Best Budget PLC Platform for Simple Discrete Application

While I like the idea of trying out new things, you have to take the cost of retraining, plus stocking even more vendor's parts on the shelf. If your staff already knows AB, there's a lot of vendor lock-in pressure, so you need a very solid business case to move away from what you already have. You really need to understand, no other vendor has a ladder logic editor that compares with AB, and no matter who you move to, you'll get a lot of grumbling about the programming software from your existing people.

Plus, most of the budget controllers out there have barriers to interoperability. You really don't want to install anything right now that's not going to play nice with others.

In your case, I suggest just buying a CompactLogix and get on with it. You can do the whole thing from one controller and it'll be an easy fit with what you have. The cost of the controller is not that much compared to all the other hardware you have to buy. It still leaves you lots of options if you need to upgrade or connect to something else in the future. In a worst case, you can take the code out and dump it into a ControlLogix, so you're really future-proofed.

Now if you've really got a hankering to jump off the AB boat to something less expensive, the only one I would personally recommend looking at is Beckhoff. I know for a fact that they're significantly less expensive than AB, and you get much better performance. You're still future-proofed if you go with them. They cut costs by basing everything on commodity hardware rather than by trimming features. Beckhoff is PC based. The have very small embedded controllers that look like PLCs, and are inexpensive. You can buy a commodity industrial PC from pretty much any vendor as long as it has Windows XP and you can run the Beckhoff runtime on it (though Beckhoff's industrial PCs are comparable in price, it seems). Note that you pay more for a license for the full blown PC version than if you went the embedded route. If you really want to cut costs, I've successfully run a controller on a regular old desktop PC. With adequate cooling, daily backups, virus protection, and a recovery plan, you're still safe (true for any PC-based solution).

That's my $0.02