Experimental Physics and
Industrial Control System

"Mark Rivers" <[email protected]> · Wed, 9 Mar 2011 08:19:18 -0600

Hi Rob,

I see the source of the 20ms delay.  It is in modbusInterpose.c:

static asynStatus writeIt(void *ppvt, asynUser *pasynUser,
                          const char *data, size_t numchars,
                          size_t *nbytesTransfered)
{
    modbusPvt  *pPvt = (modbusPvt *)ppvt;
    asynStatus status = asynSuccess;
...
    epicsThreadSleep(0.02);
...

That delay was added in this commit to the code:
------------------------------------------------------------------------
r9770 | rivers | 2009-11-27 10:36:51 -0600 (Fri, 27 Nov 2009) | 1 line

Move slave address from interpose layer to modbus packet layer, allowing multiple slave address per asyn IP or serial port; add 20 ms delay per Modbus spec; changes made by Yves Lussignol
------------------------------------------------------------------------

I made the commit, but the changes were actually done by Yves Lussignol when he got Modbus RTU running with multiple slave addressess on a single serial line.  His comment says that the 20ms delay is "per Modbus spec".  

I have not actually tried to find that in the Modbus spec.  Can Rob or Yves you have a look and see if it is in the spec for all transports (TCP, RTU, ASCII) or perhaps just for the RTU and/or ASCII?  If the spec says it is transport dependent it is easy to make that change in modbusInterpose.c.  The Modbus specs are in the documentation in the module, I just don't have time to work on it today.

Thanks,

Mark

________________________________

From: Mark Rivers
Sent: Tue 3/8/2011 6:10 PM
To: 'Robert Emery'; [email protected]
Subject: RE: Asyn/Modbus Delay

This is the first I have heard of this.

Here is the first thing to do.  Set the asynTraceMask to 0x9 on the underlying asyn TCP port (AcromagDO1), and send the asyn output to a file with asynSetTraceFile.  Then look at the timestamps to see if you learn anything.  You can also then set the asynTraceMask on the asyn Modbus port (CCC:ACROMAGDO1:Modbus:Output) and send its output to a file.  By sending the asynTrace output to a file you hopefully won't mess up the timing too much, though 20ms is still a pretty short delay to be tracking down.

Mark

-----Original Message-----
From: [email protected] [mailto:[email protected]] On Behalf Of Robert Emery
Sent: Tuesday, March 08, 2011 6:01 PM
To: [email protected]
Subject: Asyn/Modbus Delay

We are using Modbus/Asyn to talk to a Acromag ES2113 digital I/O
device.  Since updating our controller software (Linux 2.6.24 to 2.6.32,
EPICS 3.14.8.2 to 3.14.12, Asyn 4.8 to 4.15, Modbus 1.1 to 2.1) the
Modbus/Asyn communication has slowed down.  There is now a .02 sec delay
in messages sent out (see below).  To illustrate this I ran a simple db
with 4 linked longout records writing to the Modbus/Asyn port and looked
at the timing with wireshark.

Any ideas why the delay and how I can get around it?

Thanks
Rob Emery
UW Medical Center

Original Timing

Time        Source     Destination           Protocol Info
0.000000    .0.33          .0.31          Modbus/TCP    query
0.002218    .0.31          .0.33          Modbus/TCP response
0.002229    .0.33          .0.31          TCP      53105 > 502 [ACK]
0.002288    .0.33          .0.31          Modbus/TCP    query
0.005003    .0.31          .0.33          Modbus/TCP response
0.005166    .0.33          .0.31          Modbus/TCP    query
0.007680    .0.31          .0.33          Modbus/TCP response
0.008171    .0.33          .0.31          Modbus/TCP    query
0.010342    .0.31          .0.33          Modbus/TCP response

New Timing

Time        Source    Destination           Protocol Info
0.000000   .0.33          .0.31          Modbus/TCP    query
0.002168   .0.31          .0.33          Modbus/TCP response
0.003864   .0.33          .0.31          TCP      59583 > asa-appl-proto
[ACK]
0.024044   .0.33          .0.31          Modbus/TCP    query
0.026214   .0.31          .0.33          Modbus/TCP response
0.046431   .0.33          .0.31          Modbus/TCP    query
0.048319   .0.31          .0.33          Modbus/TCP response
0.068693   .0.33          .0.31          Modbus/TCP    query
0.071181   .0.31          .0.33          Modbus/TCP response

Port Configuration

## Create the Asyn IP ports
drvAsynIPPortConfigure("AcromagDO1","192.168.0.31:502",0,0,1)

## Apply The Modbus Configuration to the IP ports
modbusInterposeConfig("AcromagDO1",0,1000)

## Create Test Port
drvModbusAsynConfigure("CCC:ACROMAGDO1:Modbus:Output","AcromagDO1",0,6,59,
6,   0,  0, "Acro Test")

Test db

record(longout, Tuning:Asyn:Timing:Test:LongOut) {
   field(SCAN, "Passive")
   field(DTYP, "asynUInt32Digital")
   field(FLNK, "Tuning:Asyn:Timing:Test:LongOut2")
   field(OUT, "@asynMask(CCC:ACROMAGDO1:Modbus:Output 0 0xFFFF)")
   field(OMSL, "supervisory")
   field(VAL, "101")
}

record(longout, Tuning:Asyn:Timing:Test:LongOut2) {
   field(SCAN, "Passive")
   field(DTYP, "asynUInt32Digital")
   field(FLNK, "Tuning:Asyn:Timing:Test:LongOut3")
   field(OUT, "@asynMask(CCC:ACROMAGDO1:Modbus:Output 1 0xFFFF)")
   field(OMSL, "supervisory")
   field(VAL, "102")
}

record(longout, Tuning:Asyn:Timing:Test:LongOut3) {
   field(SCAN, "Passive")
   field(DTYP, "asynUInt32Digital")
   field(FLNK, "Tuning:Asyn:Timing:Test:LongOut4")
   field(OUT, "@asynMask(CCC:ACROMAGDO1:Modbus:Output 2 0xFFFF)")
   field(OMSL, "supervisory")
   field(VAL, "103")
}

record(longout, Tuning:Asyn:Timing:Test:LongOut4) {
   field(SCAN, "Passive")
   field(DTYP, "asynUInt32Digital")
   field(OUT, "@asynMask(CCC:ACROMAGDO1:Modbus:Output 3 0xFFFF)")
   field(OMSL, "supervisory")
   field(VAL, "104")
}

Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System