Modbus RTU - based on RS485 How to read/ write registers

Find here some information about sensors which have a Modbus RTU interface.

1) Topology and termination

1a) point-to-point connection

In that case the sensor can have an integrated termination and the sensor address doesn't matter

1b) bus operation in line topology

Each sensor must be configured with a different sensor address

only for sensors without integrated termination resistor

1c) Example for a point-to-point connection

For a reliable communication we recommend making a ground connection

2) Modbus RTU protocol

3) Function Code (field 2)

the function code must match to the requested register address, otherwise an exception Response will happen (see yellow part on the right side)

4) Examples

4a) find here one example how to read process data with our Color Sensor CSS

for reading process data we must use the function code 4, which is for read only input registers.

In our example the measurement value has 6 Byte, to read all needed values we need 3 register:
start from register address 199 until 201

each register has 16 Bit

the address directory can be found in the operating instruction of the particular sensor.

4b) how to read and change a configuration parameter

for reading parameter we must use code 3, which is for read/write register

for changing the parameter we have 2 options:

1. code 6, which is for writing one single register
2. code 16, which is for writing multiple registers (or for writing a block of serial parameters)

here You see one simple example for the color sensor CSS, how to read the status of sender configuration with code 3 and then how to change the configuration with code 6