dakotap

Clx to micro820 heartbeat

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I have remote sites with micro 820s and a controllogix at plant.. I need to set up a simple heartbeat or something to make sure communication is Okay. Currently I have been using the msg .DN and .ER to flag if something is wrong but they want a heart beat to write  a number , read a number and then compare if it’s not changing . I set up a timer and a counter for the heartbeat on the clx . Writing and reading from the 820.  but I’m having brain freeze on the logic to be able to compare to see if the number read is the same for more than let’s say 10 minutes . (Messages are being sent every 1 minute) 

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We did something like this in the PLC's on the oil system I worked on. If the HMI didn't reset the heartbeat timer within 10 minutes we shut the station down. The HMI was in Houston and the PLC's were scattered from Corpus Christi to Laredo to San Antonio. The HMI (could just as easily be your master PLC) would send a bit every X seconds. The bit would move the current timer value to a register so we could see elapsed time, then reset the timer before it tripped the done bit. If it tripped the done bit then the station tripped on Comm Fail

You basically toss a ball (bit) from one PLC (the Master) to the other PLC. If the other PLC catches the ball (bit) then it sends it back to the other PLC. Each bit transfer resets a timer. If the Master ever reaches the DN bit in the comm fail timer then comm has failed.

I have no doubt that you'll work this out.

PS - the link in the post above this has some great ideas explained in more detail than I've gone to.

Edited by Michael Lloyd

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Thank you guys! I have searched and searched I don’t know how I haven’t seen the above thread. This should be simple enough . By the way Michael I have setup 13 remote sites and all working fantastic thanks to your help previously . I just wanna add some protection Incase something goes down at a pump station . 

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When working with PLC5, SLC500 and CompactLogix, ControlLogix and GuardLogix I generally use only reads.  It is more efficient and clearer than doing writes.  Now the Micro 820/830/850 ABOMINATIONS are a departure from this.  I do all communications to Micro8XX from the COntrol or Compact logix by read and write.  My approach is simple.  I operate on a Master/Slave Hearbeat mentality as follows:  

1) Slave Reads Master Register A  into Slave Register B.

2) Slave Copies Slave Register B to Slave Register C

3) Master Reads Slave Register C into Master Register D.

4. Master Adds One (1) to Master Register D and Places the Result in Master Register A.

5. An Not Equals Statement in the Master compares Master Register A to Master Register D.  If they remain different for more than the desired Fault Timeout then a fault bit is set in Master and COde Activated to act appropriately..

6. An Equals Statement in the Master compares Slave Register B to Slave Register C.  If they remain the same for more than the desired Fault Timeout then a fault bit is set in the slave and action is taken.

This method gives you not only a communications check, but a scanning program check as well..

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