On 2025-01-31 14:44, Steve Lewis via cctalk wrote:

But for other fodder, I'm reading over the async card for the IBM 5110.
Page 12 of this IBM 5110 doc has a block diagram of IBMs version of
"asynchronous card" -
SY31-0552-3_IBM_5110_System_Logic_Manual_197902.pdf
<http://www.bitsavers.org/pdf/ibm/5110/diagrams/SY31-0552-3_IBM_5110_System_Logic_Manual_197902.pdf>
And its manual (from 1978) has some interesting paragraphs (plus how that
system was apparently obsessed with showing text-mode graphs of the error
rates; line quality is apparently still very rough by late 1970's):
SY31-0557-0_5110asyncComm_Jan1978.pdf
<http://www.bitsavers.org/pdf/ibm/5110/SY31-0557-0_5110asyncComm_Jan1978.pdf>


-SteveL


Line quality depended on what you wanted to pay for.  Voice grade lines have a few problems the main ones being the limited bandwidth and echo suppressors.  But in the late 70s when I started with TP equipment, and probably earlier you could also get "conditioned" lines that had a much wider bandwidth and no echo suppressors.  I recall  people running 56K baud and higher synchronous lines, but they did not use RS232 but rather they used a V.35 interface.  Already at that time people where using fancy means to increase speed, I recall modems that used phase shift keying and 90 degree shifts so you could encode 2 bits in every phase shift by shifting by 0, 90, 180, and 270 degrees.  In the late 70 early 80s I know for sure IBM communication controllers supported line groups where the data would be fired down multiple pipes to get  higher throughput.     I also recall dial-up conditioned lines that had a special phone set on them that you could use to talk to people at the other end, talking on them was line talking in a tunnel.

Paul.

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