Hello,

so far getting byte values is working but I'd like some opinions on the byte order.

Should byte 0 be the LSB or MSB?

Modbus being big-endian the first byte in the register (register is a 16-bit word) is the MSB. The second is the LSB.
But in my PLC when I configure the variables the first byte value at a given address is the LSB, the second the MSB.

To summarize:

PLC configuration
BYTE first AT %QB616
BYTE second AT %QB617

MODBUS register 308 (word value)
second | first

modbus4j
should this be
batchRead.addLocator("second", slaveId, RegisterRange.INPUT_REGISTER, 308, DataType.BYTE, (byte) 0);
batchRead.addLocator("first", slaveId, RegisterRange.INPUT_REGISTER, 308, DataType.BYTE, (byte) 1);
or
batchRead.addLocator("first", slaveId, RegisterRange.INPUT_REGISTER, 308, DataType.BYTE, (byte) 0);
batchRead.addLocator("second", slaveId, RegisterRange.INPUT_REGISTER, 308, DataType.BYTE, (byte) 1);

I would prefer the latter.

Regards
jmbsps

Hello,

thanks for providing modbus4j as free software.
Recently I started using modbus4j in a little project feeding some data from our house into rrdtool for long term visualisation.

I was wondering how to handle the IEC 61131-3 data type BYTE which is defined as an 8-bit bit string.
There is no 8-bit data type in modbus4j.

I know that Modbus only defines single bit or register (16-bit) operations. On the slave I can define 8-bit BYTE values and it would be great if there would be a convenience method retrieving it.

Something like the convenience methods for retrieving bit values.

batchRead.addLocator("FirstByteValue", slaveId, RegisterRange.INPUT_REGISTER, 256, DataType.BYTE, (byte) 0); //retrieves first byte from register 256
batchRead.addLocator("SecondByteValue", slaveId, RegisterRange.INPUT_REGISTER, 256, DataType.BYTE, (byte) 1); //retrieves second byte from register 256

I planned on hacking this into my own modified version of modbus4j.
My plan is to define this as java primitive short since BYTE in IEC 61131-3 is unsigned and there is no unsigned 8-bit value in java.

Is this something of general interest? Since modbus4j is free software I'd like to offer my modifications back to the project.

Regards
jmbsps

Hello,

thanks for providing modbus4j as free software.
Recently I started using modbus4j in a little project feeding some data from our house into rrdtool for long term visualisation.

I was wondering how to handle the IEC 61131-3 data type BYTE which is defined as an 8-bit bit string.
There is no 8-bit data type in modbus4j.

I know that Modbus only defines single bit or register (16-bit) operations. On the slave I can define 8-bit BYTE values and it would be great if there would be a convenience method retrieving it.

Something like the convenience methods for retrieving bit values.

batchRead.addLocator("FirstByteValue", slaveId, RegisterRange.INPUT_REGISTER, 256, DataType.BYTE, (byte) 0); //retrieves first byte from register 256
batchRead.addLocator("SecondByteValue", slaveId, RegisterRange.INPUT_REGISTER, 256, DataType.BYTE, (byte) 1); //retrieves second byte from register 256

I planned on hacking this into my own modified version of modbus4j.
My plan is to define this as java primitive short since BYTE in IEC 61131-3 is unsigned and there is no unsigned 8-bit value in java.

Is this something of general interest? Since modbus4j is free software I'd like to offer my modifications back to the project.

Regards
jmbsps