Operation Of The Communications Request For Modbus Channels - GE PACSystems RXi User Manual

8.7.1

Operation of the Communications Request for Modbus Channels

The diagram below shows how Communications Requests are executed to complete a data
read from the remote Modbus TCP device. The figure illustrates the successful operation of a
data read.
PACSystems
RXi CPU
Power flows to Open
Connection COMM_REQ in
ladder program
Command Block sent to
Interface
COMM_REQ
Status Word
Channel Open Bit is
set to 1
Power flows to Read
COMM_REQ in ladder
program
Command Block sent to
Interface
COMM_REQ
Status Word
Power flows to Close
Connection COMM_REQ in
ladder program
Command Block sent to
Interface
COMM_REQ
Status Word
Channel Open Bit is
set to 0
1. A Communications Request begins when there is power flow to a COMMREQ
2. The COMMREQ Status word (CRS word) is returned immediately if the Command
GFK-2816F
Chapter 8. Gigabit Ethernet (GbE) Interface Overview and Operation
Domain of a channel
Client
PLC
Backplane
Verify Command
Block and set up
channel to server
Return COMM_REQ
Status (CRS) Word
to CPU
Set Channel Open Bit
Verify
Command Block
and set up channel
to server
Read Request
Data Data
Data
Return COMM_REQ
Status (CRS) Word
to CPU
Verify
Command Block
and close channel
to server
Return COMM_REQ
Status (CRS) Word
to CPU
Clear Channel Open Bit
function in the client. The Command Block data is sent from the CPU to the Ethernet
interface.
Block is invalid. If the syntax is correct, then the CRS word is returned after the
transfer of data.
Domain of a TCP connection
Client
Ethernet
Server
Interface LAN
Ethernet Interface
Accept
connection
Send connection
acknowledgement
Read Request
Data
Receive Disconnect
Send disconnect
acknowledgement
August 2015
Domain of a remote server
Server
Server
Interface
CPU
This sequence must
be repeated for each
read or write request
Data
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