Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Posting-Version: version B 2.10.2 9/18/84 exptools; site whuxlm.UUCP Path: utzoo!watmath!clyde!burl!ulysses!allegra!whuxlm!nrf From: nrf@whuxlm.UUCP (Fildes N R) Newsgroups: net.dcom Subject: Re: Definition of "null modem" cable Message-ID: <832@whuxlm.UUCP> Date: Thu, 5-Sep-85 08:03:08 EDT Article-I.D.: whuxlm.832 Posted: Thu Sep 5 08:03:08 1985 Date-Received: Fri, 6-Sep-85 03:29:05 EDT References: <195@almsa-1> <196@ipso.OZ> Organization: AT&T Bell Laboratories, Whippany Lines: 42 In article <195@almsa-1> control@almsa-1 (William Martin) writes: > What is the exact definition of a "null modem" cable? Does the term > *only* mean that the connections to pins 2 and 3 on a DB25 RS232 > connector on each end are interchanged, or does it also imply > connections of other pins or the shorting or cross-connecting of > certain pins (like 1, 7, 6, and 20)? The following is the 'nrf - never known to fail' null modem. it works in many funny cases where certain signals are not provided: 20 ------------------- /------------------------- 20 \ 8 ----o-------------/ --------------------o----- 8 | | 7 ----)-----------------------------------(----- 7 | | 6 ----o o----- 6 | | 5 ----o o----- 5 4 4 3 ------------------- /------------------------- 3 \ 2 ------------------/ -------------------------- 2 The principal of this null modem is the following: As long as one side asserts DTR, the other side will get ALL signals it may possibly need to work. the only piece of equipment I have ever seen that did NOT provide DTR was the ROP port of the Datakit-VCS control computer. The only way I know of to provide null-modem support with fewer leads (<5) is to lose the ability for one end of the cable to indicate to the other that it is offline. Note: this modem is not intended to deal with EIA handshaking of printers in non-standard ways.