rees@pisa.citi.umich.edu (Jim Rees) (05/23/91)
Can anyone tell me why there is an electrical twist in most (US) modular cables? Every ready-made RJ-11 cable I've ever seen has the plugs attached on the same side of the cable at each end. This results in an electrical twist in the cable, so that pin 3 at one end is connected to pin 4 at the other. For phone line use this is usually not a problem, but I just wonder why they don't put the plug on the other way around at one end, to make a straight cable. I got to thinking about this recently when I started using these cables for RS-232. If you're clever about pin assignments, this twist can be useful for RS-232. So is there a reason for it, or is it just manufacturing convenience?
John Higdon <john@zygot.ati.com> (05/25/91)
rees@pisa.citi.umich.edu (Jim Rees) writes: > Every ready-made RJ-11 cable I've ever seen has the plugs attached on > the same side of the cable at each end. > So is there a reason for it, or is it just manufacturing convenience? Yes and yes. Bulk cable (for putting the ends on yourself) has a ridge on one side. By standardizing which way the ridge goes into the crimper, it is easy to accurately and speedily put modular plugs on the cable without worrying about whether you have "end A" or "end B" in your hands. Modular receptacals come in two flavors: receptacal and instrument. There is an assumption of pair rotation between the two, hence the tip (green) and ring (red) will match at their respective ends, regardless of the actual color of the conductor in the cable, which can vary depending on which way it is "facing". John Higdon | P. O. Box 7648 | +1 408 723 1395 john@zygot.ati.com | San Jose, CA 95150 | M o o !
"Barton F. Bruce" <Barton.Bruce@camb.com> (05/26/91)
In article <telecom11.394.4@eecs.nwu.edu>, rees@pisa.citi.umich.edu (Jim Rees) writes: > Can anyone tell me why there is an electrical twist in most (US) > modular cables? > I got to thinking about this recently when I started using these > cables for RS-232. If you're clever about pin assignments, this twist > can be useful for RS-232. > So is there a reason for it, or is it just manufacturing convenience? The wall to the phone cords have the wires flipped, and early on ModTap had made EIA to modular adapters for such cables, but soon realised their mistake and stopped. Many people have modular patch panels and can route EIA signals from one part of a building to another. There is NO way be be sure there will be an odd or and even number of cords in between. If the plugs are crimped on so the wires go straight through, and all adapters are wired that way, you will have no problems. That is the way the DATA industry generally does it. In many sites 'PHONE" cords have caused needless confusion and are BANNED. With polarity guards built into modern TT dials, and ringing being across the line and not to ground, many sites make life simple and ALL modular cords, both EIA DATA and 'phone' cords are wired straight through. FWIW most folks use six or eight wire cords for everything, and cords that come with phones are generally four or even two wire, so the problem ones are easy to spot.
Macy Hallock <macy@fmsys.uucp> (05/26/91)
>> Can anyone tell me why there is an electrical twist in most (US) >> modular cables? >> I got to thinking about this recently when I started using these >> cables for RS-232. If you're clever about pin assignments, this twist >> can be useful for RS-232. > If the plugs are crimped on so the wires go straight through, and all > adapters are wired that way, you will have no problems. Not always true. Phone people generally think in terms of all punch on type jumper wiring up to the wall jack. This is straight thru type operation ... until the base cord to the instrument is reached. Now, most telecom equipment is designed to take this type of wiring scheme and reflects it in their documentation. If the phone system is using direct connections to its terminating blocks, then the system is setup for one reversal ... the one in the base cord to the set. If the system uses RJ11 or RJ14 type connectors, it usually expects two reversals (one at the system's connection to the building wiring and one at the instrument base cord). Use of intermediate patch panels has been a problem to many, including my company. We have tried using staight thru path cords on intermediate panels with some success ... and try to use an oddball length (read short) to help make them easy to tell from standard base cords. Our unofficial policy is: two and four conductor cords are always standard reversal type base cords. Six and eight conductor cords are either, but straight thru cords are to be labeled "DATA" or "STRAIGHT" or "PATCH" at both ends. Most straight thru patch type cords are three feet long here. We are looking for a source of reasonably priced cord stock in a distinctive color for use in making up straight thru cords ... red or blue would be ideal ... we have a lead on getting black, white and brown stock, but have not yet received pricing. Bear in mind that some electronic phones and data equipment can be messed up by a reversal. > That is the way the DATA industry generally does it. In many sites > 'PHONE" cords have caused needless confusion and are BANNED. With > polarity guards built into modern TT dials, and ringing being across > the line and not to ground, many sites make life simple and ALL > modular cords, both EIA DATA and 'phone' cords are wired straight > through. FWIW most folks use six or eight wire cords for everything, > and cords that come with phones are generally four or even two wire, > so the problem ones are easy to spot. This may be true on sites manned and operated by data people who are not trained/acclimated to the telecom way of things. Here, we find we must live with both worlds. Frankly, I would like to see us use DEC type RJ connectors exclusively for data, but this is not really practical ... much of the wiring we install is set up to be used either way. I have a high regard for AT&T's standards and many of the WE designs, but I think they blew it on this one ... of course I think we all only thought in terms of phone lines and tip/ring when modular phones first came out in the 70's ... and WE though only in terms of its own production efficiency. I have spent many an evening working out RS-232 DB-25 to RJ-45 adaptor wiring schemes with the operations guys ... I'd like to see how others are setting up their RJ type wiring schemes, especially in multiuse wiring schemes involving LAN, RS-232, balun and phone operations. I will happily publish a summary ... email me at macy@ncoast.org ... fax is +1 216.778.6239 Macy M. Hallock, Jr. N8OBG 216-725-4764 Home macy@fmsystm.UUCP macy@NCoast.ORG Note: macy@ncoast.org is best reply path to me. uunet!aablue!fmsystm!macy [No disclaimer, but I have no real idea what I'm saying or why I'm telling you]