How does 3 plc's communicate each other

As I said I'm very new for this environment. I have seen one of the line in my company has 3 different plc on a single line. slc 500, micrologix 1400 and control logix. I doesn't how do they communicate each other. I have been searching in ladder logic where they communicate and found nothing help neither in rslogix 500 nor in rslogix 5000. The sequence is that initial two steps are controlled by slc 500 and then 3 steps by micrologix and again few steps by slc 500. Lastly control logix is coming into picture. Can some one guide me how actually I have to identify the code.

I'm having hard time even understanding the ladder logic in rslogix5000

Thank you so much appreciated for yours reply

Rehu

Get out the prints. Maybe they're communicating via hard-wired signals.

Search the logic of all PLC programs for MSG instructions. Note the channel identified in the MSG instructions (assuming you find them) Note the IP addresses and/or node numbers used by the Ethernet channels (assuming that they are using ethernet for communication.)

Check the wiring first. How are your devices connected. Are they all on Ethernet network?

Look for MSG instructions in you logic files.
Look for produced/consumed type tags in your control logix program.

Hi Rehu,
As previous posters have suggested, it would be helpful to understand how the PLC's are physically connected. I'm not sure what your level of knowledge is, so please forgive me if this relpy is simply too basic. Commonly, PLC's are connected by some form of data bus, of which there are many. Ethernet is becoming the most common, but there are many. All your PLC's may not even be on the same network.
For some simple applications, you might just have the inputs of one PLC detecting the state of outputs on another PLC.
As other posters have said, look in the Logix 500 code for MSG instructions. These do not work on their own, however. There is a "setup screen" associated with each one (a button on the MSG instruction itself) which defines the target device and path, where the data to be sent is in the local controller and where it is being sent to in the target controller and the protocole being used to send it. All this information is stored in Message and Routing Information Files - look in the data files listing on your logix500 project.
In the logix 5000 project, there may be no indication at all in the ladder logic: It is possible that the logix500 message instructions write data directly to an array data structure set up in your controller tags. To see if this is happening, click on the "logic" tab and then select "Map PLC/SLC messages...". This will show you the name of the tag that is the target of the logix500 MSG instruction.
There are a lot of ways communication between these PLC's could be set up, depending on the protocol being used. A lot of this is guesswork without additional information, but hopefully will give you a couple of things to check.