[Simh] pdp11 - console input with high bit set

[Timothe Litt]

On 25-Jul-20 13:47, Paul Koning wrote:
>>  But by the mid to late 70s, i.e. with the glass TTY it started to fall from favor.   I don't know why, but I would suspect this was because dedicated lines started to supplant telephone circuit-based connections and single-bit error detect was not useful.  It did not happen that often.
> It could be that glass TTYs were computer peripherals, and typically close to the computer or connected by a modem that was pretty clean.  The older devices tended to be on current loops, possibly quite long ones with debatable signal quality.

How often you got parity errors was a function of modem generation and
line quality - acoustic couplers from your house in the country were
good for frequent parity - and mult-bit - errors.

By the 80s modems got much smarter.  While the 103 was pure bit-by-bit
FSK, in addition to coding improvements, later modem protocols (e.g.USR
HST in 186, then V.42) provide error correction (including
retransmission).   In between, V.34 does a lot of work in the initial
handshake to adapt to line characteristics.

So the error rate became essentially zero (though latency could be
unbounded).

Except in some weird cases (I won't mention which computer rooms had
modems several hundred feet from the computer's interfaces...), the
modem - host and modem - TTY would be within the RS232 limit of 25 ft.

OTOTH, parity in hardware was cheap - though software often didn't
handle parity errors very effectively.  Software used (IBM) or moved to
8-bit characters as I8n came along.  So unless parity was used in HW, or
you had a non-byte architecture (e.g. the PDP-10 which treats "bytes"
between 1 and 36 bits equally), it was inconvenient.

In any case, while the short modem - DTE interface was still a
vulnerability, once you have an error-corrected path, "parity was for
farmers."

Current loops - especially when optically coupled - were actually quite
good.  Extending RS232 beyond the 25 ft spec could get problematic. 
Yes, 3,000 ft was quite possible.  But sensitive to environment.  I had
a number of notable cases where customers complained about long line
(RS233) issues - and switching to current loop modems (typically > 20ma)
resolved them.  Hint: you don't want to run long RS232 lines around
elevator machine rooms, or industrial factory floors, or ...

Like anything, it helps to read, understand, and conform to the
specifications...


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