PDP-8/A bus levels?

Tony Duell ard at p850ug1.demon.co.uk
Mon Oct 10 12:04:32 CDT 2005

> I am trying to figure out the voltages on my PDP-8/A (Omnibus)
> logic lines. DEC's documentation, as usual, is confusing at best
> on this (and other) subjects, and sometimes apparently
> self-contradictory...
> DEC used suffixes -L and -H to mean active-low and -high
> respectively (that is, a logical assertion). They also say when
> interfacing to the Omnibus that standard TTL levels are used. But
> elsewhere in the manual it says Logic 1 levels are near 0 volts
> and logic 0 is near 5 volts! So does this statement apply only to
> the bus signals that are already active-low?

They've tried to over-explain it, and confused everybody :-)

> Am I correct or not in interpreting, for example, an address line
> named "MA 11 L" to be a logic 1/true/asserted when 0-0.8 volts and
> logic 0/false when 2.0+ volts (TTL levels)?


>From my expeirience with Omnibus (and Unibus for that matter), yes. An 
address (or data) line is a logical 1 bit when it's a TTL low. That is, 
for location 0 in memory, all the address lines are TTL high.


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