Repair methods (was Cromemco 3101/Beehive B150 score)

On 6/4/06, Tony Duell <ard at p850ug1.demon.co.uk> wrote: Well... as someone who considers his own knowledge of analog circuits to be inferior to his own knowledge of digital circuits, let me say this - while I _know_ that at the lowest level, all digital circuits are really just collections of analog parts, in practice, as long as you aren't working with a design at the edge of the envelope, you can ignore 90% of analog theory and still have an understanding that works on the circuit in front of you. For example - if you are building a clock out of TTL, if you follow the recommendations from the various chips data sheets about fan-out, etc., it's not that difficult to come up with a design that requires nothing more elaborate than a logic probe to debug. In fact, if you build an electronic debounced momentary switch, you can use a DVM to "scope" the circuit, pulsing it as needed to step through the states. OTOH, I've seen mysterious problems take a long time to solve when *completely* overlooking the analog aspects of a TTL circuit - one in particular was undershoot and ringing with a 74S409 DRAM controller hooked up to a field of 32 4164s. The eventual solution was to insert 33 Ohm resistors in-line on the CAS and RAS lines to dampen the undershoot to where the ringing didn't cause a problem - it was still there, but at an amplitude that didn't have a negative effect. I was amused to see the identical solution 2 years later in a Commodore "slap-on-the-front" RAM expansion for the Amiga 1000 - the card was literally the connector, one capacitor per DRAM, the DRAM, and a couple of 33 Ohm (38 Ohm?) resistor packs. So there are certainly many cases where it _is_ important to have a good grounding (pun intended) in analog theory (transmission lines, parasitic capacitance, hidden inductance, RF effects, etc), but for a lot of TTL work, pretending that everything is exactly +5 and ground will still get you a working circuit. When it _doesn't_ work, though, be prepared to pull out all the conceptual tools you can. Obviously, the faster the clocks and the more complex the design, the analog side does start to rear its head earlier. -ethan