serial printer problem on 3b2-600
Leslie Mikesell
les at chinet.chi.il.us
Thu Aug 30 00:21:39 AEST 1990
In article <506 at mtndew.Tustin.CA.US> friedl at mtndew.Tustin.CA.US (Steve Friedl) writes:
[XOFF skid on 3B2 EPORTS...]
> At 19200bps it is reportedly 16 or 32 character times (don't
>remember which), and for some devices -- the HP LaserJet comes to
>mind again -- this is just too much. The only solutions are to
>slow down the printer, use hardware flow control, or get a real
>computer like an NCR Tower :-).
Another way that should work would be to add one of those serial buffer
boxes that are commonly used between PC's and printers. They can
be found for <$100 in the smaller buffer sizes, which should be cheaper
than dumping the computer (at least in the short run).
> To really do this right would require that the UART have
>special "watch" character so it would issue an interrupt if the
>mentioned character arrived in the port. Then the onboard CPU
>could do scanning as usual with occasional forays to service a
>port that just got XOFFed. Alas, this wasn't done on EPORTS.
No, to do it right the on-board CPU should have sufficient capacity
to service the received character interrupt from the UARTs under
all conditions. Micro-CPU's are not prohibitively expensive for
this.
[...software/hardware flow control are mutually exclusive]
> If anybody out there knows of ANY reason why this is the
>case, even a bad reason, I would love to hear it. I want very
>much to believe that AT&T is not full of sh*t, but so far that's
>the only conclusion I'm able to come to.
Well they do have that reputation to maintain.
Les Mikesell
les at chinet.chi.il.us
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