PDP-8 logic levels

[Tom Del Rosso]

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.

 

[Bob Vines]

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 2supply
voltages?

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

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

 

[Tom Del Rosso]

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.

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."

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

 

[Tauno Voipio]

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.

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."

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.

 

[petrus bitbyter]

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.

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

petrus bitbyter

 

[gregz]

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

 

[[email protected]]

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.

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

 

[[email protected]]

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?

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.

 

[Tom Del Rosso]

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!

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

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 it means so he conceals his ignorance by
using quotation marks.

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.