On 01/02/2025 02:31, roger arrick via cctalk wrote:
In 1977, at age 16, I went to work for Noakes Data Communications in Irving
Texas.
We built an 8080 industrial computer, made modems, and repaired lots of comm
gear.
RS232 was what we lived and breathed. And back then almost all the control
signals were actually used, not just jumpered or ignored.
I remember thinking at the time that the bipolar signal levels were such a
waste of time for office and personal computers. They should have just gone to
a TTL version for everything local like printers, and modems, and keyboards and
terminals. Back then we had to use 1488 and 1489 level converters with +/-12v
power supplies. Such a costly hassle. Of course, many years later we got
MAX232 with 4 .1uf caps and 5v which solved the cost problem.
I still have my blue breakout box from that year. It cost something like $300
at the time which was the price of a used car 🙂
I documented the company here:
https://www.rogerarrick.com/noakes/
[https://www.rogerarrick.com/noakes/noakes_label.png]<https://www.rogerarrick.com/noakes/>
Noakes Data Communications - Irving Texas -
1970s-1980s<https://www.rogerarrick.com/noakes/>
Noakes Data Communications. 1 May 2023. Click images for larger view 1973-1987
Noakes Data Communications 3330 Stovall St, Irving, TX 75061 214-790-1050
www.rogerarrick.com
I certainly agree TTL would have made sense for microprocessors but
earlier computers ran at much higher voltages, and lots of them :-)
It was always a pain getting -ve on non-Intel/Zilog machines like 6502.
Why? 'cos Z80s already had it for dynamic RAM anyway.
We've all been talking about the voltage being +/-12V but the standard
was much higher (15V or 25V IIRC depending on the year). But that had
two effects - the input pins needed to stand the voltage swing, but the
output voltages could be whatever you liked - including +/- 5V. Quite a
lot of them were at this level in microcomputers. Why? Because the
voltage would drop in the cable and the receiver really didn't care as
long as it was one side or the other of GND by the time it arrived. A
long cable starting at +15V would look the same as a short one starting
at +5V.
IME +/- 12V was a de-facto standard on microcomputers because they
already had a +12V and -12V rail, along with +5V. The ubiquitous 4116
DRAM needed +5V, -5V and +12V so +/- 5V was a popular option too.
These days you can, of course, get RS-232 driver chips that take TTL in
on one side, +5V, and derive the correct voltages internally. For only a
few pence. Problem gone (except they tend to be SMD and a PITA to mount
on strip-board. Grrr).
Regards, Frank.
