Allen-Bradley 1747-KE, User Manual

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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.
AB Parts