Instrumentation & PLC symbol combinations

Why not use an already existing standard?

There are plenty to choose from.

ISA 5.1 (2009 or later) is "the bible" for definitions and rules for P&ID instrumentation lettering.

I don't think you can put together a list that will not be modified by the instrument engineer. The representation of a PLC and other instrumentationon a P&ID will be custom determined for every project as depending on what is required, where PLC is located, who is the instrument engineer, and what PLC manufacturer is used.

I as a mechanical engineer prepared P&ID's with input from instrument engineer. I gave first shot based on copy of previous project P&ID's but in all cases the instrument engineer modified it for the specific project. For instance for a speed controlled VFD motor the P&ID would show the VFD with some instrument symbols inside and also some outside going to maybe a DCS or a PLC and also in-between and to the pump motor itself. With a VFD motor most of the control is buiilt into the VFD panel but auxiliary functions such as monitoring and changing setpoints, or interlocks with other equipment may be in a remote PLC or DCS. This is all project specific.

It may be benificial though to collect P&ID's of previous projects and to discuss up front with mechanical and instrument engineer what is the closest P&ID to the one they envision they will configure the system on this project to. I always started a P&ID by copying an existing P&ID from a previous project most closest to the one at hand, and modify rather than drawing from scratch which saved a lot of drafting time.

However some things are more simple like the instruments themselves. For instance there is always certain instruments associated with any instrumentation loop. For instance for level there is always a level sensor (LE) of many different possible types, a level tranmitter (LT or LIT) which transmits the sensor signal to a remote controller, and a contoller (LC or LIC) which recieves the signal and sends its own signal to operate the final control element such as a control valve (LCV). Each of these items will be represented by individual instrument bubbles on the P&ID. But when you get in to PLC and DCS or other special equipment displays they are all custom for project.