Chapter 8. Modbus/TCP Server
138 PACSystems* RX7i & RX3i TCP/IP Ethernet Communications User Manual GFK-2224Q
If the CPU module does not receive an address map from Machine Edition, Ethernet interfaces within the PLC
will respond to Modbus/TCP clients with Exception Code 4, Slave Device Failure. This same exception code will
also be returned when the PLC’s hardware configuration is cleared.
8.3 Modbus Function Codes
This section summarizes the mapping of PACSystems reference tables to Modbus addresses by the Modbus
function codes supported by the Modbus/TCP Server. The mapping shown in this table assumes that the PLC is
configured to use its default reference table sizes.
Modbus Function Code
Modbus PLC
Table Start Address Length Start Address Length
1
5
15
Read Coils
Write Single Coil
Write Multiple Coils
0xxxx 1 32768 %Q00001 32768
2 Read Discrete Inputs 1xxxx 1 32768 %I00001 32768
3
6
16
22
23
Read Holding Registers
Write Single Register
Write Multiple Registers
Mask Write Register
Read/Write Multiple Registers
4xxxx 1 1024 %R00001 1024
4 Read Input Registers 3xxxx 1 64 %AI00001 64
7
8
Read Exception Status
Diagnostics
n/a N/A N/A N/A N/A
20
21
Read File Record
Write File Record
6yxxxx 1 0 %W00001 0
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