Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Posting-Version: version B 2.10.1 6/24/83; site mit-eddie.UUCP Path: utzoo!linus!philabs!cmcl2!seismo!harvard!think!mit-eddie!hoffman%pitt@csnet-relay.arpa From: hoffman%pitt@csnet-relay.arpa Newsgroups: net.ham-radio.packet Subject: My previous comment about the TAPR TNC-1 RS232 Message-ID: <4986@mit-eddie.UUCP> Date: Thu, 15-Aug-85 12:50:34 EDT Article-I.D.: mit-eddi.4986 Posted: Thu Aug 15 12:50:34 1985 Date-Received: Mon, 19-Aug-85 20:33:22 EDT Sender: daemon@mit-eddi.UUCP Organization: MIT, Cambridge, MA Lines: 49 From: Bob HoffmanThanks 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