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Old February 5th, 2018, 12:06 PM   #1
krishna431
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Communication loss between InTouch display and PLC

Hello All,
I am using Automation Direct PLCs which are interfaced to communicate with InTouch HMI. The communication is done through modbus using the System Management Console which is part of InTouch.
Now the issue I'm facing is - I display some information about the PLCs including the scan times of all the PLCs I have connected on my HMI screen. It's not necessary, but its something I like to see now and then. I have noticed that one of my PLC's (lets call it PLC1) scan times repeated goes to 0 and back to whatever value it was on. This is every few seconds, and it is inconsistent. It's difficult to find what is causing this issue, as the communication between PLC1 and the HMI is done the same way as all the other PLCs I have networked, and they all behave perfectly. I am communicating with PLC1 for monitoring and controlling other outputs, and they all work fine, with no communication loss as far as I can tell.
Is there any reason why this value display on the HMI screen could be going to 0 and back? (And yes - I have made sure that I'm not using the same tag in different places)
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Old February 5th, 2018, 12:56 PM   #2
bernie_carlton
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To help us please state which CPU, which native address in the CPU and which Modbus address.
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Old February 5th, 2018, 12:59 PM   #3
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Sure thing. I'm using Automation Direct T1H-DM1E CPUs. The address in the PLC is N28 and the modbus address is 400151.
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Old February 5th, 2018, 02:22 PM   #4
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N28 is not a Modbus accessible memory location. It is an ordinary register only accessible using a native Do-more protocol.

Here are the system locations of relevant scan time locations:

DST1
$ScanTime
Status
Contains a filtered average of the $ElapsedTicks.

DST2
$MinScanTime
Status
Contains the length of time (in microseconds) of the shortest scan since the last STOP mode to RUN mode transition.

DST3
$MaxScanTime
Status
Contains the length of time (in microseconds) of the longest scan since the last STOP mode to RUN mode transition.

DST4
$ElapsedTicks
Status
Contains the number of microseconds for the last scan.

How did you determine this particular Modbus address was proper to view the scantime?
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Last edited by bernie_carlton; February 5th, 2018 at 02:28 PM.
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Old February 5th, 2018, 03:00 PM   #5
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@Bernie_carlton, I am copying the scan time to the N28 address and sending it over modbus. The N register is modbus accessible in the Do-More software. I use the N register for communication all the time.
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Old February 5th, 2018, 03:37 PM   #6
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Are you possibly copying to the modbus address in two separate locations in your code? That would exhibit the behavior you've described if the second register was zero.

Intouch will normally just hold the last read value so it's unlikely to be a comms issue
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Old February 5th, 2018, 03:57 PM   #7
bernie_carlton
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Oh, so the PLC is the master. Well then, anything can be the source. You may want to post at the AutomationDirect forum or the Host Engineering forum.
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Old February 5th, 2018, 05:28 PM   #8
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@Saffa,
nope - i already made sure to check that. Also, I'm getting the value at the address -- the problem is that the value goes back and forth from 0 and I don't know why.
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Old February 5th, 2018, 06:46 PM   #9
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Have you monitored the scan time directly in the PLC to see if it varies the same? If it does definitely post on the Host Engineering forum. http://forum.hosteng.com/ They make the Do-more.
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Old February 6th, 2018, 10:03 AM   #10
krishna431
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Yes, I have monitored the values in the PLC and they are fine. I am aware of the Host Engineering forum, just thought I'd check here first since this forum has been helpful in the past.
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