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F0-04THM thermocouple problems

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  • F0-04THM thermocouple problems

    Hi Everyone, I have 2 DL05 PLC's using directnet to run a heat recovery system.
    The slave PLC has the F0-04THM card in it collecting water temperature data. Once every few days something with the thermocouple card freaks out and all 4 channels date will read BCD 2767 and screw up the system. To fix it i simply have to power cycle the system.
    Any ideas?? programming glitch? communication ?
    Please help, Thanks in advance.

    Ryan LaSee

  • #2
    What firmware do you have in this DL05 PLC and what datecode on the F0-04THM? Are you monitoring the T/C burnout bits (SP600 -603)?


    • #3
      I have firmware 5.10 on PLC (master and slave). Im not to sure about the date code.. i think its P1927c??
      I am not monitoring the burnout bits in the program. I do not have that feature enabled.
      If there is a bad connection or open circuit on one of the channels will that cause irradic behavior? will that cause the channel with the glitch to show a burnout bit? I cleared the plc memory and rewrote the setup info to the slave on sunday night... so far it has been ok. but it was very irradic last week. i need more than 3 days operation to feel comfortable with it. a week or 2 would be nice.


      • #4
        I had a thermocouple system on an o6, I used the value 2767 to indicate when the thermocouple for that channel had been disconnected. While the channel was open, the value was always 2767. Maybe not much help, but that was my experience.


        • #5
          It seems like the ambient room temp has something to do with it. right now it is 42f. when i power cycle the system it reads correct for a few seconds then goes to 2767.
          During work hours the ambient temperature is 70-90f. The system seems to work well then. Has anyone ever seen a problem like this?


          • #6
            What type of thermocouple wire are you using?

            What color wires do you have connected to the terminal block CH+ and CH- from that thermcouple wire?

            Is your thermcouple wired directly to the F0-04THM terminal block or does it go through a thermocouple extension block?

            Try shorting one of the inputs with a piece of copper wire between CH+ and CH-, this should read terminal block temperature (ambient plus a few degrees). What reading do you see for this channel?

            Note that you will need to monitor the temperature value as BCD Double Word. The 2767 value you are seeing is most likely 32767 which is burnout indication.


            • #7
              The system is using 4 K type thermocouples. Orange and Yellow wires. Yellow +, Orange-. Is wired to the terminal block accordingly.
              There are no extension blocks or additional wire. All 4 are wired directly to the card.
              When the system is working all the channels read temperature accurately. When it gets screwy it suddenly will show very irradic measurements all over the scale or will just set to 32767 on all 4 channels. Im moving forward assuming that i have a imtermittent thermocouple. My question now is.. If one of them is bad and opens (burnout) shouldnt the card give the burnout indication on that thermocouple only?, instead of all 4 channels at once.
              Tonight I wiggled and gently pulled on all the wires and connections hoping to induce the problem.. no luck. Also Thanks for the 32767 tip... i hadnt realized that was the burnout indication.. read the manual right.


              • #8
                Short out the F0-04THM channels with copper wire and monitor the data and verify that you are still getting 32767. If you do not see this value change with the channels shorted then we need to figure out where this leakage voltage is coming from. When you have all the T/C disconnected measure AC Volts from the PLC ground to each T/C. If any voltage is present this will bias the analog switch and make the readings incorrect. It may take sometime for the voltage to build up, depending on your process.

                [This message has been edited by FACTSTech (edited 02-06-2007).]


                • #9
                  get a new module, these modules have lots of problems... I'm after returning 3 within the past few months. Just sent one back today because it would not let the DL06 go into run mode


                  • #10
                    Alright.. I shorted out each channel. They all read near ambient room temperature. When i disconnect the wire from each channel they indicate burnout on each channel just the way they are supposed to.
                    There is not a glimmer of any voltage from any open thermocouple wire to ground.
                    I wrote a bit of logic to store the ambient room temperature. Every time the card looses its mind the temperature is around 50F. I am going to install a small heater in the electrical panel to keep it at 65F. If after a week it does not screw up.. Automation Direct is geting the card back.
                    Does anyone have any further thoughts??


                    • #11
                      ryanlas, Are your T/C insulated or grounded? If you are using grounded T/C, try using insulated T/C. I have ran into problems with grounded T/C, particularly on water pipe temp measurements. Hope this helps.


                      • #12
                        Please zip up your code and send via email to In the mean time make sure what program is in the PLC is saved to disk. Then initalized scratchpad, and clear ALL PLC memory. Then reload your saved program and power cycle the PLC.


                        • #13
                          Did this ever get resolved...I'm having a simular issues but using non-contact infared temperature sensors with type K output.

                          I'm suspecting one sensor to be bad... but I need to be sure. When this sensor is powered, ALL channels go if they all burned out. I remove this sensor and install a type K probe instead....all is normal. Also, when this channel is not connected (open), it's the only channel showing high (burnout).

                          I'm just REALLY stumped as to why it would effect the other channels. If this is even the case.


                          • #14
                            An infrared temperature detector is not a thermocouple, it is an electronic device.
                            With that being said, ALL THM modules have burnout detection circuitry, this circuitry interferes with T/C calibration devices and infrared type detectors.
                            We "ping" the junction (send a current pulse to the T/C) the circuitry then detects.
                            Now for the bad news, I have encountered some customers that have had issues using the infrared detectors, the "ping" may have destroyed the device, this depends on the output drive circuitry.

                            To solve this issue you MUST disable the calibration enable circuitry, on the
                            F0-04THM this is a setting in ladder, other series have jumpers.

                            Stray voltages can affect the other channels, on the F0-04THM wiring diagram the negatives must be connected as shown. The 05/06 series are NOT isolated interfaces to the backplane. Proper wiring of grounds is very important, not to send ground current through the 50 pin backplane connections.


                            • #15
                              Thank you for your responce.

                              I have the THM wired as shown in the manual. There are two IR transmitters currently on CH1 and CH2 of the THM. I have disabled the Burnout detection, I believe. (Setting the Burnout detect bit to "1" is "off", correct?) The manual is only clear on the bit to work with. There's no mention as to which state is which until you get down to the ladder examples where it's mentioned that burnout detection is "on" by setting to "0".

                              CH2 is working just fine. CH1 is the problematic area. This channel is also the longest run of the KX 20ga wire (about 45 feet). Now, are the ga and length ok? CH3 is the shortest run (about 12 feet), and has a thermocouple at the end of the ext wire. This channel will also have an IR transmitter soon. I'm trying to determin if I have a dead sensor or not. So far, it looks like I do.. that's a bummer because they are about 4 weeks backorder.

                              I get steady temp readings from a thermocouple on CH1. So, I'm thinking that grounding MAY not be an issue. I'm trying to be sure about this.

                              Again, thank you for your help so far.