31 Jan
2025
31 Jan
'25
4:19 a.m.
I can provide some details of implementation, but not the design phase,
from the early 1970s era.
I worked as an FE on a site with three Univac 418s ( 4 us cycle time,
18-bit bus) for Bell Canada from 1971-1975, which were connected to a
store-and forward message switcher using Teletype machines with 85A1
controllers. I believe that the system was installed in 1968.
The system drove 64 teletype lines all over Canada, and certainly
current-loop was the flavor-du-jour when it came to long distance
telegraphy, I remember getting some nasty shocks from the 130VDC on the
lines needed to drive the 60 mA current loops.
I would say that the current loops themselves were the main factor that
limited baud rate - each line had mercury-whetted relays as it left our
floor to drive the line down eight floor to the telegraph operating centre.
There were no crystals, clocking was done using RCL networks, and one of
my routines was to check and adjust the baud rate using a scope on all
lines. There were separate tunable inductors on a card in each serial
line controller for actual baud rate and duty cycle. I know this full
well because one Sunday morning, during scheduled down time, I went
through and adjusted all these the wrong way - adjusting the baud rate
coil for duty cycle and vice versa. Needless to say, I found God that
morning as I panic-strickenly tried and found my mistake and managed to
correct it just before my boss arrived after he was called in due to the
delayed startup! AFAIK he never found out what I did, but the yards and
yards of console paper with error messages proved I was not making it
up! The console of the 418 was a Teletype machine also, running at 110Baud.
Each serial line was controlled by two rows of cards, mounted in a 6U
panel 23 inches wide. There were no UARTS! There were 16 lines in each
CTMC cabinet (Communications Terminal Mode(?) Controller) sometimes
called SCS cabinets (Serial Communications Subsystems).
Next to the CTMC cabinets, We had a cabinet full of Bell 103 modems,
used for dialling out on to the TWX (Teletypewriter Exchange) network
. I believe there were some modems for the new high-speed 300 baud,
but I don't think they were used in production while I was there. Some
300-baud GE Terminettes appeared later which I heard of from old
friends. Incidentally, TWX lines were regular telephone lines, and had a
regular telephone number iun adidtion to their TWX number. For example,
in Toronto, the (610) 491-xxxx TWX number was translated to (416)
962-xxxx. I really impressed my work mates by dialling in to our
maintenence room and waking them up with a lot of Bells during the night
shift :-)
AFAIK, there were only 3 sites with Univac 418s in Toronto. Ours at
Bell, CN/CP Telecom had one doing the same job, and there was one at
Police HQ running the traffic lights around the city.
One time I had to get parts from the Univac FEs at the other telecom
company and showing my Bell ID at the door was strictly guarded while
the FE got the parts - such was the competitiveness between telecom
companies at 110 Baud!
Wonderful memories, thanks for triggering them!
cheers,
Nigel
On 2025-01-31 03:20, Steve Lewis via cctalk wrote:
Hey all! So, I've found myself studying up on
RS-232 this year for a few
reasons.
I'm mulling over doing an RS232 themed talk at June VCF. Not a super
exciting topic, but I do think that RS232 has an interesting history: In
the SAGE relationship, and as a follow up to (essentially) prior telegraph
communication.
From what I've read, "50 baud" was a kind of an initial goal to beat,
since
that's what the top telegraph operators could achieve (in small burst,
probably not all day). And those operators did have to also deal with
things like start/stop "bits". Maybe it wasn't an intentional goal, but
just that it establishes why "50 baud" is generally the lowest we ever see
mentioned (or, if you go slower than that, might as well use the older
tech).
Then 75/110/130 baud to have digital-systems interoperate with classic
mechanical teletypes. Going any faster and those systems jam up or
overheat? These weren't yet called "serial ports", so I'm not sure
what a
late 50s system would even call their equipment that facilitate this data
exchange (since I'm not sure what kind of crystal-clock they even had
yet).
Then, was it the SAGE program that demonstrated the idea of doing this kind
of data exchange across copper phone lines? That is, the idea of computers
collaborating not just in a room, but across long distances (miles)? And
doing so by using an audio tone presentation? (they settled on around
3100MHz, which ended up translating to 300 baud? hence, that's basically
why the first digital to digital system data exchange settled on that baud
rate, which was reliable on both 50 and 60Hz power systems, and
meaningfully faster than prior 110 baud - so a good milestone to turn it
into a product, which was the Bell Model 103?).
I couldn't find much details (like a manual) on the Bell 101 equipment
(anyone seen one or have a manual?). But I did find the Bell 103 manual -
the photo of its innards is grainy, so I don't understand how the Bell 103
did 300 baud without a UART (and one of the pinout lines I see did run
power, so not sure if that's-yet RS232 or not; I know RS232 was evolving
right at that same time circa 1962). I've about the 1970ish TR1402
initial DIP UART, with anything prior being an experiment (like a full
board concept by DEC).
I know from 1962, both RS232 and ASCII standards still took maybe another
decade to really gain traction as standards (at least, from what I've
read). Getting the world to comply with any standard always takes a lot of
effort (for a practical reason of everyone still having invested in the
older tooling that was still functional). But it's interesting how those
two standards are still in use (not in their original form, but least the
1967 revisions) - extending from Baudot.and late 1800s-tech on telegraphs.
Does anyone know of any grocery stories using RS232 in the 1960s? I think
barcode scanning was just introduced in that era. I can just imagine a
smart grocery store owner, in the backroom programming their minicomputer
for payroll and inventory management. In FORTRAN and without a CRT?
Actually, in the 60s, I think included software would be negotiated with
the provider of the computer (well, I'm not sure how that differed between
minis and mainframes).
I know early microcomputers used RS232 for keyboards (1974-1976 era). The
IBM PC keyboard is essentially another form of serial.
Well, sorry for the rambling - have other RS232 related questions, but
first wanted to focus on the historical aspects (and see if I'm somewhat on
the right track at least).
-Steve
--
Nigel Johnson, MSc., MIEEE, MCSE VE3ID/G4AJQ/VA3MCU
Amateur Radio, the origin of the open-source concept!
Skype: TILBURY2591