A simple system debugger board

Hi I've been thinking for some time about something simple to help fix old machines. For the most part, the ROMs can be scanned and the RAMs swap as well as the CPU but the failure in things like the various address decoders and such often require specific code to make it easy to check. I've been thinking that one way to deal with this is two write small test loops for testing specific parts and then blowing these in EPROMs. This works well and I've used this method in the past but usually it requires specific knowledge ( schematic and/or manuals ) about that specific machine. What I've been thinking of is a simpler idea that requires less knowledge about the machine. One just needs a little knowledge about the processor. The idea is to make a simple socket header to go between the processor and the rest of the machine. All it would have is some simple circuit to determine read or write from the processor. It would have data buffers such that it would write through from the processor to the machine. On reads, it would intecept the data and replace it with an instructiion provided by some dip switches. It would not effect addresses, just let the processor free run. One could then easily probe a lot of circuits with just a logic probe. I'm not sure which single instruction would be best but for a 8080, a push or pop are quite useful. The entire circuit would just be a little glue logic and a few bus buffers. Any other thoughts? Dwight _________________________________________________________________ Hotmail is redefining busy with tools for the New Busy. Get more from your inbox. http://www.windowslive.com/campaign/thenewbusy?ocid=PID28326::T:WLMTAGL:ON:…