PC Hardware in a Nutshell, Second Edition

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.

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 ...