I think Ron is not doing justice to the ControlLogix (CLX) here, or at least not to his usual high standard.
PLC5 is based on a system that was first introduced in the early 80's. Yup...it is now over 25 years old, and still going strong. Stable, capable, with a very large installed based and lots of people familiar with it, the PLC5 system has to be one the most successful automation products of all time.
However it does have significant limitations. The inflexible 1771 IO system, the limited comms options and non-symbolic tag programming are just several that pop to mind. It is simply inevitable for any vendor that intends to stay in business to produce new products. I seem to recall from my time selling AB that customers tend to either be on the phone nagging for when the latest new feature will be released, or the type that grizzle everytime something new comes along that they aren't familiar with. Oh well such is life. At some point in the future the PLC5 system will reach its "used by date". I have no idea when, but one day it will be. At that point most AB users will have made the transition to ControlLogix, and will gladly never look back.
The question of asynchronous IO update is simple. In the PLC/SLC system the processor is the master of the all IO modules which act as simple slaves. By contrast the CLX backplane is like a peer network were all the modules are equals and produce/consume data to each other. This advantage of this is that multiple processors, multiple comms modules and very efficient data models become possible. IO modules are intelligent entitities that produce data according to their own configurations, and thus data is consumed by the CPU's asynchronously to the programs scans. Ron's point about IO data buffering is valid but certainly not the mountain he makes of it. In fact IO buffering has become a standard feature of all my programs. Actaully I even use buffering in my SLC programs these days even when not strictly necessary because it allows me better organisation of my logic.
The are the main developments in CLX over PLC:
1. More powerful system Task/Program/Routine organisation scheme.
2. All data is named and organised by symbolic tags.
3. The ability for the user to create their own data structures called User Defined Tables. Once you start using these you never look back. Huge productivity gains.
4. Far more powerful language sets, now including native on-line editing for Ladder, Function Block, SFC and Structured Text. The SFC language is far more advanced than that found in the PLC 5.
5. I think Ron's "bizzaro" ladder constructs are wonderful. They actually correspond to C langauge constructs quite well and allow logic statements to be compactly associated together, and more information displayed on the screen at a time.
6. Communication are far more open and flexible. The CPU is no longer the center of the comms paths. Comms modules can be added to your hearts content and with some limitations, bridging and routing throughout the whole system is native.
Now I have to temper this rant with the observation that CLX is not perfect. Most users have a list of improvements they are hoping/waiting for; but the reality is that few of them would EVER consider going back to the PLC5 if they had a choice.
And elledge...welcome to PLCS.net. !!!