Malfunctioning VT240 - help please

Jon Elson elson at
Wed Jun 10 16:31:35 CDT 2020

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).


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