7474 Chip Replacement QUESTIONS

From: Wirehead Prime <wirehead_at_retrocomputing.com>
Date: Thu Mar 12 18:41:45 1998

In repairing the SWTPC 6800 MP-A CPU board, I've discovered I THINK that
a 7474 which is used to generate NOT HALT to the CPU (and to halt it
under certain circumstances) is keeping the CPU halted all the time even
though the bus says there's no reason for it to be halted.

Here's why and I want someone to tear apart my logic before I desolder
the 7474 and replace it:

Preset and Clear are high, which makes them inactive.
The clock is pulsing high (IE present positive edges) to the clock input.
D, which is high, should be clocked into Q so that it should be high as
well under these circumstances, telling the CPU to NOT HALT.

Instead, Q is low and NOT Q is high which is the reverse of what SHOULD
be the case. What's the deal? Bad 7474? Or is my thinking SCREWED UP?

IF my thinking is correct and my suspicion that the 7474 is bad, is it
safe to replace it with a 74LS74? A friend of mine and I had a long
conversation about what you can replace with what and I've forgotten
what he told me about that.

If I'm goofy in the head, let me know and let me know why.

Thanks...

Anthony Clifton - Wirehead
Received on Thu Mar 12 1998 - 18:41:45 GMT