Chapter 19: Ports and Connectors^485
A serial cable, like all PC cables, is made u pof a wire cable that has as few as 2 wires
and usually not more than 20 wires, but this can vary with special applications. A cable
with 8 wires is very common. The wires in the cable are colored to make it easier to find
the same wire on each end of the cable. This is important because on each end of the cable
a connector is attached by soldering the necessary wires to the back of pins in the connec-
tor. When the connector is plugged onto a matching, but opposite, port, the pins of the
connector make contact with the holes of the port to complete the connection. The PC and
the peripheral device attached to the cable can then send signals back and forth to com-
municate and control the transmission of data.Asynchronous Communications
Asynchronous communications is what a PC uses to connect with a printer, modem, fax,
and other peripheral devices. Loosely translated,asynchronousmeans without synchroni-
zation, which on the PC means without regard to clocking signals. The transmitter and
receiver of an asynchronous communications session operate independently and are not
synchronized to a common clock signal or each other. Data blocks are separated by arbi-
trary idle periods on the line, as illustrated in Figure 19-6.Asynchronous Data Blocks
The data blocks in asynchronous communications are fixed in size and format. The eight
bits of ASCII characters is preceded by a start bit and followed by one or two stop bits.DB-25 Pin DB-9 Pin Use
1 Ground
2 3 Transmit
3 2 Receive
4 7 RTS (Request to Send)
5 8 CTS (Clear to Send)
6 6 DSR (Data Set Ready)
7 5 Signal Ground
8 1 Carrier Detect
20 4 DTR (Data Terminal Ready
22 9 Ring IndicatorvTable 19-2. DB-25 and DB-9 Connector Pinouts