GE 90-30 PLC Switch User Manual

GFK-1084B Chapter 3 Programming Communications Requests
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 and a pointer to a location in memory that con-
tains the Command Block. There is also a fault output on the COMMREQ Function
Block that indicates certain programming errors. See Section 2 for details.
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 a pointer to 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.
See Section 2 for a description of the common area of the Command Block and Section 3
for details on the Data Block area and Channel Commands.
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.
Up to 16 channels (numbered 1-16) can be established by a client PLC. 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.
See Section 3 for more information.