Structure Of The Communications Request; Commreq Function Block; Commreq Command Block; Modbus/Tcp Channel Commands - GE RX3i User Manual

Chapter 9. Modbus/TCP Client

Structure of the Communications Request

The Communications Request is made up of the following elements:
▪
The COMMREQ Function Block (ladder instruction)
▪
The COMMREQ Command Block
▪
The Channel Command
▪
Status Data (COMMREQ Status word, LAN Interface Status and Channel Status bits)
▪
The logic program controlling execution of the COMMREQ Function Block
The figure below illustrates the relationship of these elements:
CONTROL
LOGIC
INITIATES
COMMREQ
FUNCTION
BLOCK

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.

Modbus/TCP Channel Commands

The Channel commands are a set of client commands used to communicate with a server. Up to 32
can be established. The channel number is specified in the Command Block for the Channel command. The
channel can be monitored using the Channel Status bits. The 32 Client connections of an Ethernet interface are
shared between all Client protocols. For example, if 16 Client connections are used for SRTP Channels, there
are 16 Client connections available for Modbus/TCP Channels. Any given channel can be assigned to only one
protocol at a time.
140 PACSystems* RX7i & RX3i TCP/IP Ethernet Communications User Manual
COMMREQ
FUNCTION BLOCK
INPUTS
AND
OUTPUTS
COMMAND BLOCK
FOR COMMREQ
FUNCTION
COMMAND
BLOCK
ADDRESS
Figure 54: Phases of a COMMREQ Execution
COMMREQ
COMMREQ
STATUS WORD
COMMREQ
STATUS
STATUS
WORD
CODES
ADDRESS
STATUS BITS
DETAILS
OF THE
LAN INTERFACE STATUS
CHANNEL
AND CHANNEL STATUS
COMMAND
Location in PLC memory
specified when configuring
the Interface using
Configuration Software
BITS
channels
16
GFK-2224Q