Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Posting-Version: version B 2.10.3 4.3bsd-beta 6/6/85; site ucbvax.BERKELEY.EDU Path: utzoo!watmath!clyde!cbosgd!ulysses!ucbvax!nosc.arpa!embick%tetra From: embick%tetra@NOSC.ARPA (Edward M. Embick) Newsgroups: mod.telecom Subject: Re: RS-232 problem Message-ID: <8603200314.AA10618@ucbvax.berkeley.edu> Date: Wed, 12-Mar-86 12:41:03 EST Article-I.D.: ucbvax.8603200314.AA10618 Posted: Wed Mar 12 12:41:03 1986 Date-Received: Fri, 21-Mar-86 06:49:49 EST Sender: daemon@ucbvax.BERKELEY.EDU Organization: The ARPA Internet Lines: 29 Approved: telecom@xx.lcs.mit.edu We have a similar setup here in CSC's complex in San Diego. We are running 3 wire communications, but unlike you we are running pins 2 (TD), 3 (RD), and 7 (SG) only between the computer and the terminals. The RS-232 25 pin connectors have pins 4 and 5 and pins 6, 8 and 20 shorted at the terminal end and at the host. We run our lines several hundred feet in a very noisy environment without serious problems. The terminals are operating at 9600 baud. I believe the problem you are experiencing is due to the transmit line (pin 2) not being terminated at the data terminal end. It acts like a very long antenna. Any electrical noise, or something like a login prompt from the host, will be transmitted to the host. The host, upon receiving the garbage, will respond with something if a terminating character is detected. This response goes out over the receive line (pin 3) and is picked up by the transmit line (pin 2), etc., etc. etc. This also can occur when a terminal is powered off and the UART completes the connection between pins 2 and 3, as well as with an AB switch. We solved the problem by having the AB switch short pin 2 to pin 7 on the line to the host that is not connected to the terminal. This does no damage to the host port, and keeps it from talking to itself. ------------------------------------------------------------------------------ Ed Embick (the more paths I make, the more paths they break! waaaaaaa....) Computer Sciences Corp. embick@noscvax.UUCP or 4045 Hancock St. {akgua,allegra,decvax,ihnp4,ucbvax}!sdcsvax!noscvax!embick San Diego, CA 92110 MILNET: EMBICK@NOSC (619) 225-8401 x516