Re: A Modbus problem relating to byte order and endian

[Maren Purves via Tech-talk <tech-talk at aps.anl.gov>]

Aloha,

this may not so much have to do with the version of asyn and modbus but with the architectures and/or operating systems of the computers involved. I remember running into problems like this when we switched from Solaris (on Sun machines) to Linux (on x86 machines) (about 20 years ago, with the IOCs staying the same) - when for array readouts displayed as images we got 'striped data'.

In your case recompiling on the system you're running it on may help, I think at the time I may have implemented byte-switching in transfer, but this being 20 years ago I don't remember the details.

Maren Purves
Head of Instrument and Telescope Software
East Asian Observatory / JCMT

On Mon, Nov 6, 2023 at 9:09 PM Chiang, Liang-Chih [江良志] via Tech-talk <tech-talk at aps.anl.gov> wrote:

Hi, EPICS mates,

I have a motor driver(Oriental AZ series) with built-in encoder.

Its RS-485 2-wire interface is adapted by a MOXA Nport 5150A to ethernet(TCP/IP).

I use Modbus to read its encoder position.

I tested it on my own computer,  and successfully read out the encoder position.

However after transfered to a production EPICS/IOC server, the encoder position value became strange.

Please notice that the versions of modbus module are different.

On my computer(asyn-4-26, modbus-2-7), the read value is:

decimal -271,289

hex FFFB DC47

On EPICS/IOC server(asyn-R4-42, modbuus-R3-2), the read value is:

decimal 1,205,664,767       

hex  47DC FBFF

Following is my settings and EPICS record:

## drvAsynIPPortConfigure("portName", "hostInfo", priority, noAutoConnect, noProcessEos)

drvAsynIPPortConfigure("AZPORT01","192.168.2.61:4001",0,0,1)

## modbusInterposeConfig("portName", linkType, timeoutMsec, writeDelayMsec)

modbusInterposeConfig("AZPORT01", 1, 1000, 100)

##drvModbusAsynConfigure("portName", "tcpPortName",slaveAddress, modbusFunction, modbusStartAddress, modbusLength, dataType, pollMsec, "plcType")

drvModbusAsynConfigure("AZPORT01_CH1ENC",   "AZPORT01",    1, 0x03,  0x00CC, 0x02,    6, 100, "AZPORT01")

record(longin, "$(P)$(R)Xenc") {

    field(DTYP, "asynInt32")

    field(INP, "@asyn($(PORT)_CH1ENC 0)MODBUS_DATA")

    field(SCAN, "I/O Intr")

}

By the way, if I changed the "dataType" from 6(INT32_BE) to 5(INT32_LE), the value became:

decimal -599,261,189       

hex  DC47 FFFB

Best regards.

LiangChih Chiang

NSRRC beamline group

email: chiang.lc at nsrrc.org.tw