Path: utzoo!utgpu!jarvis.csri.toronto.edu!rutgers!apple!arc!steve From: steve@arc.UUCP (Steve Savitzky) Newsgroups: comp.sys.ibm.pc Subject: Re: RS-232 protocol primer (long) Message-ID: <399@arc.UUCP> Date: 28 Jun 89 00:57:27 GMT References: <589@megatek.UUCP> <45900244@uxe.cso.uiuc.edu> Reply-To: steve@arc.UUCP (Steve Savitzky) Organization: Advansoft Research Corp, Santa Clara, CA Lines: 49 In article <45900244@uxe.cso.uiuc.edu> mcdonald@uxe.cso.uiuc.edu (Doug McDonald) writes: > >What's a DCE or a DTE? I never use those words. To be a useful standard, >it should specify the sex of the connector for each particular type >of equipment, by its common name: [list of random equipment deleted] DCE = Data Communication Equipment = modem, cable, or whatever DTE = Data Terminal Equipment = CRT, computer, or whatever. Terminal, in this case, means something that "terminates" a connection, that is, the thing at the end of the line. Like bus terminal. The standard is written so that you can take a computer or terminal and connect it directly (or via a 1-1 male-female extension cord) to a modem. Connector sex is specified (I don't remember which is which). >And, of course, the question of the function of pins 2 and 3 has to be >worked out!!! Pin 2 = Transmitted data (DTE -> DCE) Pin 3 = Received data (DTE <- DCE) A cable with same-sex connectors and pins 2 and 3 swapped is called a "Null Modem" because it replaces two modems and a telephone system, letting you connect two DTE's. There are two flavors of null modem depending on whether you cross-connect the modem control signals so the two DTE's can play flow-control games, or loop them back because one or both of them is too stupid to handle them correctly. A lot of the unstandardness of connectors called "RS232" comes from microcomputer manufacturers who didn't understand the standard making the connectors on their computers look like DCE's so that you could connect a terminal to the computer using a 1-1 cable, usually made with ribbon cable and press-on connectors. Now, of course, it's hard to tell what you're going to meet up with. I always have a set of male-male, female-female, and both kinds of null modem adaptors handy. Doesn't everybody? -- Steve Savitzky | apple.com!arc!steve ADVANsoft Research Corp. | (408)727-3357 4301 Great America Parkway | #include Santa Clara, CA 95054 | May the Source be with you!