Re: Modbus write/read data error help.

[Mark Rivers via Tech-talk <tech-talk at aps.anl.gov>]

Hi Susheng,

Your device uses the RS-485 communication standard. You appear to be using a USB port on your Linux machine. That means you need a USB to RS-485 converter to talk to the device. Is that what you are using, and if so what make and model?

Your IOC configuration is OK, but you should simplify it. You are using 2 drvAsynModbusConfigure commands, each reading just 1 register. You should change this to create just 1 driver that reads 2 registers. 

Sometimes the device returns nothing, and sometimes it returns 4 bytes.  I suspect it could be the hardware converter or wiring. 

You should add asynTrace commands on the underlying serial port so you can see the complete communication with the device.

Mark

Sent from my iPhone

On Jun 10, 2025, at 5:59 PM, Xin, Susheng via Tech-talk <tech-talk at aps.anl.gov> wrote:



Hi Modbus expert,

 

I am working on a Modbus RTU Temp & humidity sensor. Here is the driver configuration and error message after running the IOC. It seems the write/read length has error.

I am new to Modbus and don’t know how to make the correction. The last picture is the device protocol.   

 

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Susheng Xin

Brookhaven National Laboratory

Tel: 631-344-8477