Wonderware Derived Tags

I was wondering if I could set up In touch to pass one tag from one plc to another plc based on the tags in wonderware.

Factorytalk derived tags type idea.

I want an analog value to pass through the HMI into another PLC.

Is this possible.

eluder said:

I was wondering if I could set up In touch to pass one tag from one plc to another plc based on the tags in wonderware.

Factorytalk derived tags type idea.

I want an analog value to pass through the HMI into another PLC.

Is this possible.

Click to expand...

Yes. Create two IO tags, one for each PLC. Then use a timed or event driven script to make the consumer PLC tag equal to the producer PLC tag.

Let say first tag is analog_1 and second is analog_2

Would the script be as simple as
analog_1=analog_2

or something like that.

Yup it's as simple as that. I have inherited a system with about 20 tags in a script doing that as a means for writing some remote Site Intouch tags back to the client's RTU system. I don't like it though, mainly because:

a) it's a waste of 10 tags (maybe not an issue if you've got the 60k licence)
b) I think PLCs are much more reliable than Windows Machines & Intouch, so using PLC-to-PLC comms is in my opinion the better option.
c) Loss of a PC or rebooting etc means that those tags are now dead until Intouch is back up. Fine if it's just a little monitoring application, but if they're being used for control then you should consider implications of that script not running.

Saffa said:

Yup it's as simple as that. I have inherited a system with about 20 tags in a script doing that as a means for writing some remote Site Intouch tags back to the client's RTU system. I don't like it though, mainly because:

a) it's a waste of 10 tags (maybe not an issue if you've got the 60k licence)
b) I think PLCs are much more reliable than Windows Machines & Intouch, so using PLC-to-PLC comms is in my opinion the better option.
c) Loss of a PC or rebooting etc means that those tags are now dead until Intouch is back up. Fine if it's just a little monitoring application, but if they're being used for control then you should consider implications of that script not running.

Click to expand...

Valid points. Still, the HMI can provide communication between different brands and models of PLCs that don't have native capability to communicate directly with one another. It is also fairly easy to monitor a heartbeat pulse between the PLCs to detect and react to a communication failure. This should be done even if you are messaging between two identical PLCs without using an HMI as a virtual router.

I use a simple heartbeat..
I run a timer in the PLC and use Intouch to reset the timer register at a certain value of the timer register.
If it times out sound an alarm.
ie...Run a 5 second timer in the plc and reset the timer register from Intouch to 0 when the count is >= 2 seconds (using a "Data Change" script with a tag called "Heartbeat" that has the "Item" assigned to the timer register).
You can also set "Log Data" on "Heartbeat" tagname to log speed of communications, by monitoring the timer register value change. This works great with a millisecond based timer.

If you want Intouch to sound the alarm then reverse the procedure, run the timer in Intouch and send the count to the PLC and let the PLC logic do the reset of the Intouch timer.


A word of caution here..Be careful how you set the "Log Deadband" on the timer register, when logging it to history.
If you set the "Log Deadband" to "0" on a millisecond based register, and you are running a "Heart Beat" on multiple PLCs) your hard disc will be very busy.


This should probably be another thread but..
I always warn people, when they are Logging "Real" tags, to make sure they set a reasonable "Log Deadband" on the tagname.
If you log a lot of "Real" Tags with a "0" in the "Log Deadband" you may be logging data that is changing on the order of 10-6 counts (0.000001) or more (when you maybe only need to see in a 0.1 (10-1) change....like temperature or flow). This will make make a HUGE log file every day.