# PDP-8 logic levels

Discussion in 'Electronic Design' started by Tom Del Rosso, May 6, 2013.

1. ### Tom Del RossoGuest

From the wikipedia article on the PDP-8:

"In the 8/S two different logic voltages were used, an inexpensive way to
increase the fan-out of the inexpensive diode-transistor logic."

What's that mean? Normally there are 2 logic levels. Do they mean 2 supply
voltages?

I know DTL modules in the Apollo guidance computer each had complementary
outputs, so maybe that's what they're talking about.

2. ### Bob VinesGuest

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

3. ### Tom Del RossoGuest

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

4. ### Tauno VoipioGuest

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.

5. ### petrus bitbyterGuest

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

petrus bitbyter

6. ### gregzGuest

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

7. ### Guest

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

8. ### Guest

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 something? Well that's not too different from the kind of people writing articles for Wiki these days, complete shaved apes!
You can look at the 8S series [gawd-awful] circuits and see they're using two different logic power supplies for internal pull-up/down biasing. This is what the /quote/ is referring to, and the Wiki author has no idea what itmeans so he conceals his ignorance by using quotation marks.

9. ### Tom Del RossoGuest

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.