Unfortunately, there is not an industry standard for this. What people have suggested are all various ways I've seen it done too, even within the same company. They all have strengths and weaknesses to their approach to help people. The biggest issue with many of these is that, over time, the system falls apart as people don't understand it (because it's not well documented and people aren't trained, so they have to figure it out like you are doing), and maintenance activities usually drive changes that don't always get documented well. So, in my past company, one thing we did after working with our operations and maintenance folks, was to approach it differently. Instead of using some scheme to label wires that is captured on some cable/conduit schedules and have little to no meaning (there is good reason to approach it this way when designing a new system), we instead decided to use ISA standard equipment labels. For example, in your setup, you mentioned different pressure transmitters coming into the PLC. What we would simply do is label the wire with the instrument tag (i.e. PT1000) and the different wire designators (i.e. +/-). The overall cable here might have PT1000 on the cable on both ends, and a PT1000+, PT1000- on the 2 wires that go to/from this. In the drawings, this same label was used, and where it went to/from different drawings, we always cross-referenced with the FULL drawing number and line number to easily located each side as well. Then, in the PLC program and the SCADA System, this same instrument tag was also used. So, from the drawings (P&ID's, loop diagrams, etc...), to the physical cables, to the PLC and SCADA systems, this was all the SAME label. The best part about this is that when the technician was working an issue, we can look at the SCADA or PLC program and be talking about the exact same thing immediately. If we have a problem with a failed I/O channel, we just simply move it. It remains the same everywhere, and the drawing just needs to adjust the location it lands, not any labels associated. Imagine you did this with some label that uses a drawing/line number, and you move it. Now you need to relabel everything to match, or deal with it not following the convention because of it, which drives more confusion. I can say, it's not maybe the best, but the label and programs all match, and it's almost as if it was self-documented. It has led to fewer issues during support calls, and a quicker resolution time in most cases, especially when drawings aren't kept up to date (or if you have drawings at all). The only downfall of this is in design, where you need to know this information to properly complete drawings, and because a lot of them use CAD tools that autogenerate this, this method would require a lot of manual drafting time. From my experience, this has been the most widely accepted and easy to follow for engineers, operators, and technicians, even those with little to no experience.
