====== 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.

{{ :comfilehmi:simplemodbus:simplemodbus.png?nolink |}}
===== Simple MODBUS Function Codes =====

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

^Function Code (Base 10) ^Description^
|[[.:Modbus_Code1:|Code 1 - Read Coil]] |Read Bits|
|[[.:Modbus_Code3:|Code 3 - Read Registers]] |Read Words|
|[[.:Modbus_Code5:|Code 5 - Write Coil]] |Write Single Bit|
|[[.:Modbus_Code6:|Code 6 - Write Register]] |Write Single Word|
|[[.:Modbus_Code15:|Code 15 -Write Multiple Coils]] |Write Multiple Bits|
|[[.:Modbus_Code16:|Code 16 -Write Multiple Registers]] |Write Multiple Words|



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===== 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)
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===== Simple MODBUS MCU Program =====

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

[[.:simpleMODBUS_Source:|Simple MODBUS MCU Source Code and Explanation]]

[[comfilehmi:index#ComfileHMI_Connectivity| Back to ComfileHMI]]