Serial problems on DecStations?

[George Robbins]

In article <432 at wattres.UUCP> steve at wattres.UUCP (Steve Watt) writes:
> In article <6152 at umd5.umd.edu> hans at umd5.umd.edu (Hans Breitenlohner) writes:
> >In article <423 at wattres.UUCP> steve at wattres.UUCP (Steve Watt) writes:
> >>I have discovered why this doesn't work:  If you hang an oscilloscope off of
> >>the serial port on the DECStation 3100, you see a voltage swing of -5v to +5v
> 
> >Personally, I would consider anything that can not operate on +/- 5 Volts
> >broken.  (I assume that is under load, not open circuited).
> 
> Just curious:  Does anybody know of an "RS232 signal booster" of some sort?
> Like where to buy one, or schematics or something?

Yes, you can get "RS232 Line Extenders" that are probably just an RS232
receiver and driver with a power supply.  Black Box and others have them,
but you'd want to be sure they handle modem control and not just transmit/
receive signals.

You can do the same with some 1488's and 1489's from Radio Shack...

Note that the threshold voltage for 1489's can be controlled by a resitor
between the "response control" pin and one of the power supply rails.  I'd
halfway suspect the modem is doing something like this to shift the threshold
to prevent no-connect "chatter", otherwise the threshold should be only a
volt or so...

-- 
George Robbins - now working for,     uucp:   {uunet|pyramid|rutgers}!cbmvax!grr
but no way officially representing:   domain: grr at cbmvax.commodore.com
Commodore, Engineering Department     phone:  215-431-9349 (only by moonlite)