First of all, let me apologise, I am new to VB 6.0....I come from the world of electronics and LabView and PLC programming ( Shudder ). I didn't give enough information in my first post...

App3, which communicates with the microcontroller, receives the command that starts either the Cal run OR the Data Acquisition Run. It also needs to know if those runs ( fail/pass which at this time is simply a text box nasty message.

I could run all three apps from the keyboard, 'cause i can see the message in App3 text box saying " Start normal run OR start cal run ". App1 run takes from 15 min up to 4 hours. App2 process takes about 2 minutes always...App3 runs as fast as you can hit the keys.

A typical sequence of events would be:
App3 displays Run Normal,
I Goto App1 and press Start Normal Run btn.
When App1 writes the data.txt file, I go to App2 and load
that file, it processes it and writes a result.txt file. I
Go to App3 and load that result.txt file, it sends it to the Microcontroller.

The cal run is almost the same as above.

At this time there is no way for App1 or 2 to let the Microcontroller know there was an error.

To combine all three apps together is the best solution, But App2 is the real barrier there...It was written over a period of 10 years by 3 different programmers, I think parts of it were even written in earlier versions of VB. Plus it calls huge libraries of FORTRAN AND c++ .dllS. The other two applications are fairly small, like you could create them in 3 months or so...

It sounds like this " shell " idea is the way to go...Is there some book I could buy, or just scan the archives of this forum.