22.3 Serial Cables

If you have problems getting a serial port to talk to a peripheral, you're probably using the wrong cable. That's not surprising because there's no such thing as a standard serial cable. Serial cables differ in the connectors used on each end, the number and type of wires that are connected end to end, the pinouts (which pin on one connector is connected to which pin on the other), and the connections made internally within each local connector, if any. With permutations, there are literally millions of ways you could build a serial cable. Fortunately, only a handful are commonly used.

22.3.1 Common Serial Cable Types

Commonly used serial cables fall into one of the two following general categories:

Straight-through serial cables

Straight-through serial cables are used to connect unlike devices (DTE to DCE). A straight-through cable is just what it sounds like?each pin on one connector connects to the corresponding pin on the other. On a DB25-to-DB25 or DB9-to-DB9 cable, this means that each pin on one connector connects to the same pin number on the other. On a DB9-to-DB25 cable, the wires connect different pin numbers, but the same signal. For example, DTR (Pin 20 on the DB25) is connected to DTR (Pin 4 on the DB9). Almost any cable with a DB9 connector connects all nine pins. DB25 cables may have all or only some pins connected, but the existing connections are straight-through.

Cross-over serial cables

Cross-over serial cables are used to connect like devices (DTE to DTE, or DCE to DCE). Cross-over cables come in an amazing variety of pinouts, some reasonably standard and others specific to one particular type of connection?e.g., an HP LaserJet serial port to a DB25 PC serial port. The term null-modem cable is often misused to mean any cross-over cable, but a null-modem cable is really just one variety of cross-over cable.

PC serial ports are usually configured as DTE. Modems, mice, trackballs, digitizers, and scanners are usually DCE devices, and so connect to a PC with a straight-through cable. Serial printers and plotters are usually DTE devices, and so connect to a PC using some form of cross-over cable. Connecting two DTE PCs also requires a cross-over cable.

Any bidirectional serial cable must connect at least TD, RD, and SG (Pins 2, 3, and 7 on a DB25). Those to be used with hardware flow control require additional connections. Table 22-4 through Table 22-6 show the pinouts for some common serial cables.

Table 22-4 shows the simplest possible bidirectional straight-through cables, which may be used to connect a DTE (e.g., a serial port) to a DCE (e.g., a modem). These cables are not commonly used because they make no provision for hardware flow control.

Table 22-5 shows the simplest possible bidirectional cross-over cables. These cables, which also make no provision for hardware flow control, are commonly used to attach DTE terminal server ports to DTE terminals in a host environment, but are uncommon in the PC environment.

Table 22-6 shows the connections for typical straight-through cables used to link DB25M and DB9M serial ports to a DB25F modem. Some programs, notably some bulletin board software, also require that Pins 12 and 23 be connected, which rules out using these programs with a DB9 serial port.

Table 22-7 lists the pinouts for a DB9-to-DB25 adapter cable, which allows a DB25 serial cable to connect to a DB9 serial port. We generally keep only straight-through, 25-wire, DB25-to-DB25 cables on hand, which can be used with such adapters to connect anything to anything.

The following tables list the pinouts for serial cables that you can use with the MS-DOS InterLink or the Windows 9X Direct Cable Connection (DCC) utilities. Because a serial link is much slower (about 10 KB/s), use one of these serial cables to link the computers only if parallel ports are not available on both or if the operating system does not support direct parallel connections. The pinouts for parallel InterLink/DCC cables are provided later in this chapter.

Although Windows NT does not support InterLink, DCC, or direct parallel connection, you can also use these serial cables to connect a Windows NT computer and a Windows 9X computer via Windows NT RAS. For detailed instructions, see Microsoft article Q142065, Connecting Windows NT to Windows 95 with a Null-Modem Cable.

Table 22-8 shows a DB9F-to-DB9F serial cable for use with InterLink or DCC. Pins 1 and 6 are shown bridged because some motherboards cause a "Cable not connected" message to appear unless CD is asserted by being bridged to DSR (so that the voltage that appears on DSR anytime the modem is ready also appears on CD to spoof the DTE into believing that a carrier is always present). You can leave Pin 1 disconnected if this is not the case with your motherboard. However, it doesn't hurt to have Pin 1 connected, even if your motherboard does not require CD be forced high, so the pinouts shown result in a more generally useful cable. Table 22-9 shows a DB25F-to-DB25F serial cable and Table 22-10 a DB9F to DB25F serial cable, both for use with InterLink or Direct Cable Connection.

^[1] Pins 6 and 8 are bridged per the explanation in the preceding text.

^[2] Pins 6 and 8 are bridged per the explanation in the preceding text.

22.3.2 Cable Guidelines

Keep the following issues in mind when you need to buy or build a cable:

• Buy the cable if you can. Any cable you need is probably a standard item, and will be cheaper and better than one you build. If you need an unusual pinout, order a custom cable from Black Box, DataComm Warehouse, or another company that produces cables to order.

• Use the shortest cable possible for more reliable connections, particularly at high speeds. If you need a long cable, buy a high-grade one made with low-capacitance wire.

• Don't judge cables by appearance. Mass-produced cables appear similar, but vary in quality. Custom-built cables are usually of high quality, and priced accordingly. You'll get no more than you pay for, and sometimes less.

• Don't assume that a cable connects every pin that appears on the connector. Mass-produced cables usually have pins in each position, while custom-built cables often have pins only in the positions that are actually connected. Nearly any DB9 serial cable connects all nine pins, but cables with two DB25s may have anything from three to 25 pins connected, with nine and 25 wire connections most common. Any decent mass-produced cable lists the number of wires and how they are connected on the packaging, and sometimes on the connectors.

22.3.3 Making Cables

You'll probably never need to make a cable, but these guidelines will serve you well if you do:

• Create a grab bag of patch boxes, male-male and female-female 9- and 25-pin gender changers, 9-to-25 adapters, and so forth. You can get these things from Global, DataComm Warehouse, and similar places. They cost only a few dollars and occupy little space. Buy two of each, and add a straight-through 25-wire DB25-to-DB25 cable to the bag. The next time you desperately need a cable and all the stores are closed, you can use these parts to cobble together any cable you need.

• Use these parts to make a jerry-built cable that works, record the pinouts, order a custom cable made to those specifications, and leave the jerry-built cable in place until the new cable arrives to replace it.

• If you're running cables through walls to wall jacks, make those cables straight-through and label both ends accordingly. No one will remember the cable pinouts later, and testing pinouts on a cable when only one end at a time is accessible is almost impossible. Do the custom pinouts in the drop cable that connects the wall jack to the equipment, and label both ends of that cable, too. Note that a plain cable is a lot easier to run through walls and floors than one that already has connectors attached.

• To make semipermanent quick-and-dirty cables, keep a supply of RJ-to-DB connectors on hand. They are available from Global and other vendors in DB9 and DB25 male and female versions, and have a standard 6P6C ("RJ11") or 8P8C ("RJ45") jack on the back. The jack is prewired to loose pins, which you can insert as appropriate into the DB connector. You then use standard flat satin cable to link the two connectors. If six or eight wires are enough, you can make a custom cable with these connectors in about two minutes flat. We've made a lot of "temporary" cables this way that are still in use 10 years later.

22.3.4 Determining Pinouts on an Unknown Cable

At some point, you may find yourself looking at a cable with no idea how it's pinned out or whether it's usable for what you want to do. When that happens, proceed as follows:

Just try it

You can connect any RS-232 pin to any other pin without damaging anything. The worst that can happen is that nothing will work.

Look for a label

Labels may be unambiguous. One labeled "HP LaserJet" is probably just what you need to connect a serial HP LaserJet. Short labels may mislead. A cable labeled "null modem" may not have the pinouts you need for your "null modem" cable. Also, some cables are asymmetric?both ends may use the same connector, but the pinouts differ and the two ends are not interchangeable.

Identify the active pins visually

Custom-made cables often have pins only for active lines. By determining which pins are connected on each end, you can often make a reasonable guess about the actual pinouts. If the visible pins correspond to a "standard" cable type, chances are good that it is that type of cable.

Disassemble the connectors

Remove the connector hood and note the connections, including local loopbacks within the connector body. By noting which pin number is connected to which color wire, you can trace the pinouts end to end. Mass-produced cables ordinarily use molded connectors, making this method impossible to use.

Use a continuity tester or DVM

Assuming a male-to-female cable, insert one probe into hole 1 of the female connector, and then brush the other probe quickly against each of the pins on the male connector, noting the pin number each time the tester beeps to indicate continuity. Repeat this process for each hole of the female connector. On a female-to-female cable, it takes a bit longer because you cannot simply brush against the pins on the second connector. On a male-to-male cable, it may seem that you need three hands to keep everything lined up, but after some practice it takes only a couple of minutes to determine a pinout.

22.3.5 Loopback Plugs

In days of yore, every PC technician carried a set of loopback plugs for testing 9-pin serial ports, 25-pin serial ports, and parallel ports. Nowadays, it's pretty rare to see a loopback plug, but they can still be invaluable in diagnosing port problems.

In essence, a loopback plug (also called a wrap plug) is simply a connector of the appropriate type with internal wiring that connects paired pins such as TD and RD. When the loopback plug is connected to a serial port for testing, outbound signals from the serial port (e.g., TD) are looped back to the corresponding inbound pin (e.g., RD). If a signal does not appear as expected, either the outbound pin or the inbound pin is not doing its job.

High-end diagnostic software (such as CheckIt) often includes loopback plugs, either standard or as an option. You can also buy a combination DB25/DB9 loopback plug from most electronics distributors for $5 or so. If you have the components on hand to make cables, it's easy enough to make your own loopback plugs, using an empty D-shell connector and crimp-on, solder-on, or wirewrap pins. The exact pinouts required differ according to the diagnostic utility you are using, but most utilities can use a loopback plug which uses the following standard pinouts:

DB25F loopback plug

• 2 to 3 (TD to RD)

• 4 to 5 to 22 (RTS to CTS to RI)

• 6 to 8 to 20 (DSR to CD to DTR)

DB9F loopback plug

• 2 to 3 (RD to TD)

• 7 to 8 to 9 (RTS to CTS to RI)

• 6 to 1 to 4 (DSR to CD to DTR)