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comfilehmi:simplemodbus:index

What is Simple MODBUS?

Simple MODBUS is a protocol created by COMFILE Technology, implementing a simplified subset of the standard Modbus RTU protocol.

The ComfileHMI is the Modbus master; the MCU is the Modbus slave. The master sends a query to the slave, and the slave responds.

Simple MODBUS Function Codes

Simple MODBUS supports the following 6 function codes. A Word is 16-bits.

Function Code (Base 10) Description
Code 1 - Read Coil Read Bits
Code 3 - Read Registers Read Words
Code 5 - Write Coil Write Single Bit
Code 6 - Write Register Write Single Word
Code 15 -Write Multiple Coils Write Multiple Bits
Code 16 -Write Multiple Registers Write Multiple Words


Simple MODBUS Memory and Address Space

Simple MODBUS supports two memory and address spaces: one for coils and one for registers. The memory is allocated in the MCU's memory.

Memory Space Number of Bits Addressing (Base 10)
Coil 1-bit 00001 ~ 09999
Register 16-bit (1-word) 40001 ~ 49999

Bit addresses range from 00001 through 09999. Address 00000 is invalid. The address put out on the wire will be this address minus 00001 (e.g. Address 00004 will result in a Modbus frame with start address 00004 - 00001 = 00003)

Word addresses range from 40001 through 49999. Address 40000 is invalid. The address put out on the wire will be this address minus 40001 (e.g. Address 40005 will result in a Modbus frame with start address 40005 - 40001 = 00004)

In Simple MODBUS, addresses that begin 1 or 3 are not valid (e.g. 10001, 30001)

Simple MODBUS MCU Program

The following program implements the simple MODBUS protocol on the on the MCU.

Simple MODBUS MCU Source Code and Explanation

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comfilehmi/simplemodbus/index.txt · Last modified: 2016/12/20 21:34 by COMFILE Technology