Rockwell-automation 1747-KE,D17476.12 DH-485/RS-232C Interface Module User Manual download pdf (Page 37)

Publication 1747-UM005B-EN-P - March 2006

Communicate with the Interface Module 3-11

DH-485 Non-Token Passing Devices

The DH-485 network also supports non-token passing slave devices.

The module communicates with these slaves using a special PLC

command (CMD) byte.

Application programs communicate with non-token passing slaves via

Send and Receive Data (SRD) messages on DH-485. The SRD message

is a link layer service provided on DH-485. The Programmable

Controller Communications Command Set (PCCC) has been extended

to provide SRD messages by setting the PCCC CMD byte to 09. The

SRD message cannot be used in slave-to-slave communication.

A detailed description of the packet is found in the DF1 Protocol and

Command Set Reference Manual, publication 1770-RM516.

The format of the application layer data within the packet is

completely dependent on how the destination device is implemented.

Refer to the destination device’s user manual for this information.

Communicate with a

Modem

The hardware handshaking option must be enabled for the module to

properly control a modem.

The module continually asserts DTR while it is waiting for a call.

Under this condition, the modem answers a call and asserts DCD

upon connection. When the module detects DCD, communications

can start.

After detecting DCD, the module continues to monitor the DCD line.

If DCD goes off, the module restarts the 10-second timeout. If DCD is

not restored within 10 seconds, the module initiates the hang-up

sequence. This feature lets the remote node to redial in case the

connection was lost due to a fault in the phone system.

This handshaking is necessary to guarantee access to the phone line.

If the handshaking protocol is defeated by improper selection of

modem options or wiring of communication cables, the modem may

still answer a call. But if the connection is lost, the modem will not

hang up. It will then be impossible for the remote node to reestablish

the connection because it will get a busy signal.

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Rockwell-automation 1747-KE,D17476.12 DH-485/RS-232C Interface Module User Manual Page 37