mouse@mcgill-vision.UUCP (09/06/87)
In article <650@ora.UUCP>, tim@ora.UUCP (Tim O'Reilly) writes: > In article <8998@brl-adm.ARPA>, art@acc.arpa writes: >> For asynch RS-232 cables, I recommend the following: >> SD 2 -------------\/----------------- 2 SD >> RD 3 -------------/\----------------- 3 RD >> RTS 4 --+ +-- 4 RTS >> CTS 5 --+ +-- 5 CTS >> DSR 6 ------+ +------ 6 DSR >> SG 7 ------|------------------|------ 7 SG >> DCD 8 ------o------\/----------o------ 8 DCD >> DTR 20 -------------/\----------------- 20 DTR > Note that the cable shown here is a null modem cable for connecting > two DTE (Data Terminal Equipment) interfaces. DTE-DCE (i.e. terminal > to modem) connections would not cross lines 2 and 3, and 8 and 20. In practice, the distinction between DTE and DCE seems to have all but disappeared. The only reliable way I know of to tell which some equipment behaves as is to hook it up, and if it doesn't work, try inserting a null modem cable and see if it cures it. (Of course, for a permanent connection, make or get a cable wired the right way (that is, whichever way works)!) > [However,] I do want to point out one drawback of the cable shown > here, which has nothing to do with DSR/DCD/DTR: > A cable with CTS and RTS jumpered together will work fine for normal > terminal operation (people don't type all that fast) but if you try > to upload text from a PC using a cable like this, the receiving > system will tend to lose characters. [Further explanation of RTS and > CTS.] If there is any chance your cable will have to carry data from > the terminal to the host at computer speeds rather than at human > speeds, you should be sure to include RTS and CTS. [Null-modem > cables should cross the lines, other cables straight through.] Or make sure you have either a hardware/firmware silo on the interface or a machine fast enough to keep up with full-speed input. One reason RTS/CTS/DCD/DTR/DSR signals are little-understood is, I expect, because many terminal cables are made with four-conductor wire just because it is what is on hand. Such cables must of course use two conductors for pins 2 and 3 (crossed or not). This leaves only two wires, one of which must be taken for some sort of ground (1 or 7 [*]), leaving just one wire. But just one wire isn't enough for a signal in each direction. Fortunately, such cables tend to be for real terminals, not computer-to-computer links. (To be sure, we use them that way, but we are aware of the potential problems and make sure the machines can take it....) [*] I notice the original cable didn't have pin 1, Protective Ground, connected. Fie for shame. der Mouse (mouse@mcgill-vision.uucp)
ron@topaz.rutgers.edu.UUCP (09/17/87)
It is not generally a good idea to hook up pin-1. Especially true when you're running from different power sources. -Ron
gwyn@brl-smoke.ARPA (Doug Gwyn ) (09/21/87)
In article <14824@topaz.rutgers.edu> ron@topaz.rutgers.edu (Ron Natalie) writes: >It is not generally a good idea to hook up pin-1. Especially true when >you're running from different power sources. You could hook up just one end of it. However, shielding of these cables is not usually necessary.
brian@casemo.UUCP (Brian Cuthie ) (09/21/87)
In article <14824@topaz.rutgers.edu>, ron@topaz.rutgers.edu (Ron Natalie) writes: > It is not generally a good idea to hook up pin-1. Especially true when > you're running from different power sources. > > -Ron Ron's right. This has also been changed in EIA 232-D (note it's no longer RS232). Pin one should only be connected at one end (I can't remember if it's DTE or DCE). Cheers, Brian ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Brian Cuthie CASE Communcations Inc. Columbia, MD 21046 (301) 290 - 7443
root@hobbes.UUCP (09/25/87)
+---- Doug Gwyn (VLD/VMB) writes in <6453@brl-smoke.ARPA> ---- | Ron Natalie writes: | >It is not generally a good idea to hook up pin-1. Especially true when | >you're running from different power sources. | | You could hook up just one end of it. However, shielding of these | cables is not usually necessary. +---- Beware of running cables near florescent lights - if you do, you almost are forced to ground ONE of pin-1's ends. BTW - I've seen systems working at 9600 baud with ONLY pins 2 and 3 connected!! Don't ask me how... -- John Plocher uwvax!geowhiz!uwspan!plocher plocher%uwspan.UUCP@uwvax.CS.WISC.EDU