Only 1 size of register ( e.g. 16-bit, 32-bit) is allowed within the address range defined for Coils and Discrete Inputs (see Map Addresses on Settings → Edit Settings... ). To get this error you may be adding a register that is in the range defined for coils/discrete inputs and has a different size to other registers in that range. Alternatively, you may be defining an address range for coils/discrete inputs that includes registers defined with different sizes.

If the address ranges overlap with input or holding Registers, then registers in the overlapped part are also restricted to one size.

If your slave device uses coils/discrete Inputs, ensure all are defined in ModSlaveSim with a consistent size (e.g. all int16). If your slave device does not use coils/discrete Inputs, set the number of addresses for coils and discrete inputs to 0 (Disc: No or Coil : No in Map Addresses on Settings → Edit Settings...).

The register address is not within the address range defined for holding or input registers, discrete inputs or coils (see settings in Map Addresses on Settings → Edit Settings... ). Check that you have correctly entered the address range, especially where your slave device uses a non-zero base address, or overlays coils/discrete inputs with holding/input registers.

Addresses must be unique. This error can occur when you add a register or modify the setting for Word Registers (in the Set 32/64-bit Values panel on Settings → Edit Settings... ). (see Set 32/64-bit Values).

Ensure the Type (e.g. int16) is the same as the master.

Check/Uncheck Little Endian ( in Set 32/64-bit Values on Settings → Edit Settings... ) to make it consistent with the master (see Set 32/64-bit Values).

Check/Uncheck Word Registers ( in Set 32/64-bit Values on Settings → Edit Settings... ) to make it consistent with the master (see Set 32/64-bit Values)

Check/Uncheck C/D Swap (in Map Addresses on Settings → Edit Settings... ) to make the order of bytes of a 16-bit register consistent with the master (see Map Addresses).

Check/Uncheck C/D Swap (in Map Addresses on Settings → Edit Settings... ) to reverse the order of bytes of a 16-bit register and make it consistent with the master (see Map Addresses).

Check/Uncheck Little Endian ( in Set 32/64-bit Values on Settings → Edit Settings... ) to reverse the order and make it consistent with the master (see Set 32/64-bit Values).

Start logging to get more information to help with the following points (see Logging).

Ensure that ModSlaveSim has a register defined at the correct address for the master request (see Map Addresses or Mapping coils/discrete inputs overlaid with holding/input registers).

Ensure that the address of the ModSlaveSim register is within the range defined ( check Map Addresses in Settings → Edit Settings...) for the Modbus type of the master command. For example, ensure it is in the coil range for a 'Read Coils' command (see Map Addresses).

Ensure that you press the Apply button after entering your address mapping settings (in the Map Addresses panel of Settings → Edit Settings... ).

If the master command is for more than one register, ensure that there are enough ModSlaveSim registers defined (check the slave register display area). If the master command is for coils/discrete inputs and they are overlaid, you might want to look at Mapping coils/discrete inputs overlaid with holding/input registers.

If the master command is for more than one register, ensure that the ModSlaveSim register addresses are consecutive on the slave register display area of the main dialog. However, if you are using 32/64-bit values and Word Registers is checked, then not all consecutive addresses are displayed (see Set 32/64-bit Values) Mapping coils/discrete inputs overlaid with holding/input registers if using overlaidCoils/Discrete Inputs).

Ensure that the Write checkbox for the register is checked (see the register display area of the main dialog) if a write command is sent from the master (see Slave Registers - DIsplay and Editing ).

Start logging to get more information to help with the following points (see Logging).

Ensure that the Type of the defined register is consistent with the master (see Slave Registers - DIsplay and Editing ).

Check/Uncheck Word Count ( in Set 32/64-bit Values of Settings → Edit Settings... ) to make it consistent with the master (see Set 32/64-bit Values).

Ensure that the Port is not in use by any other server.

If ModSlaveSim is run under a Unix operating system by an ordinary user (not super-user) then the Port needs to be above 1023 and the Modbus master should be configured to use that port.

The address of the Host can be 'localhost' if the slave and master are on the same machine, or can be left empty to accept connections via any network interface. Otherwise check with your System Administrator for the address of your machine and modify if necessary.

Ensure Packet type is TCP (on Edit Interface of Settings → Edit Settings... ).

Ensure the Slave Id is consistent with the master.

Start logging to get more information to help with the following points (see Logging).

Ensure the Slave Id is consistent with the master.

Ensure a cable is connecting master and slave interfaces.

Start logging to get more information to help with the following points (see Logging).

If master and slave are on same machine, ensure the ports are different.

Ensure Packet type is RTU or ASCII and is consistent with the master.

Ensure each of Speed, Parity, Data bits, Stop bits, RTS control is consistent with the master.

Ensure cable is appropriately wired if using RTS control: Flow control.

Ensure no other software is using the same port.

Try increasing the memory allocation by providing the -Xmx option when you run the program, for example :

specifies a maximum memory allocation size of 80 megabytes. So the full command is:

If you get a low memory error it tells you the current size, so you will need to increase memory above that size.