hoffman%pitt@csnet-relay.arpa (08/15/85)
From: Bob Hoffman <hoffman%pitt@csnet-relay.arpa>
Thanks to Harold Price, NK6K, I got a speedy reply to my comment/question
about the use of the CD line on the TAPR TNC-1. Here's the forwarded
reply, comments in [] are mine:
>From bellcore!csisb!harold Thu Aug 15 04:39:28 1985
To: pitt!hoffman
Bob,
Here's the answer to your TNC hardware question, straight from the
designer. Please pass it on to net.ham-radio.packet or wherever the
original request went. tnx.
-----------
[From Lyle Johnson, WA7GXD, TAPR President]
Harold,
The strange use of the CD line in TNC1 is well documented.
See Hardware Chapter 5 page 18 and 19. [I missed this (oops)]
The RS-232C spec calls for a "0" level to be greater than +3 volts.
It also specifies that the input impedance of a device should be
greater than 7k ohms. If our 1489 IC on the TNC is 7k and the attached
device is 7k, we have a 3.5k load to ground. But our pullup is
6.8k returned to +12 v. So, the point should never see less than
about +4.0 volts.
So, it SHOULDN'T be a problem.
It is, however, somewhat flakey. TNC 2 does not have this "feature"
of allowing an input or an output. Part of the reason we did it this
way was becasuse we knew of some folks who have the TNC attached to
a telephone modem at a remote site on a hill and operate packet radio
via telephone!
Anyway, the solution Bob came up seems to work fine...
(anyone got an egg scraper..there's some on my face...)
Lyle
---------------
bellcore!csisb!harold
----------------------------------
Many thanks go to Harold and Lyle for their prompt reply.
--
Bob Hoffman, N3CVL {allegra, bellcore, cadre, idis, psuvax1}!pitt!hoffman
Pitt Computer Science hoffman%pitt@csnet-relay