GE COMMRQ Function Block and Channel Commands in PLC Communication - ic693cmm321

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COMMREQ Function Block

The COMMREQ Function Block is the ladder instruction that triggers the execution of the Channel command. In the COMMREQ Function Block, you specify the rack and slot location of the Ethernet Interface, a task value, and the address of a location in memory that contains the Command Block. There is also a fault output on the COMMREQ Function Block that indicates certain programming errors.

COMMREQ Command Block

The COMMREQ Command Block is a structure that contains information about the Channel command to be executed. The Command Block consists of two parts:

Common Area - includes the address of the COMMREQ Status word (CRS word).

Data Block Area - describes the Channel command to be executed.

When the COMMREQ function is initiated, the Command Block is transferred to the Ethernet Interface for action.

SRTP Channel Commands

The Channel commands are a set of client PLC commands used to communicate with a server PLC.

Advantages of Channel Commands

The advantage of Channel commands is their ability to establish a channel to execute multiple periodic reads or writes with a single initiation of a COMMREQ function. A Channel command can also be used to execute a single read or write.

ƒUp to 32 channels (numbered 1–32) can be established by a Series 90-70 Ethernet Interface.

ƒUp to 16 channels (numbered 1–16) can be established by a Series 90-30 Ethernet Interface. (If the Series 90-30 Ethernet Interface supports Modbus/TCP Channels, the 16 available channels are shared between SRTP and Modbus/TCP.)

The channel number is specified in the Command Block for the Channel command. The channel can be monitored using the Channel Status bits and the Detailed Channel Status words.

GFK-1541B Chapter 3 Programming Channel Commands 3-3