Modbus Query

A guide on how to frame a Modbus query

Modbus query command

[[Modbus query 1],[Modbus query 2],.....]

Things required to make Modbus query

• Slave ID

• Function code

• Starting address

• How many registers/coils want to be read

• Data type

• Index

Modbus query format

[ Slave ID, Function code, Starting address, No. of registers/coils want to read, Data type, Index]

How to choose function code

It depends on starting address of the register or coil.

Parameter	Register / Coil Address	Function Code
Coil status	00001 on words	1
Input Coil status	10001 on words	2
Holding register	40001 on words	3
Input holding register	30001 on words	4

How to choose starting address & number of registers

• For eg: if we want to read the parameters like Watts Total, Watts R phase, Watts Y phase, Watts B phase, and VAR Total from the Elmeasure EN8400 Energy meter then its address is from 40101 to 40110. so now the starting address would be 100 & no. of registers want to read would be 10.

• NOTE: The number of register would be 2 in order to read single parameter of data type float, UINT32, INT32 & for rest it would be 1.

How to choose the data type

• Supported data types & formats are:

  Datatypes case no	Datatype	Remark
  0	Float ABCD
  1	Float DCBA
  2	Float BADC
  3	Float CDAB	Generally, the datatype mentioned in the datasheet will be without data format[A, B, C, D sequence], for example, datatype is float. In that case, use this format[default].
  4	UINT ABCD
  5	UNIT DCBA
  6	UINT BADC
  7	UINT CDAB	Generally, the datatype mentioned in the datasheet will be without data format[A, B, C, D sequence], for example, datatype is UINT32. In that case, use this format[default].
  8	INT ABCD
  9	INT DCBA
  10	INT BADC
  11	INT CDAB	Generally, the datatype mentioned in the datasheet will be without data format[A, B, C, D sequence], for example, datatype is INT32. In that case, use this format[default].
  12	INT AB	Generally, the datatype mentioned in the datasheet will be without data format[A, B sequence], for example, datatype is INT16. In that case, use this format[default].
  13	INT BA
  14	UINT AB	Generally, the datatype mentioned in the datasheet will be without data format[A, B sequence], for example, datatype is UINT16. In that case, use this format[default].
  15	UINT BA

• In case of Coil status or input Coil status data type is not mandatory, we can choose default data type 0.

• Note: In some exceptional cases even after selecting the [default] data format, we will observe that we are not receiving data from the slave in that scenario we have to do a trial & error by selecting a different data format until we will receive the data.

Index

Index number is nothing but a number of queries you have? The first query index is 0.

Example Queries

Schneider meter EM6436

UART: {"action":"modbus", "mb_mode":"1", "baud":"4800", "data_bits":"8", "parity":"EVEN", "stop_bits":"1", "mb_interval":"2000", "mb_timeout":"1000"}

[[1,3,3000,20,3,0], [1,3,3030,20,3,1], [1,3,3060,20,3,2], [1,3,3090,20,3,3], [1,3,3120,4,3,4], [1,3,3150,4,3,5]]