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comms port failure

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I am running the latest version of Lookout in a very simple setup controlling three Modbus nodes on a USB / 485 converter port. It all works perfectly and was installed on two machines. There was a problem with the first machine in that the control pc inbuilt 485 port had a problem which caused me to swap to the USB interface. This fixed the problem so I used it on the second machine. What is happening now is the second machine will operate correctly for a random period of time then lose all communication through the port. The client notes that the activity indicators on the nodes are dead and the Modbus stats show response failures. Re booting the complete setup cures the problem for a while whcih seems to be anything from two to 60 minutes. Stragely enough the first machine is now performing the same tricks having worked without issue for several days. The modbus nodes are one of 8 outputs, one of 16 inputs and one of 2 analogue inputs. Needless to say it worked perfectly when I was on site for a few days and the job is half a days plane ride away.


Apart from changing the port address everything is as per the Lookout defaults.

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Accepted by topic author GDI

Let's log the communication data on the serial port and see what happens.

Select Option->Serial Ports, choose the COM port the Modbus uses. Enable the "Diagnostic File Settings", input file name, and check "Value in hex". Then the Modbus communication on that COM port will be logged to the file. Run the process until the problem happens. Attach the log file here.

Ryan Shi
National Instruments
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Thanks Ryan

As a last resort I had the client increase the receive gap to 50  under options > serial ports. He said that following that the machine ran several cycles without further incident. Great if that is the whole story, however I also found out in a round about way that there were power supply problems going on at the same time. The modbus nodes are on a seperate unmaintained supply to the computer, which was being shorted briefly due to the movement of a chain drive. This tripped the breaker thus the modbus nodes went dead. The thing moves at one rev per hour so no one noticed it stopped and it auto resets, so the modbus nodes come good coincidently with the computer reboot. With a reboot the system starts from scratch, someone pushes the start button the next chain link comes round and it shorts again. Sometimes one link sometimes several links, depending on how the chain hangs. Just to top it off the wire being shorted was the neutral, so if they had wired it correctly rather than swapped active neutral the problem would not have been noticed


There are names for these kinds of problems (clients) that I won't repeat here.


Hopefully that is the end of it, as I am told a similar problem was found on the earlier machine, which makes sense as it failed after a long period.


Thank you for your time

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