GE PACSystems RX7i, User Manual

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Chapter 9. Modbus/TCP Client
GFK-2224Q January 2017 171
9.6 Differences between Series 90 and PACSystems Modbus/TCP
Channels
This section lists the known differences between the Series 90 implementation of Modbus/TCP Channels and
the PACSystems implementation.
1.
On the 90-30 CMM321 if a Modbus error response is received for a Modbus/TCP channel, the Ethernet
interface closes the TCP connection and updates the CRSW with an appropriate error code. For
PACSystems Ethernet, the Modbus error response results in an updated CRSW with an appropriate error
code but the TCP connection is NOT closed.
2. A CRSW of 0x8390 (Invalid Server Memory Type) is returned when an invalid Modbus Function code is
specified for the CMM321. For PACSystems Ethernet, an improved CRSW of 0xB690 (Invalid/Unsupported
Modbus Function Code) is returned.
3. The TCP connect timeout (i.e. the amount of time to wait for the Remote server or Gateway to establish a
TCP connection with a Modbus/TCP Channel) is 90 seconds on the Series 90 and 75 seconds on
PACSystems. An error is returned in the CRSW for the Open Modbus/TCP Connection COMMREQ when this
timeout occurs.
4. The station manager command “stat m” on the Series 90 results in displaying “Closed” for specific Closed
channels while PACSystems Modbus/TCP Channels results in displaying nothing for a specific Closed
channel.
5. When sending a Close Modbus/TCP Connection COMMREQ, the PACSystems Modbus/TCP Client will return
a success CRSW (0x0001) while the CMM321 module returns an error CRSW.
6. The rules for Endian conversion when transferring between Word and Bit types of memory are different in
order to make these types of conversions consistent.
CMM321 Modbus Client Endian Conversion Example
For example, depending on the direction of the transfer, the end-to-end values result in bytes being swapped
for CMM321 Modbus Client. This can be seen in the example table below.
Memory
Location /
Type
Memory value
example
Transfer
Direction
Memory
Location /
Type
Resulting Value After
Transfer Notes
Client Bit
%M16-%M1 =
0x4321

Server Word %R1 = 0x4321
End-to-end bytes
un-swapped
Server Bit
%M16-%M1 =
0x4321

Client Word %R1 = 0x2143
End-to-end bytes
swapped
Client Word %R1 = 0x4321

Server Bit %M16-%M1 = 0x4321
End-to-end bytes
un-swapped
Server
Word
%R1 = 0x4321

Client Bit %M16-%M1 = 0x2143
End-to-end bytes
swapped