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plc serial -27.6

Modem

Computer

Null-Modem

Three wire

Figure 27.4 Common RS-232 Connection Schemes

Common connectors for serial communications are shown in Figure 27.5. These connectors are either male (with pins) or female (with holes), and often use the assigned pins shown. The DB-9 connector is more common now, but the DB-25 connector is still in use. In any connection the RXD and TXD pins must be used to transmit and receive data. The COM must be connected to give a common voltage reference. All of the remaining pins are used for handshaking.

plc serial -27.7

Commonlyusedpins 1 -GND (chassis ground) 2 -TXD (transmit data) 3 -RXD (receive data) 4-RTS (request tosend) 5 -CTS (clear to send) 6 -DSR (data set ready) 7 -COM (common) 8 -DCD (Data Carrier Detect) 20 -DTR (data terminal ready)

Other pins 9 -Positive Voltage 10 -Negative Voltage 11 -not used 12 -Secondary Received Line Signal

Detector 13 -Secondary Clear to Send 14 -Secondary Transmitted Data 15 -Transmission Signal Element Tim

ing (DCE) 16 -Secondary Received Data 17 -Receiver Signal Element Timing

(DCE) 18 -not used 19 -Secondary Request to Send 21 -Signal Quality Detector 22 -Ring Indicator (RI)

DB-9

1-DCD 2-RXD 3-TXD 4-DTR 5-COM 6-DSR 7-RTS 8-CTS 9-RI

Note: these connectors often have very small numbers printed on them to help you identify the pins.

Figure 27.5 Typical RS-232 Pin Assignments and Names

The handshaking lines are to be used to detect the status of the sender and receiver, and to regulate the flow of data.

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