Interfacing and wiring numbering PLC's

Robot IO to PLC IO interfacing.
I was at a company that use the following markings on there panels.
Robot Inputs - S1,S2,S3 etc
Robot Outputs - X1,X2,X3 etc

PlC Inputs - T1,T2,T3 etc
PLC Outputs - Y1,Y2,Y3 etc

I do see a pattern but I was taught to use PLC IO, X or I for inputs and Y or Q for outputs. What are the forums thinking? Is there an actual rule or could one number it anything.

Other question is a robot output say X1 is connected to a PLC input say T1 via a terminal. Then there is a wire number change is this correct? Their rule is outputs always win so a PLC output Y1 would be marked Y1 at the PLC and then also at the terminal Y1 one side and S1 other side but the robot output X1 would go straight to the PLC as X1 which confused everything.
Confusing yes but is there another way.

Also this could be applied to interfacing 2 PLC's together there must be some kind of wire number changing Is there a rule?

The short answer is no, there is no "rule" and no, there is no best way.

I am personally in favor of just about any system within a given facility that is consistently followed - and in my experience almost none are.

The one key to any system is that all wire numbers that attach to any given enclosure are unique. I say it that way because remote systems where communications are used to pass information from one enclosure to another (ie. Ethernet, Devicenet, etc.) can have repeating wire numbers that should not be confused with others.

My recommendation would be to select one device as the "central" device and wire your I/O based on it. So, if you choose the PLC as the central device, all wires between it and the Robot would be labeled X and Y (or I and O). On the Robot side you would then see wire numbers that are both X and Y based as well as S and T based for those devices unique to the robot and not directly part of the PLC system. If you choose the robot controller as the central device, the PLC would then have a mix of X/Y and S/T wire numbers.

In the end, IMHO, it really doesn't matter so long as you have unique wire numbers in each enclosure and everyone understands and follows the thought process.

Steve

I agree, the wire numbers in the enclosure must be unique. The plant wire numbers can be an extension of the enclosure wire numbers to for example a limit switch. Plant wire numbers between between enclosure can be double identified, (The cabling contractors hate this), where you have the start wire number and the destination wire number. ie start is 1234 and end is 4567 gives a plant cable of 1234/4567.