Cat Genset modbus problems
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  1. #1
    Join Date
    May 2009
    Posts
    620

    Cat Genset modbus problems

    Need some advice on what to do with this.

    Situation is this, I have two modbus loops with four generators on one and three on the other. We had some initial trouble with spotty timeouts on the generators and on the generators themselves we would get a amber caution light at the time we didn't know what that was but we worked on decreasing point refresh rates and increasing modbus timing and it all went away for about six months but now is back in only one generator. Mostly what I am asking for is a sanity check on my timing values below.

    The problem today: On the loop that has four generators I have ONE genset that has some points that come back with timeouts and then will work fine on the next poll. On the generator itself it has the amber caution and according to the generator service tech the alarm is "Logic Scada comm not active." All the other generators are fine, all points coming back no errors at all. Anyone think this is a problem with the generator? I'm thinking yes.

    Details:

    Generators are Cat with logic EMCP 3.3, which is pretty standard. This is one of those rare situations where we are talking to the generators directly and not through some ATS switchboard.

    Modbus 485, 9600, 8N1,
    Response Time-Out: 600 ms -----go even further?
    Intercharacter Delay: 40 ms -----decreased to default 20 no change
    Interpacket Delay: 50 ms -----increased to 90 no change
    Maximum Retries: 5

    In the equipment file we are polling 22 analog points with refresh times of 1 minute to 3 minutes for the less important stuff. Also are polling 9 digital points with refresh times of 30 seconds. IMO we aren't asking for a lot of information here.
    Scott Jalbert
    Harris Integrated Solutions

    Formerly Liebert (Emerson Network Power)
    Expressed opinions are my own

  2. #2
    Join Date
    Feb 2005
    Location
    Edmonton, AB Canada
    Posts
    596
    Good day Maxburn,

    Although I know nothing of your generators and your modbus client, I do know a little about RS485 and modbus and so I would do the following troubleshooting:

    1. Can you isolate the suspected piece of equipment from the RS485 bus? If so, then try communicating with that generator alone and see if it communicates. If it does communicate, then you may have some RS485 issues. If it does not, then there is something odd with the equipment

    2. Try using a program like modscan and see if you can read any data from the equipment if you direct connect to it (via a computer to RS485 adapter, etc). If no communications, then the equipment may be faulty. If there is communication then there is something wrong with your RS485 bus.

    3. If you are suspect of the RS485 bus, then place a bus sniffer and analyze the Modbus packets for problems. I would think that this would be less likely, as you stated that other devices are communicating successfully on the same bus.

    4. Like (3) if you are suspecting the bus and you have an oscillscope, then scope the RS485 signals to see how they look. They should be relatively nice square pulses. If they are not, then you could have bus problems.

    A few questions...
    a. What type of cabling are you using?
    b. What is the maximum cable length?
    c. Where is the offending equipment in the topology (i.e. last device, etc)
    d. Do you have a beginning and end terminations?
    e. Have you connected the RS485's reference lead?

    Cheers,

    Sam

  3. #3
    Join Date
    May 2009
    Posts
    620
    I guess you were right, it was the basics that were the problem here. They re-terminated the connection on that generator which must have been lose and it fixed the problem. I was off track in the timing was an issue here but I am interested to learn the CAT equipment being the slave here was throwing an alarm also, must have been seeing the CRC errors. First I have seen that the slave will throw an alarm with bad comms.

    I have never thought to look at it with an Oscope, I bet that would have shown something here.
    Scott Jalbert
    Harris Integrated Solutions

    Formerly Liebert (Emerson Network Power)
    Expressed opinions are my own

  4. #4
    Should of used Loytec.

  5. #5
    Join Date
    Aug 2007
    Location
    Western Colorado
    Posts
    497
    Fifth. You dating Sys or what?
    RealEyes
    Realize
    RealLies!




  6. #6
    Quote Originally Posted by rad1 View Post
    Fifth. You dating Sys or what?
    I love him.

  7. #7
    Join Date
    Feb 2005
    Location
    Edmonton, AB Canada
    Posts
    596
    Quote Originally Posted by MaxBurn View Post
    I guess you were right, it was the basics that were the problem here. They re-terminated the connection on that generator which must have been lose and it fixed the problem. I was off track in the timing was an issue here but I am interested to learn the CAT equipment being the slave here was throwing an alarm also, must have been seeing the CRC errors. First I have seen that the slave will throw an alarm with bad comms.

    I have never thought to look at it with an Oscope, I bet that would have shown something here.
    Good day Maxburn,

    Indeed sometimes the simple things can cause the most headaches... and it does not help when the wiring worked for a while and then started to act up! I know I have gone in circles a few times over events like this. Now, I check all the basics first before I look further.

    A portable oscilloscope is a nice tool to have. I have found a number of issues with it like AC coupled noise from an internally fried step-down transformer, data com errors, timing errors, you name it. Also it helps if you get a scope that is battery powered, as AC powering the scope can fry it if the circuit you are analyzing has a ground fault. I have a very nice and very expensive Tek for my desk and a Fluke Scopemeter for field service.

    Cheers,

    Sam

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