Experimental Physics and
Industrial Control System

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<== Date ==>		<== Thread ==>

Subject:	A Modbus problem relating to byte order and endian
From:	Chiang, Liang-Chih [江良志] via Tech-talk <tech-talk at aps.anl.gov>
To:	"tech-talk at aps.anl.gov" <tech-talk at aps.anl.gov>
Date:	Tue, 7 Nov 2023 07:09:13 +0000

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

Replies:: Re: A Modbus problem relating to byte order and endian Maren Purves via Tech-talk

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ANJ, 07 Nov 2023