29 Mar
2005
29 Mar
'05
12:31 p.m.
On Mon, 2005-03-28 at 22:41 +0100, John Honniball wrote:
Jules Richardson wrote:
This implies that some buffer chip that feeds to 8530 is
faulty. It may be a 74LS244, and 74LS245 or something similar.
Try to trace bit 6 of the 8530's data bus back to a buffer.
Right, no buffer unfortunately.
It'd appear that one of the big LSI chips handles the data bus to the
SCC 8530 chips, which is also common to the clock IC, the board's ID
PROM, and also an octal D-type flip-flop IC (as an input). the latter's
responsible for latching the diagnostic LEDs on the edge of the board I
think.
I'd be surprised if the system would boot (which it appears to be doing
fine) if the LSI chip was busted and the machine couldn't talk to the ID
PROM (and probably the RTC chip too), but I suppose it's possible. I
don't recall it having lamp #6 of the diag LED array jammed on either.
So, it's *probably* the SCC but I think I'll take the board back to the
museum next time I'm there and as Dave said, stick a 'scope on it just
to be certain.
One SCC drives serial ports A and B, and another drives C and D.
Unfortunately it's probably more trouble than it's worth to try and fool
the system into swapping those over via a few hack wires on the board to
see if it fixes things - as there's an interrupt chain it's not as easy
as just swapping chip select lines.
cheers
Jules