Assorted Null-Modem Cable Information Here's how to construct a null modem cable, used to connect 2 PC's by their serial ports. This allows you to transfer files from one PC to another at up to 115,200 baud, providing a fast and easy way to transfer files which are too large to fit on diskettes, or solving the problem of transfers when 2 PC's have incompatible disk drives. A suitable cable can be purchased already made, or you can make one yourself. In addition, you will need some type of software to manage the job. There are commercial packages out there (LapLink and FastLynx are 2 examples), or you can get the appropriate software through shareware sources, or free one from other sources. One such program is PC Magazine's ZCOPY, available from their sources, or as printed in PC Magazine, February 28, 1989 issue. It's probably about the same price, and less fuss to buy the proper cable from a decent computer shop. If your like me, however, you like making your own. All the necessary parts are available at Radio Shack or a similar store. I paid about $13 for all that was needed at Radio Shack. Additionally, you will need: Electrical or similar tape. Helps. Small blade or phillips screwdriver (for the connector you buy) Tweezers or forceps Sharp knife or wire stripper Soldering iron & solder (if using the solder type) Connector Crimping Tool (for AMP brand connectors) Wire cutters Eight or nine conductor cable works well, and allows for addition of connections if they should be needed in the future. To use the crimp type connections, you need the tool to crimp the pins onto the wires. If you plan on making several cable in your lifetime, then the tool is well worth it. Otherwise, you're probably better off paying someone else to make it. Scout your local electronics store. I've had good results with the connectors from made by AMP, sold by your finer electronics stores. Also, Belden cable is some of the best stuff to buy, if your retailer sells it. If the computers you are connecting have 9 pin serial connectors, you need the female (with holes) connectors on both ends of your cable. (IBM's and compatibles have male connectors for their 9 pin serial ports). If yours are female, make sure the connector is not a video connector instead of a serial port connector. 25 pin COM ports on IBM's and clones are typically male, also. Whether you make a cable with 9 pin or 25 pin connectors (or one at each end), pre-made adapters can be purchased to convert from 9 pin to 25 pin, and vice versa. I made all mine with 9 pin ends, and use 9 pin to 25 pin adapters with good results. TIP: Buy enough cable! As long as you're making it, make a length of about 8 or 10 feet. I've made cables, when using shield, about 35' in length with no problems. Better a little too long than too short. Avoid excess length, however. TIP: If you end up with Radio Shack parts, wrap electrical tape around the cable at each end to fit underneath the connector. The connectors and cable at Radio Shack don't fit real snug together, allowing the cable to slip out and put stress on the electrical connections. You'll see what I mean when you are closing the connector together. TIP: If you are using Radio Shack cable, try to use the shield (the braided wire around the outside of the cable as the ground connection. This task is made easier if you use Belden cable - it comes with a wire that runs along the shield, which is foil instead of that crummy braided wire. By using the shielded part of the cable as the ground connection, it helps protect from electrical noise. This is more important with longer cables. ************************************************************** The following is part of the docs for FastLynx: -------------------------- Cable Specifications ---------------------------- Following is a description of the pin connections for a FastLynx 7-wire serial cable. The cable is a 4-headed cable with a 9-pin and 25-pin female connector on both ends. The cable is wired as follows: 9 pin 25 pin 25 pin 9 pin ----- ------ ------ ----- pin 5 pin 7 <----> pin 7 pin 5 (Ground - Ground) pin 3 pin 2 <----> pin 3 pin 2 (Transmit - Receive) pin 7 pin 4 <----> pin 5 pin 8 (RTS - CTS) pin 6 pin 6 <----> pin 20 pin 4 (DSR - DTR) pin 2 pin 3 <----> pin 2 pin 3 (Receive - Transmit) pin 8 pin 5 <----> pin 4 pin 7 (CTS - RTS) pin 4 pin 20 <----> pin 6 pin 6 (DTR - DSR) The ground wire is connected to the same pin on both ends. The last three wires are a reverse of the prior three. Following is a description of the pin connections for a FastLynx parallel cable. The cable has a male DB25 connector at both ends. 25 pin 25 pin ------ ------ pin 2 <----> pin 15 pin 3 <----> pin 13 pin 4 <----> pin 12 pin 5 <----> pin 10 pin 6 <----> pin 11 pin 15 <----> pin 2 pin 13 <----> pin 3 pin 12 <----> pin 4 pin 10 <----> pin 5 pin 11 <----> pin 6 pin 25 <----> pin 25 The second set of 5 wires is the reverse of the first set. ****************************************************************** The following cable will allow transfers using LapLink III. However, it doesn't support the feature of installing the software from the remote. The FastLynx cable above does work with all the features of FastLynx. The following cable merely transmits and receives data. It cheats by jumping connections at each end to trick the computer into thinking it's connected to another computer. The FastLynx cable above allows the 2 PC's to actually communicate. However, I haven't gotten LapLink III to install software from remote with FastLynx's cable, either. FastLynx does it just fine when using a FastLynx type cable. Update 11/13/90: See diagrams at end of file to make a true Laplink III cable. The instructions are identical to the FastLynx cable. IMPORTANT! See * ³ ³ Connector 1 ³ Connector 2 ----------- V ----------- Transmit Data 2 <================\ /-------------------> 2 Transmit Data \/ Receive Data 3 <-----------------/\====================> 3 Receive Data RTS 4 <ÄÄÄÄÄ¿ ÚÄÄÄÄÄ> 4 RTS ³ ³ CTS 5 <ÄÄÄÄÄ´ ÃÄÄÄÄÄ> 5 CTS ³ ³ DSR 6 <ÍÍÍÍÍØÍÍÍÍ» ÉÍÍÍÍØÍÍÍÍÍ> 6 DSR ³ º º ³ Ground 7 <-----³----º-----------------º----³-----> 7 Ground ³ º º ³ CD 8 <ÄÄÄÄÄÙ º º ÀÄÄÄÄÄ> 8 CD º See See º DTR 20 <ÍÍÍÍÍÍÍÍͼ ** ** ÈÍÍÍÍÍÍÍÍÍ> 20 DTR Explanation: ------------ *Connect pin #2 of one connector to pin #3 at the other end. This is known as a "pin 2 to 3 crossover". That way one computer receives what the other is transmitting. At each end, connect pins #4, #5, & #8 together. Also at each end, connect pins #6 & #20. ** If you are using a nine pin connector, this connection is not needed as there is no pin #20. A connection to pin #6 is not needed. TIP: Before you get too far, cut off about 3/4 inch of cable off one end of your length of cable. Then, strip the insulation and foil from this piece or use tweezers or forceps to remove the 9 wires from inside. Strip the insulation off both ends of 4 of these wires, 6 if making a 25 pin connector cable. These short pieces of wire will be needed to make the jumpers at each connector. Twist one end of each of 2 wires together, and solder them both pin #5. Then one wire can go to pin #4, and the other to pin #8 as in the diagram. In case you want to know: ------------------------- TD = Transmit Data RD = Receive Data RTS = Request To Send CTS = Clear To Send DSR = Data Set Ready CD = Carrier Detect DTR = Data Terminal Ready I received instructions from a friend one day on how to construct a null modem cable, and decided to add some comments of my own. Then, I stumbled upon the FastLynx documentation. I've made their cable, and it works quite well (at least the serial cable does. I think I made the parallel cable a little long. Back to the crimping tool!). So, this text is a culmination of all three - the original file, my comments and ideas, and part of the FastLynx documentation. ************************************************************** Following is part of the Laplink III documentation ************************************** PARALLEL HIGH-SPEED CABLE- that is used with the printer end of a printer cable! DB25 CENTRONICS MALE FEMALE SHIELD --- SHIELD 2 -------- 32 3 -------- 13 4 -------- 12 5 -------- 10 6 -------- 11 10 --------- 5 11 --------- 6 12 --------- 4 13 --------- 3 15 --------- 2 25 -------- 30