I want to use an "always high" digital tag sent from VTScada to the PLC as a comms status bit.

I've tried creating a numeric calculation tag and entering the PLC bit address in the address field, but this doesn't work (I'm assuming because the numeric calculation only outputs analog, but correct me if I'm wrong).

I have tried to use a digital tag with start condition, but not sure what to put in the expression. Entering "1" doesn't work. Would appreciate any advice.

Thanks.

You can use a digital I/O tag and set its data source to a calculation tag. The trick to this will be that the bit should be alternating in a fashion which works for your 'heartbeat' as I don't expect it to continuously write the same value if the calculation is static.

For example, a calculation expression such as "Now(5)%2" will alternate between 0 and 1 every five seconds.

Thanks for the reply.

I have an alarm set using the driver quality, and have been able to use this with an alarm status to act as a "connection active" bit. However, the alarm is set to trigger when quality is below 95%, so when comms reconnect it takes some time for the alarm to turn off.

I also have a comm indicator on the main screen, which does pretty much exactly what I want ... it turns on immediately when there is a connection. I cannot figure out what tag this is triggering off of though, as I'd like to use this as the "connection active" bit and send it to the PLC. Under "tag" in the idea studio, it only gives the driver name, and not which property is turning it on or off.

Is this a tag I can use and link to with a digital output?

You can look at the value of the port or driver tag to get results in VTScada showing the (ex:[Path\To\Driver]\Value) and use that to create custom alarms on VTScada's ability to connect to a driver or connect over a network.

Note that when the comm indicator's value changes, you have likely already lost your connection to the PLC so you probably won't be able to send any updated values to it unless the connection is intermittent.

When the connection is good, the value won't change so you likely won't see any regular values written to the PLC to use that like a 'heartbeat'.

Does that make sense?

Hi Dave, yes that makes sense, was thinking about this as well and if there is a workaround, but probably a heartbeat makes the most sense.

In your expression above, you use "Now(5)%2". I have not seen this expression before, is a description of how to use this located in the help somewhere? Thanks.