There are several tools available for that purpose. Search the help file on "security".
A lot of IMO unscrupulous OEMs do this primarily to force end users to pay out for service calls for things that would otherwise be easily fixable by the end user. They often claim it is to protect some kind of proprietary knowledge but I think they speak with forked tongue. And I feel justified in saying this having seen what is in a fair number of these "proprietary" chunks of code.
There probably are a few applications where what is coded is legitimately proprietary, but IMO they are extremely rare. The two examples that I have run across in the last almost 30 years involved ladder logic implementations of PID loops back before PID loops were commonly available as blocks in PLCs.
You might be able to concoct a scenario where it is a reasonable thing to do, in the real world, mostly not.
There are two things you can do. The first is called Source Protection. This allows you to lock certain routines or Add-On Instructions so that they cannot be edited or viewed. (You can actually allow viewing-only, if you like, but your question implies you don't want your code seen.)
The second is called Logix CPU Security. This prevents someone from uploading the project or going online.
Both methods, and some much more complicated stuff about securing access when you're in a networked environment, are in the Logix5000 Controllers Security Programming Manual.
(Look out for spaces in that link. Control.com has a nasty habit of adding spaces into long blocks of text, so when you copy-paste the link it doesn't work. If there are any spaces, delete them before you hit "enter" in your browser address bar.)
I have to agree with Bob on this one....And you will solve the problem of my reading your code because I will strongly council against buying anything that can't be adequately maintained. And If purchased, I've been known to replace such black boxes with maintainable code. I have had someone argue that their cardboard baler or conveyor held valuable trade secrets, but they usually argue that on the way out.
I had a customer about 7 years ago that had a simple machine with a 20-year old Toshiba PLC where the relay outputs kept dying and new ones were no longer available, so I sold them on a new PLC and MMI. The OEM was still in business so I called and asked if they'd give me a copy of the software and the guy tells me that software is their IP and the only thing keeping their company in business. So I ended up writing it from scratch and it was like 80 rungs or something. Pretty sad if 80 rungs is all that's standing between his company and oblivion. He must have been right though, and someone must have got a hold of the "secret" code, cause they're out of business now.
