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plc serial - 27.6<br />

Modem<br />

Computer<br />

com<br />

txd<br />

rxd<br />

dsr<br />

dtr<br />

com<br />

txd<br />

rxd<br />

dsr<br />

dtr<br />

Modem<br />

Computer<br />

Computer<br />

A<br />

com<br />

txd<br />

rxd<br />

cts<br />

rts<br />

com<br />

txd<br />

rxd<br />

cts<br />

rts<br />

Computer<br />

B<br />

Null-Modem<br />

Computer<br />

A<br />

com<br />

txd<br />

rxd<br />

dsr<br />

dtr<br />

cts<br />

rts<br />

com<br />

txd<br />

rxd<br />

dsr<br />

dtr<br />

cts<br />

rts<br />

Computer<br />

B<br />

Three wire<br />

Computer<br />

A<br />

com<br />

txd<br />

rxd<br />

cts<br />

rts<br />

com<br />

txd<br />

rxd<br />

cts<br />

rts<br />

Computer<br />

B<br />

Figure 27.4<br />

Common RS-232 Connection Schemes<br />

Common connectors for serial communications are shown in Figure 27.5. These<br />

connectors are either male (with pins) or female (with holes), <strong>and</strong> often use the assigned<br />

pins shown. The DB-9 connector is more common now, but the DB-25 connector is still in<br />

use. In any connection the RXD <strong>and</strong> TXD pins must be used to transmit <strong>and</strong> receive data.<br />

The COM must be connected to give a common voltage reference. All of the remaining<br />

pins are used for h<strong>and</strong>shaking.

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