How do I read values spanning multiple registers over the Modbus protocol?

I am using the Industrial Communication Toolbox to communicate with external hardware. I want to read a 32-bit floating point number, but this value is spread across two (not necessarily adjacent) 16-bit registers. How do I achieve this?

Answer 1

[MathWorks Support Team]

There are two possible approaches for reading values which span multiple registers. These examples both specifically read 32-bit floats from 16-bit registers, but this is just for illustration and the process works with any register/value types.

Approach 1:

If the two 16-bit registers are on consecutive addresses, then the standard "read" function will combine their values automatically when reading a 32-bit value from the lower register's address.

For example, this will read a 32-bit floating point number by combining the 16-bit registers at addresses 2000 and 2001:

floatValue = read(m,'holdingregs',2000,1,'single');

More information on the "read" function can be found here:

https://www.mathworks.com/help/releases/R2023a/icomm/ug/modbus.read.html

Approach 2:

If Approach 1 does not work (for example, if the registers are not at two consecutive addresses), then each register can be read independently with "read" and then combined into a 32-bit value with the "typecast" function.

For example, this will read a 32-bit floating point number by combining the 16-bit registers at addresses 2000 and 3000:

reg1 = read(modbus, 2000, 1, 'uint16');
reg2 = read(modbus, 3000, 1, 'uint16');
floatValue = typecast([reg1 reg2], 'single');

More information about the "typecast" function can be found here:

https://www.mathworks.com/help/releases/R2023a/matlab/ref/typecast.html