Communication between 3 Allen Bradley PLCs

Hello everyone,

I have 3 identical processes controlled by 3 Allen Bradley PLCs (1769-L33ER CompactLogix 5370). All 3 share some identical sensors.

I need to start automatically 3 different motors, but 1 at a time. All 3 motors/plc share identical sensors, so right now if the right conditions are true, all 3 motors will try to start at the same time, but this is not what I want.

The process should behave like this:
If "Auto Mode" is on in one motor, this one will turn on and off depending on conditions.
If "Auto Mode" is on in motor 1 and motor 2, motor 1 has to start first and once it stops, the next one will be available so if conditions are again the right ones motor 2 should turn on and eventually turn off. After this, motor 1 becomes available again and so on.
This behavior would be the same for 3 motors with "Auto Mode" on. First motor 1, then motor 2 and eventually motor 3.

The problem I have is, how can I choose with what motor start if all 3 depend on the same conditions and besides that, they are controlled by independent PLCs?

I was thinking of creating a consumed/produced variable. So if variable is equal to 1, motor 1 is the one with the permission, once it stops this variable changes to 2 and it is consumed by plc 2 that now gives the permission to motor 2....and so on. But I'm not really sure if that would work.

Thanks in advance for any help you can provide.

Are the 3 PLC's communicating with each other

Your P/C tag idea would work but you need to READ from the other 2 PLC's as well. You have to have one of the PLC's be the master counter that knows when a motor has run and increments the variable that the motor run logic depends on.

Gotta say the arrangement sounds a bit off. There must be a reason the sensors are common but motor control is not?

When you say" share identical sensors" do you mean that the 3 separate plc's share the same sensor or each have the same type sensor.

Why cant it all run on one PLC is the redundancy to cover for a plc failure

You probably need to work on a couple more scenarios with regards to communication faults, but you could have one of the PLC's calculating the start conditions and send over the command to each individual PLC to start said pump.

In the event of communication fault you would either revert to manual, or have a hardwired signal to know whether the other pump is running.

I would also put your logic to decide which motor to run based on running hours, this way you can level usage.

roxusa said:

When you say" share identical sensors" do you mean that the 3 separate plc's share the same sensor or each have the same type sensor.

Click to expand...

I meant 3 separate plc's share the same sensor.
Sorry for the confusion.

TheWaterboy said:

Your P/C tag idea would work but you need to READ from the other 2 PLC's as well. You have to have one of the PLC's be the master counter that knows when a motor has run and increments the variable that the motor run logic depends on.

Gotta say the arrangement sounds a bit off. There must be a reason the sensors are common but motor control is not?

Click to expand...

I believe in case one plc fails, the other ones can fulfill the demand.

roxusa said:

Why cant it all run on one PLC is the redundancy to cover for a plc failure

Click to expand...

Yes sir.

So will a sensor failure take out all 3

That was my thought too. Redundant PLC's but a single sensor for all PLC's is shooting yourself in the foot. But alas we have to play the cards we are given.

Kinda sounds like homework not a real world scenario (three PLC's sharing three sensors). Can you show what you've done already?
Most of the people on this site won't help with homework unless the person doing the work shows that they are truly making an effort to learn what they are doing. That can actually be said for anyone posting but no one here wants to give a student a free ride.
If it's not homework, showing what you've done so far will still be a big help and I would suggest to the powers that be that the PLC's sharing common sensors is a bad idea. It runs the risk of one sensor bringing an entire system down.

does not sound like a real world problem. and if it is why would you do that.

What are the PLC's CompactLogix or ControlLogix
if so then put the sensors on remote I/O (ethernet)
have all 3 processors read the same remote I/O directly. you can have multiple processors read the I/O but only one can write to them (Master)
This way if one processor in off line the other 2 can still have access to the sensors

if it a smaller processor then you could do a round Robben A writes a word to B, B writes the received word to C and C writes the received word to A
Each processor will update the passed word with the sensor data
Down side if one processor fails all 3 are down you may just as well use 1 processor for all machines.