Xref: utzoo comp.dcom.modems:3562 comp.sys.ibm.pc:25949 Path: utzoo!attcan!uunet!amdahl!oliveb!pyramid!csg From: csg@pyramid.pyramid.com (Carl S. Gutekunst) Newsgroups: comp.dcom.modems,comp.sys.ibm.pc Subject: Re: RS-232 vs. V.35 Keywords: maximum distance Message-ID: <61976@pyramid.pyramid.com> Date: 9 Mar 89 23:54:32 GMT References: <1766@umbc3.UMBC.EDU> <4287@ingr.com> <7548@polya.Stanford.EDU> Reply-To: csg@pyramid.pyramid.com (Carl S. Gutekunst) Followup-To: comp.dcom.modems Distribution: na Organization: Pyramid Technology Corp., Mountain View, CA Lines: 32 In article <4287@ingr.com> goodloe@ingr.com (Tony Goodloe) writes: >If somebody can point me to the page number in (one of the several >standards associated with) the V.35 spec that talks about distance, I >sure would appreciate. The CCITT recommendations are very specific about what the cable has to look like; see Recommendation V.35, Appendix II and Recommendation V.28, chapter 3. The problem is that the "cable length" is defined in terms of impedance and capacitive load, since these are qualities that vary tremendously based on the type of cable being used. For example, Black Box sells what they call "V.35 cable" that is specified at 12pf and 0.03 ohms per foot. Given that cable and Pyramid's present V.35 Line Interface Module that has active slew rate limiting (don't ask :-)), I could run 200 feet of cable and be in compliance. On the other hand, if I went down to Radio Shack, bought a Centronics printer cable, clipped off the ends, and put M34 connectors on it, the maximum might be very close to 50 feet. I am well aware that what *works* is an entirely different issue from what's standard. But I just don't accept the "if it works, it must be OK" philosophy. Someone wanted to know, "why do you care what the standard says?" The reason *anyone* cares is realibility. If I push the margins to the edge to make a brand new installation work, what happens when the cables and equipment age? No, the standards are very specific just for this very reason, so that you can install something and expect it to work not only now, but 20 years from now, and when you switch out an old modem or an old length of cable and replace it with something else. And the standards *are* very specific; they just aren't specified in language that is useful to the average system administrator. Which is what keeps people like me employed. :-)