On 06/10/2020 12:48 PM, Charles via cctalk wrote:
That leaves the unlikely possibility that one of the octal
TTL devices, or ROMs. has developed a weird internal
pathway that only interferes with DAL3 & 1 on some bit
patterns, but not all the time. Seems like a zebra rather
than a horse. The only part that drives multiple low-order
DAL lines at once besides the E19-22 ROMs is the E55 LS245.
Quite possible that this could happen when a specific device
is driving the bus -- or that NOBODY is driving the bus in
that state. When it is stuck at the ~1V level, try a
resistor of about 1 K to ground on one of those lines. If
it moves several hundred mV lower, it is a TTL open
circuit. If it doesn't change at all, it is a bus
contention (TWO drivers driving at once).
Jon
