Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Posting-Version: version B 2.10.1 6/24/83; site wateng.UUCP Path: utzoo!watmath!wateng!padpowell From: padpowell@wateng.UUCP (PAD Powell) Newsgroups: net.periphs Subject: Re: How long can an RS232 line be? Message-ID: <1537@wateng.UUCP> Date: Fri, 12-Oct-84 07:31:49 EDT Article-I.D.: wateng.1537 Posted: Fri Oct 12 07:31:49 1984 Date-Received: Sat, 13-Oct-84 05:03:30 EDT References: <487@oddjob.UChicago.UUCP> Organization: U of Waterloo, Ontario Lines: 47 Actually, you can run an RS232 line as far as you want. What is critical is the signal strengths remaining at the far end. The standard (ANSI RS-232C), which I don't have beside me just now, states what the maximum input voltage levels and max/min impedances are. In addition, it specifies the minimum "received" signal values for a 1 and 0 value, together with the receiver max/min impedance. The problem is to figure out what YOU can get away with. The usual DRIVER for an RS232 signal is an integrated circuit (ARRG! mem fault- core dump! I can't remeber the number), which generates a +11 to -9 volt signal. The matching receiver will detect a 1 volt signal, and has a hysterisis input, centered on about .5 volts, with about .2 volts hysterisis. Note that these values are typical, and can vary all over the landscape. The limiting factor on distance is your cable and speed. The longer the cable, the more total capacitance, the slower the driver can drive the signal. The following combinations have been tried here at Waterloo. 1. Unshielded Bell 4 conductor cable, using (TX,ground) (DX,ground) in the pairs. RTS, DTR, etc. were unused. 19.2K bps- 150 meters 9600- 400 meters 1200- ran out of cable, at least 2 KM. 2. Shielded 50 pair cable: TX, RD, with ground, rest jumbled together. About the same distances. By the way, as an interesting experiment, I tried using the new RS422 drivers from Texax Instruments, in a 4 KM loopback mode. Ran it at 9600 for a week with only a couple of bit errors. This was in differential mode. I made up a small 6 component PC board that has a 422driver/receiver, and a small regulator, to convert from 422 to TTL levels (which are compatible with the RS232 receivers HINT), and use these instead of long distance modems. The power supply is a $3.00 calculator battery replacement. UGLY? Yerp. But it sure is cheaper than $200 for a line driver. I have gotten paranoid lately and added 2 optical isolators for comfort. If you want isolation, you have to get a power source from the unit; pin 25 of the RS232 connector is unused, and is usually a good place to stick a tap to the +12 or +5 supply. Patrick Powell