17 Jan
2023
17 Jan
'23
1:51 p.m.
On Jan 17, 2023, at 10:17 AM, Bill Degnan via cctalk
<cctalk(a)classiccmp.org> wrote:
...
I found a good general description of how the negative / positive relates
to the PDP-8 and how peripherals fit into the picture.
https://homepage.cs.uiowa.edu/~dwjones/pdp8/models/
"The "negibus" or negative logic I/O bus used -3 and 0 volt logic levels
in
92 ohm coaxial cable" - a M531 would be used if attaching to a computer
with a negative bus. A G742 indicates the computer using the TU56 had some
form of positive logic bus.
The positive I/O bus, or posibus, was a 100 ohm bus clamped between 0 and 3
volts with TTL drivers and receivers.
Interesting to see the terms positive and negative logic used this way. I'm used to a
different usage, where "negative" logic simply means the lower (less positive)
voltage is used to represent the boolean value "true" (1) rather than
"false" (0) as it does for "positive" logic. For example, the CDC
6000 series machines are "negative logic" in that sense: they represent boolean
true as 0 volts and boolean false as +6 volts.
paul