Malfunctioning VT240 - help please

Charles charlesmorris800 at
Wed Jun 10 22:08:42 CDT 2020

On 6/10/20 4:31 PM, Jon Elson wrote:
> 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

After much Googling, I discovered/remembered that the RQDX3 M7555 floppy 
controller card in my PDP-11/23+ system has a T11 CPU on board!

So I pulled the card and popped the T11 into the VT240. Guess what - the 
terminal still doesn't work!! Craptastic. At least it's not the most 
expensive and rarest part on the board... but now I'm really stumped. 
This isn't my first rodeo - in fact back in the 80's I used to design 
microprocessor systems for a living, and have continued to keep my hand 
in repairing my video arcade games and a PDP-8 system, among other 

Meanwhile... the T11 DAL lines are only connected to a few parts that 
can drive onto that local bus. Time to have a look at the glue logic for 
the DRAM selects. Although the ROM chip selects seem to work, maybe the 
DRAM or something else actually IS conflicting despite the mixed signals 
(pun intended) ;)

Time to break out the logic analyzer, and start burning pairs of 27256 
EPROMs with test programs. Maybe initially just fill them with NOP's 
(000240 octal) with a jump to zero at the end!

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