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To answer John's question: Yes, the window of the EPROM is covered with black electrical tape. I've found this to be a particularly useful way to do it, since the electrical tape doesn't stick "too hard" to be easily removed, and tends not to gum up the window either. To answer Calamity Jane: Yes, these are EPROM that worked in this circuit until very recently. None of the hardware changed from the working to non-working scenario. In fact, very little of the code even changed between times when it did and did not work. What makes me believe it is the chip is the fact that I can program the identical code into a more recent EPROM, and everything works fine. All my CS lines are tied correctly, as are the unused interrupt pins on the HC11. The decode logic for a 32k ROM is very simple - basically an inverter on AD15 for -CS, and a NAND of E and R/W for the -OE signal. The delay times, according to the data sheets for these part should be less than 10ns. The 27c256 I am using are 120ns parts, but my master clock is only 4MHz, which means E is 1MHz. A 120ns part should be plenty fast to meet the timing criteria. I believe I have plenty of bypass caps on the board, but I'll try adding some more. Thanks to all! --- WinnonaD <> wrote: > It sounds like "it's running on FFs". > > Did these chips work before and don't work now (in > this same board)? > > If it's the first time around in this board, these are > basics, but good to check as they are the most likely > causes: > > 1- Are all the unused CS lines tied firmly to +5v or > ground? > > ROMs can vary in the loading and "float point" of > various inputs. > > 2- Does the address-decode leave enough access time > and is it free of noise or extraneous swithing? > > Some ROMs can be way faster than average and respond > to extraneous switching; some are slow and need the > FULL specified access time. Some are more sensitive to > noise. > > 3 - Ensure you have a Vcc/Gnd bypass cap. > > Usually if some chips work and others don't, there's > something borderline in the board circuitry. > > Calamity Jane > > __________________________________ __________________________________

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