NIModbuss.dll Cannot Write to Register

[Satcom]

Hello,

I am trying to communicate to a power control board using the NIModbus.dll downloaded from the NI website. I am using the RS232 com port on my PC. The com port is connected to a NOXA RS232 to RS485 converter (2-wire) and then to my power control board. The power control board uses Modbus protocol.

I can read from all of the required registers using Read Input Registers or Read Holding Registers command. My problem is that using Write Single Register and Write Multiple Registers causes a timeout error 6101.

Example:

Function Code: Read Input Registers

Starting Address: 475

Quantity: 1

Returns: 1581

This is correct. We have verified this with other software here.

Function Code: Write Single Register

Starting Address: 475

Quantity: 1

Returns: Timeout Error

Any suggestions?

[RavensFan]

Are you sure you can write to this input register or is it read only?

Per the Modbus spec.  http://www.modbus.org/docs/Modbus_Application_Protocol_V1_1b.pdf (see page 19), you can write to holding registers (40,000 series), but you can only read from input registers (30,000 series)

[Satcom]

Thanks for the reply.

Yes, the register address is 40475 actually.

We have connected a different computer (laptop)  to the NOXA device with RS232 COM port. That laptop has a custom VB program communicating with Modbus. The custom VB program can change the register value.

[RavensFan]

Try register 474.

The registers in the modbus libraries are 0 based although the device would start them at 1.

[Satcom]

I cannot write to many of the registers in the 40000 series. I have tried many including 474, 476. All write commands end up in a time out fault.

[RavensFan]

You said you were using NIModbus.dll you downloaded.  Where is that?  Do you mean .llb?

[Satcom]

Sorry, yes llb not dll