I have an old customer that has an AV2 and a Tstat-EX. He wants to add an ST-PC power controller which will turn his HEPA filter on and off based on the fan being on or off on the furnace. The problem is that the only feedback that I can see for the TSTAT-EX is "Fan Call Status - Fan is Running If High, off otherwise". How do I write the conditional to turn off when low? Or is the status based on a fan call and the signal only goes high momentarily? This system was programmed initially in SB. Any help is appreciated.

Because the Thermostat is hardwired into the Furnace you can do this without any logic involved. You need to install a Relay connected to GH on the stat and when GH goes high your relay trips and will then trigger the HEPA filter. If you want the HEPA to turn ON if the fan is running (low, med, or High) then you would require 3 Relays wired in an OR sequence (GL, GM, GH on the tstat).

We do this all the time with boiler pumps. Saves on programming time and is not linked to programming which could fail.

If you need to do this programmatically it would be quite ease
If Fan is Running = High
Turn ON HEPA (Close relay I am assuming)
Else
Turn Off HEPA (Open Relay)

Again Programmatically this is possible, but to do this using simple relay's will be much easier and is a fail safe solution.

If the output is high when Fan is Running you could create a global expression called Fan. In the global expression create a conditional step of that Fan is Running and use it to trigger the ST-PC on. Then use an else step in the same logic tree to trigger the ST-PC off. This should work but its been awhile since I touched SB.

SWO is also right on - a processor failure or removal for any reason kills the HEPA control. His method bypasses that possibility. A little more upfront work but you'll probably never have to revisit it.

+1 SWO