[Simh] pdp11 - console input with high bit set

[Clem Cole]

On Sat, Jul 25, 2020 at 2:29 PM Timothe Litt <litt at ieee.org> wrote:

> 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.
>
Amen ... I remember those days - calling to a GE 635 with an acoustic
coupler [and an ASR33] running DTSS in '67.


> Current loops - especially when optically coupled - were actually quite
> good.
>
That was always my impression. I remember seeing optically coupled 20 ma
links in a couple of factories style (press rooms) for this reason at the
Pittsburgh Press and the Philadelphia Inquirer.  They ran that way until
the MI folks replaced that whole thing with optical networking gear
??Protean maybe/I think?? when we replaced it all in a project the early
1980s [I was part of the original design, but not the deployment.  I just
remember going to the site to see what was there].



> Extending RS232 beyond the 25 ft spec could get problematic.
>
McNamara has a wonderful piece - "How far, how fast" which he capped at
9.6K baud.   I kind of chuckle today as current gear goes at speeds like
115.2K and the scroll menu on the 'serial' apps displays as fast as
926.6K.   But 115.K is pretty much now the standard out of the console
serial port from things that use "Cisco Pinning" of an RJ45 with RS-232E
signaling (like my new Ubiquiti GW/FW).




>   Yes, 3,000 ft was quite possible.  But sensitive to environment.
>
UCB's Berk-Net --- 2 wire plus a common ground, 9.6K lines across UC
Berkeley's campus, building to building.   No optical isolation either - so
we occasionally blew out the 488/489 drivers in the Able DMAX's that they
connected.   Hey, it worked amazing well before the first Ethernet was
installed.




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