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TXD/RXD -(transmit data, receive data) -data lines DCD -(data carrier detect) -this indicates when a remote device is present

plc serial -27.8

RI -(ring indicator) -this is used by modems to indicate when a connection is

about to be made. CTS/RTS -(clear to send, ready to send) DSR/DTR -(data set ready, data terminal ready) these handshaking lines indicate

when the remote machine is ready to receive data. COM -a common ground to provide a common reference voltage for the TXD and RXD.

When a computer is ready to receive data it will set the CTS bit, the remote machine will notice this on the RTS pin. The DSR pin is similar in that it indicates the modem is ready to transmit data. XON and XOFF characters are used for a software only flow control scheme.

Many PLC processors have an RS-232 port that is normally used for programming the PLC. Figure 27.6 shows a PLC-5 processor connected to a personal computer with a Null-Modem line. It is connected to the channel 0 serial connector on the PLC-5 processor, and to the com 1 port on the computer. In this example the terminal could be a personal computer running a terminal emulation program. The ladder logic below will send a string to the serial port channel 0 when A goes true.

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