Operation Of The Communications Request - GE RX3i User Manual

Chapter 9. Modbus/TCP Client

Operation of the Communications Request

Figure 54 below shows how Communications Requests are executed to complete a data read from the remote
Modbus/TCP device. The figure specifically illustrates the successful operation of a data read.
Domain of a channel
Client
PACSystems
RX3i CPU
Power flows to Open
ConnectionCOMMREQ in
ladder program
Command Block sent to
Interface
COMMREQ
Status Word
Channel Open Bit is
set to 1
Power flows to Read
COMMREQ in ladder
program
Command Block sent to
Interface
Data
COMMREQ
Status Word
Power flows to Close
Connection COMMREQ in
ladder program
Command Block sent to
Interface
COMMREQ
Status Word
Channel Open Bit is
set to 0
1. A Communications Request begins when there is power flow to a COMMREQ function in the client. The
Command Block data is sent from the CPU to the Ethernet interface.
2. The COMMREQ Status word (CRS word) is returned immediately if the Command Block is invalid. If the
syntax is correct, then the CRS word is returned after the transfer of data.
142 PACSystems* RX7i & RX3i TCP/IP Ethernet Communications User Manual
Domain of a TCP connection
Client
Ethernet
PLC
Backplan Interface
Verify Command
Block and set up
channel to server
Return COMMREQ
Status (CRS) Word
to CPU
Set Channel Open Bit
Verify
Command Block
and set up channel
to server
Read Request
Data Data
Return COMMREQ
Status (CRS) Word
to CPU
Verify
Command Block
and close channel
to server
Return COMMREQ
Status (CRS) Word
to CPU
Clear Channel Open Bit
Figure 55: Illustration of Phased Operation of a COMMREQ
Domain of a remote server
Server
Server
Interface
LAN
Ethernet Interface
Accept
connection
Send connection
acknowlegement
Read Request
Data
Receive Disconnect
Send disconnect
acknowlegement
Server
CPU
This sequence must
be repeated for each
read or write request
Data
GFK-2224Q