!pr1
Speaking of Slow Chips.......................Robert H. Bernard

William O'Ryan's article (AAL June 1984) about making the 65C02 work in II+s reminds me of some other slow chip problems I have had in the past with Apples.

Years ago, I had a problem with an SSM AIO card in an Apple that traced to a slow 74LS138 at position H2.  The symptom was that every few hours the program would fly off into the weeds.  I traced it to the device select for the slot, which caused the data on the bus to be late for ROM program fetches from the card.  I was able to fix the problem in that case by swapping H2 with another '138 from a different (less critical) position.

Some time later I was able to fix a problem in another machine by swapping the ROM SELECT chip at position F12 (another 74LS138) with another '138.  There are apparently many marginal timing situations in II+s, and they are not necessarily in the oldest ones.

All this slow circuit stuff has some interesting side effects.  I personally had a number of conversations with SSM about this problem before I found the real cause, and all they could suggest was a capacitor on the clock line.  Even after I found the problem, the SSM people I talked to seemed uninterested in the fix, perhaps because they couldn't apply it directly to their product.

The unfortunate end result was that a number of organizations that previously sold or recommended AIO cards stopped doing so.  A domino effect was that our local retailer stopped pushing Anadex printers (which required the DTR signal, at that time only available on the AIO) rather than find another serial card to replace the AIO.  I always wondered if the Anadex people noticed the effect on their sales....
