Hood when you are working with a 24 T changer that needs look ahead or hot swap then it gets very complicated for VB to handle in a fast action function. as the tool changer needs to run WHILE mach is running Gcode. If you notice I said WORK AS IT SHOULD (;-) I just assumed the OP wanted it to work close to what the original machine was like. So far I have never seen a MACH tool changer(mill) work well with just mach I/O and VB. I have seen a plugin tool changer function work just a well as the commercial machines
Now if you can handle a "very" slow tool changer function it can be done
But that is just my opinion, others may vary
Terry the toolchanging I would do in the PLC, that is what they excel at in my opinion, VB would only be used to tell it to start and get told when its done.Similarly hot swap just started by a m code and the PLC would do its stuff while Mach is going on its merry way, when the time comes for the next toolchange it would all be done and the only thing Mach would do is look to make sure its correct.
As for very slow toolchangers, I think the reason the ones you have seen are that way are because the hardware is the original and was that slow on the original control. My lathes turret and post are hydraulically operated and are no faster or slower than they were on the original control.
The turret I will be replacing the front post with, one of these days, however will be much faster. The reason it will be faster is I have a servo on it and it can go forwards or backward and doesnt have to clamp at every tool like the one on the rear of the lathe does.
One day I my even get round to making a changer for my mill that I have promised myself
Hood