PDP-8 logic levels

supply

Tom,

The Wikipedia article appears to be incorrect as far as this sentence is concerned. The PDP-8/S is a negative logic machine using -3 and 0 volts. The DC voltages required by the logic are +10V and -15V. The PDP-8/S Maintenance Manual at bitsavers.trailing-edge.com/pdf/dec/pdp8/ pdp8s/PDP8S_MaintMan.pdf, page 1-3 states: "The dc voltages required by the logic are +10V and -15V. All logic is solid state; transistors and diodes operate on static logic levels of 0 and -3Vdc (tolerances are 0V to -0.3V and -3.2V to -3.9V)."

A friend experienced with PDP-8 family machines stated, "I would hazard a guess this is just a misunderstanding of the fact that DEC predates integrated circuits so the newer ones [PDP-8s] are plus 3 and the power supply is +5 while the older ones are -15 power supply and the logic is -3 volts." He also pointed out that there are R-xxx [labeled] modules and there are S-xxx modules. The difference is there are different resistors. The more expensive "S" cards have less fanout but higher speed than the R-xxx modules.

He also said, "Positive stuff has only +5. The old stuff has +10 and

-15. That's not a logic level consideration. Just that some aspects of the module need the 10 to make the spec. Plus 10 volts is even "lower" than 0 volts when you are speaking to -3 or so as the reference point. In any case, a designer of R-series modules can count on a +10 voltage to help cleanup signals if need be."

Bob

The original 8 and the serial 8/S used discrete transistors and negative logic levels. The PDP8/I ("integrated circuit") version was DTL and TTL, positive 5 volt logic.

THANKS for the expansive answer. That FTP site is quite a goldmine.

DTL is resistor pull-up open-collector logic. The supplies well out of the logic signal range help getting decent edge speeds from the pull-up. The opposite polarity supply is used to guarantee off-state bias on transistor switch bases.

"Tom Del Rosso" schreef in bericht news:km6rn2$tnt$ snipped-for-privacy@dont-email.me...

That PDP8 was a digital computer using two logic levels. Though to maintain that levels positive and negative voltages were used.

petrus bitbyter

I never got the knack of using those DEC symbols. I remember them doing a voltage test changing power supply AC levels, to see if the computer passed limit testing. The power supplies just used constant voltage transformers. I got in there learning integrated circuits on PDP-15 and PDP-8I.

Greg

That's not what "negative logic" means...

ply

Do you remember the scene from Planet of the Apes when they were reading a passage from their "Bible" that turned out to be a GM repair manual or some thing? Well that's not too different from the kind of people writing articl es for Wiki these days, complete shaved apes! You can look at the 8S series [gawd-awful] circuits and see they're using t wo different logic power supplies for internal pull-up/down biasing. This i s what the /quote/ is referring to, and the Wiki author has no idea what it means so he conceals his ignorance by using quotation marks.

Rod Serling wrote that script, and if a GM manual was his choice then I have faith in his judgement.

I put the quotes on it. The words were composed by the wiki contributor. I guessed right at supply voltages, but it has no relevance to fanout obviously.