What are the Modbus request commands?

field-instrument

#1

Modbus masters sends request commands to modbus slaves, requesting slaves the datas on field variables or to write data to the slaves, not every request messages are the same. Major types of request command of modbus masters are:

Read commands:

  • Read input status

  • Read coil status

  • Read holding register

  • Read input register

All the read commands request data value from the salves. In order to request the data, a master should specify the particular slave ID and the register name to be read.

The master can read the status and coil status from the slave, but cannot make a change in the input status area.

All these read command mentioned above are to be read from different locations of the slave memory. Input status, Coil status, Holding register, Input register are needed distinct appropriate commands with their ID.

Each command is send to the slave are responded with a data value on either coil status, Holding register, Input register, or Input status.

Write commands:

  • Force single coil

  • Preset single register

These are write commands which are send by masters to write a data on a specified location on slave memory.

Force single coil: Force slave to set a value in a particular memory block of the coil memory, it would be mostly 0 or 1 bit.

Preset single register: Allow the master to set the value of 16bit memory block in the holding registers area.

A master cannot change in the value of the input memory blocks or there is no command for that.