Question re:PLC to PLC interaction.

Hello,

I'm starting out with PLC programming and had a question reLC to PLC interaction.

The situation is one where 1 PLC acts as a master and 5 other PLCs acting as slaves. (For the record they are Mitsubishi FX series)

I know that the master can write some data to appropriate registers on the slaves. My Question is "Can the master rewrite the actual program that runs on any of the slaves?"

I'm pretty sure the answer is no, but I have had a hard time tracking down this information.

Thanks in advance,

Jamie.

The terms 'master' and 'slave' generally refer to serial communications. The communications master is the entity that can initiate communications. Communications slaves can only respond to commands from the communications master. On any given master-slave network, there can only be one master at a time.

As far as their ladder logic and control of their I/O is concerned, PLCs are autonomous. Each PLC reads its own inputs and controls its own outputs. A 'master' PLC might send some data to a 'slave' PLC that can influence the ladder logic execution, but I've never run into a situation where one PLC modifies the ladder logic in another PLC. I think if I ever encountered a situation like that I'd walk (more likely run) away from it.

Hi Steve,

Thank you for the quick response. You have confirmed my suspicions reLC interactions.

The implication of your response is that any updating of the PLC programming (ie:Ladder Logic) must be done one a PLC at a time.

I have to admit that this will make my job much easier.

Thanks Again,

Jamie.

While I believe Steve is correct in that a 'master' PLC may not change the actual logic in a 'slave' PLC, don't dismiss this interaction altogether. If the 'slave' PLC's action is dependent on which product may be processed then the 'master' PLC can change the actual operation of the slave if it is allowed to change the data about the product to be processed. This type of interaction or coordination may be not only possible but even necessary for the automatic operation of a line.

I agree,

There were 2 circumstances where the PLC's were going to communicate.

1. The first circumstance was to allow the PLCs to interact or coordinate as Bernie suggested.

2. The second circumstance was to update the Ladder Logic Programming in the PLCs when a new revision of the Programming is released. (I now know that this has to be done one at a time)

Thanks for the feedback / distinction,

Jamie.

PS: I'll try to elaborate more on the situation next time I post a question.

PLC / PLC Interation

Hi Jamie,

We developed a system similar to what you are describing. We had three remote work centers reporting to a main PLC. All four PLC's were of equal capability: Mitsubishi A1S.

Communication occurred via designated registers. Each PLC had a unique group of registers, which could be used to influence the other PLC's. In the intrest of scan time, only push button and pilot light type infornmation was sent from one PLC to the others. Heavy duty calulations were performed locally.

The system worked well since the work centers were located about 75 meters apart. A two conductor cable was the only thing needed for communication.

I hope you find this information helpful.

Norbert