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Modbus Communication
User Manual
6.3
MVI56-AFC ♦ ControlLogix Platform
Liquid and Gas Flow Computer
Modbus Master
Port 3 can be configured for Modbus Master operation (Project / Site
Configuration / Port 3).
The Modbus Master command is generated from the processor using ladder
logic (Modbus master block). After the Modbus Master transaction is completed
the module is ready to receive another Modbus Master request from the ladder
logic:
The following Modbus functions are supported for Modbus Master operation:
Modbus Function Code
Description
1
Read Coil Status
2
Read Input Status
3
Read Holding Registers
4
Read Input Registers
15
Force (Write) Multiple Coils
16
Preset (Write) Multiple Registers
The module offers considerable flexibility for Modbus Master operation, allowing
the ladder logic to select one of the following data types:
Bit (packed 16 to a word)
Word (16-bit register)
Long (32-bit items as register pairs)
Long Remote (32-bit items as single registers)
Note: Long data type implements each data unit as one pair of 16-bit registers (words). Each
register contains two bytes. Long remote data type implements each data unit as one 32-bit
register. Each register contains four bytes. The proper choice depends on the remote slave’s
Modbus implementation.
Page 80 of 316
ProSoft Technology, Inc.
February 25, 2011