Installation
2476_en_I
PHOENIX CONTACT
3-9
There are two types of serial port cables that both have DB-9 (9-pin D-sub) connectors (see
Figure 3-6). One is called a straight-through 9-pin serial port cable and the other is called a
null modem cable. On a straight-through cable, it is wired as just that – straight through, in
other words, pin 1 is connected to pin 1, pin 2 to pin 2, etc.
A null modem cable crosses over pins 2 and 3 (transmit and receive data) and also crosses
over pins 7 and 8 (clear-to-send [CTS] and ready-to-send [RTS]). A null modem cable
allows two devices to be connected together when they both function as data terminal
equipment (DTE), or when they both function as data communications equipment (DCE).
By swapping the pins, it connects inputs to outputs and vice versa for proper operation.
Equipment with serial ports can be designed as either DTE or DCE. This determines the
functions of pins 2 and 3, and 7 and 8. For example, if pin 7 is an output on one end, then it
will have to be an input on the other end. Computers are typically DTE devices while
modems and radio modems are DCE. Programmable Logic Controllers (PLCs), flow
computers and other industrial instruments could be either DCE or DTE.
To connect a DCE device to a DTE device, a straight-through cable is used. To connect two
DCE devices together or to connect two DTE devices together, a null modem cable is
required.
Figure 3-6
RS-232 wire diagrams and pinouts
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