j2mod Illegal Data Address: Resolving Register Reads

The read request spans two holding-register offsets, but the process image defines only one. The slave contains offset 47; readMultipleRegisters(1, 47, 2) requests offsets 47 and 48. Because offset 48 is absent, the request returns an Illegal Data Address exception.

Diagnose the requested Modbus range

The arguments identify unit 1, starting offset 47, and a quantity of 2. Modbus applies the quantity across consecutive offsets, so every requested register must exist in the slave process image.

Correct the register definition or read quantity

Choose the correction that matches the intended data model:

1. If the master needs only the value at offset 47, change the quantity to 1:

   Register[] data1 = tcpMaster.readMultipleRegisters(1, 47, 1);

2. If the master needs two values, add a register at offset 48 before opening the slave, then retain the quantity of 2:

   SimpleProcessImage spi = new SimpleProcessImage(1);
   spi.addRegister(47, new SimpleRegister(251));
   spi.addRegister(48, new SimpleRegister(secondValue));
   slave.addProcessImage(1, spi);

Keep the addressing interpretation consistent

Treat 47 in this code as the process-image offset requested by the API. Do not silently substitute a display-style register number from another Modbus application. The evidence shows that another simulator accepts the request, but it does not establish whether that simulator predefines offset 48 or applies a different address presentation.

Verify the correction

Reconnect the master and repeat the read with either quantity 1 or both offsets defined. Confirm that the request completes without Illegal Data Address and that the first returned register contains 251. For a two-register read, also confirm that the second returned element corresponds to offset 48.

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