Need Help Monitoring a PLC Register

[BuddyC]

This should be easy to do, but I just cannot figure out how to do it (I'm a new Lookout user).

Whenever a particular logical register in the PLC goes "False", I want my Lookout process to set it back to "True".  We are using this to indicate to the PLC whenever Lookout is not running.  The PLC will check this logical register once per second - if its "True" the PLC will set it to "False";  if its "False", then the PLC will signal an alarm.

Thanks for your help.,

Buddy

[frisk]

While making Lookout invert your PLC's register is fairly easy (create pushbutton, connect inverted plc register to reset_value and "regular" plc register to reset, that is if your PLC allows you to write to it's registers from both PLC and Lookout, which is unlikely and definitely bad practice), from my experience performing the function you require is not so simple, I gave up after 2 hours of experimenting and built some sort of workaround.

I'm eagerly waiting to have a look at some smart solution for nodeguarding (if I am correct this is how this procedure is called). Some devices have this as a built-in feature, very convenient if you need to know where's your system at.

Bye,

Darko

[Khalid]

One other way to quickly reset a bit in the PLC is to negate and write back the value that's read.  For instance, if you're resetting the coil at Modbus1.3, you can create the following connection:

Modbus1.3 = !Modbus1.3

Regards,

-Khalid

[Mike@DTSI]

Our plcs have a "Com Timeout" function, after set time in minutes expires it turns off pumps, etc.  What i did was conenct a Pulse Timer to the Poll item.  just before the poll, another time activates the PLC register to True, this resets the timer in the PLC.

[flexman]

Another option.
Send a changing value from Lookout to PLC (e.g. timer that goes on or off, or numeric that keeps varying).
If value in PLC is the same as it was last scan- increment PLC timer. If PLC timer expires then Lookout or link is dead.
I have heard this refered to this as a heartbeat.

If you choose to do this on a bit, be sure the PLC catches both on or off states when checking.

This is more programming work, but more relable than merely checking the comms link (as sometimes info is not getting across even though comms link is good).

[reandy]

Yes, I agree with Flexman, and I too call this a 'heartbeat'. I motor in Lookout that it is still able to communicate with the PLC, and I monitor in the PLC that it is still able to communicate with Lookout. Why do it both ways? When Lookout can not communicate with the PLC, I send an email message to myself (I have other things to do than to watch the system all day long). When the PLC is not communicating with Lookout, I can have logic in the PLC to take over critical functions it needs to maintain on it's own.

An easy way to do monitor Lookout in the PLC, is to have a PULSE object in Lookout. Have it pulse at the frequency you wish to be notified of lost communications. Ok, so now you've got a bit in the PLC changing states every minute from Lookout (or faster if you need) Then monitor the normaly-open state of this bit into an on-delay timer, set the preset of this timer for 150% of the lengh of time that the bit stays on. Also monitor the normaly-closed state of this bit into an on-delay timer, and again set the preset of this timer to 150% of the lengh of time that the bit stays off. Now if either of these timer's 'time-out' (use an OR statement into an output bit) you can trigger what you need to do when you loose your communications to Lookout.

good luck,

Rich