What exactly is scheduling?

[Pineapple]

Hi all,

New grad working with PLCs for the first time. Today I came across the concept of scheduling when adding new devices to a ControlNet network. Couldn't get a good explanation of exactly what scheduling was and why its necessary so I was hoping someone could explain it.

 

[AlfredoQuintero]

Hello. ControlNet is a specification which is part of the ODVA CIP (common industrial protocol) specification. In ControlNet scheduled messaging is used for process data, such as the temperature that a particular sensor with ControlNet adapter functionality can provide to a PLC which supports ControlNet scanner functionality. The schedule is calculated by an engineering tool such as RSNetworkx. The schedule as the name implies indicates when a particular MAC ID in the network can transmit scheduled data. This schedule information is programmed to every node in the network. In addition to scheduled (i.e. critical) communication, there is also unscheduled communication, also known as explicit messaging, which is normally used for non-critical communication such as diagnostics and parameterization. I would recommend you visit the ODVA website and consult Rockwell's documentation, for more details.
And yes, schedule messaging IS necessary.

 

[Operaghost]

Hey Pineapple, Welcome to the forum.

Think of it like a bus schedule. The bus has 60 minutes to go through its entire route. The schedule has been carefully laid out so that the bus does not arrive too early or too late at any stop. Sometimes it means the bus waits a little longer at a stop just to be sure it isn't too early at the next stop. The riders expect that bus to be at their stop at an exact time each and every day. And they expect it to drop them off at a specific time.

You've added a new stop on the route. The schedule then must be re-calculated to make sure everyone still gets picked up and dropped off at the proper times.

As Alfredo mentions, RSNetWorx is the tool that recalculates the schedule. When scheduling, you can choose to "Optimize and rewrite the schedule" which throws out the existing schedule and starts over. This is the most efficient method but requires all network PLCs to switch to Program mode. Or you can choose to "Merge changes into existing schedule" which is less efficient but only requires the affected PLC to switch to Program mode. Which sounds bad, but it would have had to switch to Program mode anyway to download your changes. Merge is usually the preferred option since it doesn't impact other network devices.

So with Optimize, we might adjust everyone's pickup and drop off times to get the most efficient bus route. With Merge we would need to fit that new requested stop into the existing schedule without affecting any of the other stops on the route.

In recent years you have been able to get by without scheduling as a sort of temporary workaround. But that really wasn't ideal. We could go into all the details of scheduled vs unscheduled bandwidth, and the NUT, but that is in the category of TLDR.

OG