Experimental Physics and
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<== Date ==>		<== Thread ==>

Subject:	Modbus port driver processing record with new value call back without a new value
From:	Tobin R Weber via Tech-talk <tech-talk at aps.anl.gov>
To:	tech-talk at aps.anl.gov
Date:	Mon, 12 Oct 2020 12:04:23 -0700

Hello Ladies and Gentleman,

I have been reading 16-bit holding registers using modbus (function code 3) from an acromag device (XT-1541) and a DirectLogic DL 205 Modular PLC. Using the "ai" record, and "asynInt32" for the "DTYP" field, I am able to correctly read the 16-bit values. However, the "SCAN" field is set to "I/O Intr", and the record processes every time the modbus port driver is polled (every 100 msec, in my case) regardless of the status of the value (i.e., even if the value has not changed). I see an identical issue with the "longin" record. I have tried many different things, and it appears using the "asynUInt32Digital" for the "DTYP" field with the "longin" record fixes the problem. Unfortunately, this device support is not available for the "ai" record with modbus.

My question: Is this the specified behavior? I'm sure I'm doing something wrong, but I have no idea what at this point. Below is one simple example of the issue:

Here is the st.cmd file:

st.cmd: Modbus TPC/IP communication with a Acromag XT-1541.

---------------------------------------------------------------------------------------------------------

drvAsynIPPortConfigure("AcromagHVPS","10.10.10.176:502",0,0,1)
modbusInterposeConfig("AcromagHVPS", 0, 1000, 0)
drvModbusAsynConfigure("HVPSRead", "AcromagHVPS",0,3,0,9,1,100,"AcromagHVPS")

dbLoadRecords "db/test.db"

cd "${TOP}/iocBoot/${IOC}"
iocInit

asynSetTraceIOMask "HVPSRead",1,4
asynSetTraceMask "HVPSRead",1,0x000b

---------------------------------------------------------------------------------------------------------

Here is the record that is scanned every 100 msec, despite the value not changing:

test.db: "ai" record to read modbus data.

---------------------------------------------------------------------------------------------------------

record(ai, "HVPSBOReadRecord") {
field(SCAN, "I/O Intr")
field(DTYP, "asynInt32")

field(INP, "@asynMask(HVPSRead, 1, -16, 1000)MODBUS_DATA ")
field(FLNK, "iocHeartbeat")
}

---------------------------------------------------------------------------------------------------------

epics shell: Shows that the modbus data read properly, but the "HVPSBOReadRecord" record is continually processed with a callback, despite it's value not changing. And I have verified that this record is actually being processed with FLNK's to other records as well as by setting the "TPRO" field.

---------------------------------------------------------------------------------------------------------

2020/09/10 16:03:21.561 HVPSBOReadRecord devAsynInt32::interruptCallbackInput new value=12750
2020/09/10 16:03:21.561 HVPSBOReadRecord devAsynInt32::getCallbackValue from ringBuffer value=12750
2020/09/10 16:03:21.662 HVPSBOReadRecord devAsynInt32::interruptCallbackInput new value=12750
2020/09/10 16:03:21.662 HVPSBOReadRecord devAsynInt32::getCallbackValue from ringBuffer value=12750
2020/09/10 16:03:21.764 HVPSBOReadRecord devAsynInt32::interruptCallbackInput new value=12750
2020/09/10 16:03:21.764 HVPSBOReadRecord devAsynInt32::getCallbackValue from ringBuffer value=12750
epics> 2020/09/10 16:03:21.865 HVPSBOReadRecord devAsynInt32::interruptCallbackInput new value=12750
2020/09/10 16:03:21.865 HVPSBOReadRecord devAsynInt32::getCallbackValue from ringBuffer value=12750
2020/09/10 16:03:21.966 HVPSBOReadRecord devAsynInt32::interruptCallbackInput new value=12750
2020/09/10 16:03:21.967 HVPSBOReadRecord devAsynInt32::getCallbackValue from ringBuffer value=12750

...

---------------------------------------------------------------------------------------------------------

Conversely, if I use this record, the record is only processed when the input data changes:

test.db: "longin" record to read modbus data.

---------------------------------------------------------------------------------------------------------

record(longin, "HVPSBOReadRecord") {

field(SCAN, "I/O Intr")
field(DTYP, "asynUInt32Digital")
field(INP, "@asynMask(HVPSRead, 1, 0xFFFF, 5000)MODBUS_DATA ")
field(FLNK, "iocHeartbeat")
}

---------------------------------------------------------------------------------------------------------

epics shell: Shows that the modbus read data properly, and not processing the "HVPSBOReadRecord" record continuously with a callback. I also verified that this record is only processed when the value changes via the .TPRO field, and FLNK's to other records.

---------------------------------------------------------------------------------------------------------

2020/09/10 16:01:35.160 HVPSBOReadRecord devAsynUInt32Digital::interruptCallbackInput new value=12750
2020/09/10 16:01:35.160 HVPSBOReadRecord devAsynInt32::getCallbackValue from ringBuffer value=12750
epics>

---------------------------------------------------------------------------------------------------------

I'm using:

EPICS: 3.15.6

Asyn: R4-32

Modbus: >= 2-10-2

However I have since tried EPICS base-3.15.7, modbus 3-0, and asyn R4-38.

Thanks very much for any help.

Toby

Tobin Weber

University of Washington

Department of Aeronautics and Astronautics

Research Scientist