TECHNICAL BULLETIN #87 - Rev 1.10 (9/03/96) ============================================================================== TITLE : About Null Modem Cables PRODUCT : Procomm Plus 4.x, 3.x ============================================================================== WHY USE A NULL MODEM CABLE? Computers can communicate over both modem and direct cable connections. When connecting two adjacent computers, you may use either of these methods. Although you can connect your PC to a modem with a normal serial cable, you must connect two PC's with a modified serial cable called a null modem cable. Instead of using two modems and two phone lines, you can simply connect a null modem cable between the serial ports of the two computers for a much easier and less expensive way to transfer files between adjacent PC's. WHERE CAN I BUY A NULL MODEM CABLE? Your local computer store will most likely carry null modem cables. If you have an extra serial cable around, you can buy a null modem adapter. Adapters are usually less expensive and allow you to get the most out of what you already have. If you need to buy a null modem cable, be sure to get one that is both long enough and has the proper size connectors on it. If need be, you can buy a 25 to 9 pin (or 9 to 25 pin) converter to arrive at the proper connector size. HOW DO I MAKE A NULL MODEM CABLE? A null modem is cable is not much different from an average serial cable. Null modem cables have simply been rewired to simulate a modem connection. To create a null modem cable you will need one the following: 1. A length of shielded nine-conductor cable and two female connectors of the appropriate size, or 2. A normal serial cable and a gender changer. You will take apart and rewire the gender changer; or 3. A serial cable with two female connectors. You will disassemble the connector housings and rewire the connectors. You will need a soldering iron and some solder to create a null modem cable. Whichever method you use, make the appropriate connections using one of the following diagrams. See the section following the diagrams ("How to Read the Diagrams") for an explanation of the diagram codes. Guide for a 25 pin to 25 pin null modem cable: FG 1-----------------------1 FG TD 2----------\ /----------2 TD (pin 2 is tied to pin 3) RD 3----------/ \----------3 RD (pin 3 is tied to pin 2) RTS 4----------\ /----------4 RTS (pin 4 is tied to pin 5) CTS 5----------/ \----------5 CTS (pin 5 is tied to pin 4) SG 7-----------------------7 SG DSR 6--------|\ /|--------6 DSR (pin 6 is tied to pin 8 DCD 8--------| \ / |--------8 DCD on both sides; DTR 20---------/ \----------20 DTR both are tied to pin 20) Guide for a 25 pin to 9 pin null modem cable: 25 Pin Side 9 Pin Side TD 2-----------------------2 RD RD 3-----------------------3 TD RTS 4----------\ /----------7 RTS (pin 4 is tied to pin 8) CTS 5----------/ \----------8 CTS (pin 5 is tied to pin 7) SG 7-----------------------5 SG DSR 6--------|\ /|--------1 DCD (pin 6 is tied to pin 8; DCD 8--------| \ / |--------6 DSR pin 1 is tied to pin 6; DTR 20---------/ \----------4 DTR both are tied to pins 20 & 4) FG 1-----| |-----9 RI (pins 1 and 9 are terminated) Guide for a 9 pin to 9 pin null modem cable RD 2----------\ /----------2 RD (pin 2 is tied to pin 3) TD 3----------/ \----------3 TD (pin 3 is tied to pin 2) DTR 4--------|\ /|--------4 DTR (pin 4 is tied to pin 1 DCD 1--------| \ / |--------1 DCD on both sides; DSR 6----------/ \----------6 DSR both are tied to pin 6) SG 5-----------------------5 SG RTS 7----------\ /----------7 RTS (pin 7 is tied to pin 8) CTS 8----------/ \----------8 CTS (pin 8 is tied to pin 7) RI 9-----| |-----9 RI (pins 9 and 9 are terminated) HOW TO READ THE DIAGRAMS The diagrams follow the pattern: abbreviation of line function, pin number, connection graphic, pin number of other connector, abbreviation of other connector's line function. For example: SG 5-----------------------5 SG This diagram shows that you are to connect pin 5 on the left side of the cable directly to pin 5 on the right side of the cable. The SG denotes the these are Signal Ground connections. You will find a complete explanation of all abbreviations in the glossary below. In this example: RTS 7----------\ /----------7 RTS CTS 8----------/ \----------8 CTS you are instructed to connect pin 7 on the left side to pin 8 on the right side. Pin 8 on the left side is connected to pin 7 on the right side. The fact that the lines cross does NOT mean that these two wires should be connected to each other. These are two totally independent signal paths. An example where you would connect wires together is diagrammed as follows: DSR 6---------|\ /|--------1 DCD DCD 8---------| \ / |--------6 DSR DTR 20----------/ \----------4 DTR In this example you are to connect pins 6 and 8 on the left side to each other and then connect them to pin 4 on the right side. You then connect pins 1 and 6 on the right side to each other and connect them to pin 20 on the left side. Lastly, the symbol -----| |----- indicates a line which is not connected to anything. It remains unused. GLOSSARY CTS.....Clear To Send. The CTS line is asserted by the PC (as DCE device) when it is ready to receive data. DCD.....Data Carrier Detect. The DCD line is asserted when the data link is established. DCE.....Data Communications Equipment. Refers to the modem in a computer to modem setup. DSR.....Data Set Ready. The DSR line is asserted by the DCE when it is ready to communicate with the DTE. DTE.....Data Terminal Equipment. Refers to the computer in a computer to modem setup. DTR.....Data Terminal Ready. The DTR line is asserted by the DTE when it is ready to communicate with the DCE. FG......Field Ground. A protective line used to ground the DCE. RD......Receive Data. The RD line is used by the DCE to send data to the DTE. RI......Ring Indicator. The RI line is asserted by the DCE when a ring is detected. RTS.....Request To Send. The RTS line is asserted by the DTE when it wants to transmit data to the DCE. SG......Signal Ground. The common return (and voltage baseline) for the various signal lines. TD......Transmit Data. The TD line is used by the DTE to send data to the DCE. ============================================================================== To ensure that you have the latest version of this document, compare its revision number to that of the same document on any of our support sites. This technical bulletin may be copied and distributed freely as long as it is distributed in its entirety and it is not distributed for profit. Copyright (c) 1996 Quarterdeck All rights reserved. ==============================================================================