DEC VT05 with OS/8 -- basic system rebuild?

Tony Duell ard at
Mon May 18 15:30:23 CDT 2009

> > More seriously,. I would have thought you could make hardware
> > modifications ot the serial interface PCB, possibly to gate ome of the
> > ready signals with the flow control input. Or (in this case), detect the
> > CR (or is it LF that's the problem?) and then hold the ready line
> > deasseted either for enough time for the VT05 to complete its operation
> > or until it gets some ecternal acknowledge signal.
> >
> > No, I am not suggesting this is a sensible solution to the problem,but it
> > is _possible_
> Someone suggested a microcontroller to kludge up a flow control signal
> from the VT05 earlier.
> Once such a signal is derived (however that is done), the 8/i, 8/L, and
> PDP-12 *do* have a TTL signal input that will perform this function.
> There isn't a single "serial interface PCB", however.  There's one for

Sure. The only PDP8 I have direct experience of is the 8/e on my desk, 
but I've read the PDP12 prints for fun...

> input, one for output, another for the baud rate generator, and another
> yet for the current-loop interface.  They are connected together by the
> custom wire-wrap "backplane".  So you'd need to convert the flow control
> signal to a TTL level, then apply it to the magic wire-wrap post on
> the backplane.


My comment about 'modifying the serial interface board' could be taken as 
correct in that case, though. That 'magic wirewrap pin' presumably 
conencts to one on the PCBs in the serial interface circuit. So you 
_could_ pick up the signal on that PCB (Personally I wouldn't, on the 
grounds that unwrapping a wire from a backplane pin is easier to do than 
desoldering a wire from a PCB if you ever wanted to reverse the mod). 

> The nice thing is that it could be done with minimal, reversable mods to
> the vintage gear.


Which PDP8 does the OP have? From an earlier comment I gathered it was an 
Omnibus machine, but maybe I misread that. In that case I think you do 
have to make some modification to the serial interface PCB (and it is 
likely to be one PCB on those machines)


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