Datasheet

Identifying Characteristics of Ports and Cables 65
FIGURE 1.46 The mini-Centronics connector
Serial
If standard parallel communications were similar to taking the interstate, then RS-232 serial
communications were similar to taking a country road. In serial communications, bits of data
are sent one after another (single file, if you will) down one wire, and they return on a differ-
ent wire in the same cable. Three main types of serial interfaces are available today: standard
serial, Universal Serial Bus (USB), and FireWire. USB and FireWire use increased signaling
frequencies to overcome serial’s stigma and join other serial technologies, such as PCIe and
SATA, as frontrunners in data communications.
Standard Serial
Almost every computer made since the original IBM PC has at least one serial port. These
computers are easily identified because they have either a DE-9 male or a DB-25 male port
(shown in Figure 1.47). Standard serial ports have a maximum data transmission speed of
57Kbps and a maximum cable length of 50 feet.
Serial cables come in two common wiring configurations: standard serial cable and
null modem serial cable. A standard serial cable is used to hook various peripherals such as
modems and printers to a computer. A null modem serial cable is used to hook two computers
together without a modem. The transmit wires on one end are wired to the receive pins on
the other side, so it’s as if a modem connection exists between the two computers but with-
out the need for a modem. Figures 1.48 and 1.49 show the wiring differences (the pinouts)
between a standard serial cable and a null modem cable. In the null modem diagram, notice
how the transmit (tx) pins on one end are wired to the receive (rx) pins on the other.
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