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4
4-9
GFK-1004B
Chapter 4 Programming Communications Requests
Section 3: Channel Commands
This section describes the Channel Commands. A detailed description and example of
each channel command is included. There are five Channel Commands.
1. Establish Read Channel
2. Establish Write Channel
3. Abort Channel
4. Retrieve Detailed Channel Status
5. Assign Channel Status Vector
Establishing a Channel
The Ethernet transfers data to or from another PLC using a channel. There are two
channel commands for transferring data between PLCs.
Establish Read Channel
Establish Write Channel
These Channel Commands are based on the concept of periodic data transfers. The
client (local) PLC uses a single COMMREQ function to establish a channel (connection)
to a server (remote) PLC and to request that specific data be periodically transferred
between the PLCs.
Note
To simplify the discussion of the Command Blocks, we make the
assumption that the operator/programmer is local to the client PLC and
the server is remote from this operator/programmer.
The Ethernet Interface automatically manages the establishment of communications and
the periodic data transfer. Parameters in the Command Block specify the frequency and
direction of the transfer, and the memory locations in the client and server to be used in
the transfer.
Aborting and Re-tasking a Channel
There are 4 ways a channel can be aborted.
1. When the PLC CPU is stopped, all channels in use are aborted.
2. A channel (or all channels) can be aborted by issuing an Abort Channel command.
3. A channel in use can be re-tasked by issuing an Establish Read Channel or Establish
Write Channel command for its channel number. This aborts the previous channel
operation and then performs the new channel operation.
4. A channel is also automatically aborted if a fatal error occurs.