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From: ben@val.com (Ben Thornton)
Newsgroups: comp.ivideodisc
Subject: Re: computer controlled VCR
Message-ID: <1991Feb13.030712.2105@val.com>
Date: 13 Feb 91 03:07:12 GMT
References: <754@anaxagoras.ils.nwu.edu> <9265@cognos.UUCP> <1991Feb4.183844.13097@ux1.cso.uiuc.edu> <2763@motcsd.csd.mot.com>
Organization: Video Associates Labs
Lines: 29

lance@motcsd.csd.mot.com (lance.norskog) writes:

>tmkk@ux1.cso.uiuc.edu (Scott Coleman) writes:


>>A minor note: Connecting to a ^L port is a bit more involved than simply
>>wiring up the correct cable and plugging one end into an RS-232 serial port.
>>For one thing, the ^L port operates at TTL voltage levels (0-5VDC), not
>>RS-232 levels (+/- 12V or so). 

>Ummmmm, I flunked electronics and even I know that +12V->+5V is implemented
>with 2 resistors.  What more complex things are there to know about L-ports?

Now we know why you flunked electronics.  A simple voltage divider, as you
describe will not only fail to work, it is likely to cause destruction
of the receiving input device, since nothing is done to prevent
the -12V swing of the ^L port transmitter from going below TTL ground
(0V) at the input of the receiver.  This is very bad.

This is why they make devices like the MC1489 or the 75189.

>Lance

Ben
-- 
Ben Thornton             packet:  wd5hls@wd5hls.ampr.org
Video Associates       Internet:  ben@val.com
Austin, TX                 uucp:  ...!cs.utexas.edu!val!ben
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