Experience counts for a lot here. Custom chips : These are a PAIN!. Often you can identify most of the pins by what they're connected to - CPU bus signals, RAM bus signals, memory select lines, etc. Make a pinout table, and fill it in. Interconnected custom chips : Panic. A logic analyser may help you distiguish data from enable from clock, but you will spend a long time figuring it all out. And of course the device has to be working for you to do this. Which brings me on to my next point. The time to start thinking about maintenance/repair is when the device is still working. That's when you can take dumps of the ROMs (and know they're good), can record important waveforms (and know they're correct), etc. After it's failed you have a lot less to go on. -tony