When you create a Modbus data source, you must specify the byte order. If you don't know which byte order to use, you can simply try different byte orders until you find the correct value. The following byte orders are available: ABCD, BADC, CDAB, or DCBA.
A Modbus register contains 16 bits and the register is sent in one data block. This is sufficient to store booleans (bool) or integers (int8, int16, uint8, or uint16). If long integers (int32, int64, uint32, or uint64) or floats (float32 or float64) are used, they are stored in multiple registers and sent in multiple data blocks.
Unfortunately, the byte order is not fixed in the official Modbus protocol specification. Therefore, it differs per manufacturer. It is important to select the byte order that matches the Modbus server on the PLC.
The following example shows the effect of choosing different byte orders:
- The number 123456.00 appears as follows when using AB CD:
- With a byte swap, the order is BA DC and the value is changed.
- With a word swap, the order is CD AB and the value is different again.
- With both a byte swap and word swap, the order is DC BA with another different value.