How to retrieve data size larger than Qt Modbus InputRegisters?

Question

From what I understand, the range of QModbusDataUnit::InputRegisters is range 0-65535 which is unsigned short.

The method to read 1 unit of inputregisters is as follows:

QModbusDataUnit readUnit(QModbusDataUnit::InputRegisters, 40006, 1);

The value of that will be in the reply, i.e : int value = result.value(0);

My question is that what if I have to read a value of unsigned int which is much larger of the range of 0 to 4,294,967,295.

How can I retrieve that value?

Answer 1

As you stated, Modbus input registers are 16 bit unsigned integers. So without some type of conversion they are limited to the range: 0 - 65535. For 32-bit unsigned values it is typical (in Modbus) to combine two registers.

For example, the high 16-bits could be stored at 40006 and the low 16-bits at 40007.

So, if you were reading the value ‭2271560481‬ (0x87654321 hex), you would read ‭34661‬ (0x8765) from address 40006 and 17185 (0x4321 hex) from location 40007. You would then combine them to give you the actual value.

I don't know the Qt Modbus code, but expanding on your example code you can probably read both values at the same time by doing something like this:

readUnit(QModbusDataUnit::InputRegisters, 40006, 2);

and combine them

quint32 value = result.value(0);
value = (value << 16) | result.value(1);