Path: utzoo!utgpu!news-server.csri.toronto.edu!rutgers!ucsd!ucbvax!ssc.UUCP!fyl
From: fyl@ssc.UUCP (Phil Hughes)
Newsgroups: comp.laser-printers
Subject: Re: NEC LC890 Pageprinter Shortcomings
Keywords: serial ports NEC service
Message-ID: <9007200323.AA07322@crayola.cs.UMD.EDU>
Date: 16 Jul 90 17:41:36 GMT
References: <9007120321.AA28460@crayola.cs.UMD.EDU> <50647@iuvax.cs.indiana.edu>
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Organization: SSC, Inc., Seattle, WA
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In article <50647@iuvax.cs.indiana.edu>, hess@IUVAX.CS.INDIANA.EDU (Caleb Hess) writes:
> In article <9007120321.AA28460@crayola.cs.UMD.EDU> borriell@aecom.yu.edu (Frank Borriello) writes:
> >absolutely not the case.  In fact it is a TTL port where the signals
> >vary from 0 to +5V and not -6V to +6V as RS-232 stipulates.  This is no
> 			    ^^^^^^^^^^
> The last time I looked at the EIA RS-232 spec, it gave a range of
> -3V to -20something and +3V to +20something.  I have run into a similar
> problem with a QMS-PS800 over a long wire run (about 200') where the
> voltage drop on the wire was enough to bring the voltage at the receiver
> down close to the threshold.  The QMS is now working just fine with a
> short RS-232 cable.  There is nothing wrong with using +/- 5V. for RS-232
> drivers if you abide by the cable length recommendations. 

From my handy-dandy RS-232 Reference Card (which I wrote :-) ) the
specs are >+3V for space, <-3V for mark.  This are the required voltage
levels at the receiver end.  The Maximum open circuit voltage is 25V.
One other characteristic that limits the cable length (and can be
somewhat corrected by higher driving voltages) is that the maximum
transition time is 4% of the bit time.  The line capacity (which is
also limited by the 2500pf maximum in the spec) is what slows down the
transition.  Transition time is defined as the amount of time the signal
is between -3V and +3V.

If the drivers are designed in a decent fashion you can send data
really fast over moderate length cables.  When manufacturers skimp you
get into problems.  The most common is those that don't have a -12V
supply so they run the drivers off a -5V supply.  This ends up giving
you only a volt or so to play with to cover line drop.

-- 
Phil Hughes, SSC, Inc. P.O. Box 55549, Seattle, WA 98155  (206)FOR-UNIX
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