Information Technology Reference
In-Depth Information
13.2.2 DB25S connector
The DB25S connector is a 25-pin D-type connector and gives full RS-232 functionality. Fig-
ure 13.2 shows the pin number assignment. A DCE (the terminating cable) connector has a
male outer casing with female connection pins. The DTE (the computer) has a female outer
casing with male connecting pins. There are three main signal types: control, data and
ground. Table 13.1 lists the main connections. Control lines are active HIGH, that is they are
HIGH when the signal is active and LOW when inactive.
Pin
Signal
2
TxData
13
12
11
10
9
8
7
6
5
4
3
2
1
3
RxData
4
RTS
5
CTS
6
DSR
25
24
23
22
21
20
19
18
17
16
15
14
7
GND
20
DTR
Figure 13.2
RS-232 DB25S connector
13.2.3 DB9S Connector
The 25-pin connector is the standard for RS-232 connections but as electronic equipment
becomes smaller, there is a need for smaller connectors. For this purpose most PCs now use
a reduced function 9-pin D-type connector rather than the full function 25-way D-type. As
with the 25-pin connector the DCE (the terminating cable) connector has a male outer casing
with female connection pins. The DTE (the computer) has a female outer casing with male
connecting pins. Figure 13.3 shows the main connections.
Pin
Signal
2
RxData
3
TxData
5
4
3
2
1
4
DTR
5
GND
6
DSR
9
8
7
6
7
RTS
8
CTS
Figure 13.3
RS-232 DB9S interface
13.2.4 PC connectors
All PCs have at least one serial communications port. The primary port is named
COM1:
and
the secondary is
COM2:
. There are two types of connectors used in RS-232 communications,
these are the 25- and 9-way D-type. Most modern PCs use either a 9-pin connector for the
primary (
COM1:
) serial port and a 25-pin for a secondary serial port (
COM2:
), or they use
two 9-pin connectors for serial ports. The serial port can be differentiated from the parallel
port in that the 25-pin parallel port (
LPT1:
) is a 25-pin female connector on the PC and a
male connector on the cable. The 25-pin serial connector is a male on the PC and a female on
the cable. The different connector types can cause problems in connecting devices. Thus a
25-to-9 pin adapter is a useful attachment, especially to connect a serial mouse to a 25-pin
connector.
