Modbus timeout error - RS485 communication with Protocol 3 Controller inside Despatch Oven

[ivang12]

I am trying to communicate with a Despatch oven equipped with a Protocol 3 controller. I'm using an Rs485 connection. I keep receiving a timeout error when trying to read/write to registers. The settings on the oven are set at:

Data rate: 9600

Parity Bit: None

Modbus Address: 1 

I'm using the modbus library in labview. Say I want to read the Current Mode Selected, according to the protocol 3 modbus users programming manual, the address to this register is 8280 (Dec), 0x2058 (Hex). I then input slave address 1, the starting address 8280, Quantity 1 and receive a timeout error when ran. I'm not sure where I'm making a mistake. I've tried all the read functions and they all give me a timeout error. I've also tried using 1 stop bit and 2 stop bits. I'm fairly new to labview and modbus programming. Any help or tips would be much appreciated. Thank you.

[arteitle]

NI has a newer Modbus library that I'd recommend you try instead of the old one you're using. You can find it here: https://forums.ni.com/t5/NI-Labs-Toolkits/LabVIEW-Modbus-API/ta-p/3524019

[Kemens]

ivang, have you had a chance to try out the other Modbus API that arteitle suggested? Any difference with that?

[ivang12]

Yes I've tried it and still have had no success. I've tried connecting with the despatch oven software as well and am still unable to connect. I am still trying to figure out what the issue is.

[arteitle]

If no software will work, the most likely culprit is a misconfigured serial port. Are you sure that the baud rate, parity, stop bits, and Modbus ID match between the oven's configuration and the software? Are you sure you're using the right COM port on the PC? How are the two connected?

[ivang12]

Yes I made sure many times that the baud rate, parity, Modbus ID match. I'm using a standard Rs485-USB converter cable. I assume I'm using the right COM port, as it's the only USB serial port that appears in my device manager when the cable is connected. 

[arteitle]

How is the USB-RS485 adapter connected to the controller? Sometimes the nomenclature on the signals will be inconsistent from one manufacturer to another.

[Kemens]

If you haven't already, I would suggest reaching out to Despatch support as well in getting their software to interface with the oven.  I believe once you get one piece of software working the other will likely follow. 

[ivang12 · Accepted solution]

Yeah turns out the oven wiring pins 3 &4 were flipped. Got it working now, thank you. 

[arteitle]

That's what I was thinking, sometimes they call the signals "+" and "-", sometimes "A" and "B", and sometimes they get them backwards too.