Re: MODBUS User-Defined Function Codes

[Torsten Bögershausen via Tech-talk <tech-talk at aps.anl.gov>]

On 2026-02-02 18:54, William Jamieson via Tech-talk wrote:
> Does anyone have experience using user-defined function codes alongside 
> the standard function codes that can be handled by the asyn MODBUS 
> driver? I am currently using the MODBUS driver for the standard 03 "READ 
> HOLDING REGISTERS" function while simultaneously using streamDevice 
> driver for executing user-defined functions. This is resulting in 
> conflicts between the MODBUS and streamDevice driver. I have tried 
> setting up two virtual ports pointing to the same serial port:
> drvAsynSerialPortConfigure("rs485_modbus", "/dev/ttyUSB0", 0, 1, 0)
> drvAsynSerialPortConfigure("rs485_stream", "/dev/ttyUSB0", 0, 1, 0)
I think that this is the wrong approach - according to my understanding.
Both the modbus and streamdevice should lock the asynport.
("rs485_port")
Which should the lock the underlying device, "/dev/ttyUSB0".
If you enable all asyn traces (and direct them to a file)
there should be some light comming up, if there is a problem
here in synchronizing things.
Or if there is a problem with the hardware ?


> Then using a SEQ record that disables the rs485_modbus port then enables 
> the rs485_stream port before processing the stream record. after 
> processing, it disables the rs485_stream port and re-enables the 
> rs485_modbus port.
> I disable and enable ports by writing a 0 (disable) or 1 (enable) to 
> the CNCT field of an ASYN record for each port. Example record:
> record (asyn, "modbusPort_ASYN")
> {
>      field (PORT, "rs485_modbus")
> }
>
> Not only does this switchover create a lot of I/O Error messages in the 
> IOC shell, but this still results in a garbled data readback from the 
> stream record.
> Does anyone have a successful method of making these two methods work in 
> parallel? (or some other tools that make it work) Or should I just give 
> up and use deviceStream reads/writes instead of just the custom ones. I 
> prefer to use the MODBUS driver if possible for the standard MODBUS 
> function codes.
> Thanks for any help!
>
> William Jamieson
> PPPL I&C Engineer