AcePLP – How to Read Input and Output (I/O) Schematic Diagrams

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AcePLP- All you CADders out there, today you’re in for a treat! The team at CADPRO catches up with our guest author, AcePLP engineer Elson Lau, on his area of knowledge in reading schematics!

One of the projects that I worked on with Rockwell Automation Southeast Asia Pte Ltd was to design and draft out their Programmable Logic Controller (PLC) system cabinet for their Oil and Gas client from the Middle East. I learned a lot during this attachment, and here’s a short introduction to what I picked up along the way and how to interpret the Input and Output Schematic Diagram.


1. Before you start


When doing the Input and Output (I/O) schematic drawing, you need to know the components and the tag running numbers. In different drawings, different components have different tags when connecting either power supply or terminal block.


It requires skills, be it junior or senior engineers, to understand how the components are being laid during the planning and sketching of the schematic diagram. You should get the on-site engineers to demonstrate how the different components work, as a preparation to draft the actual diagram.


2. Tagging number


The tagging number indicates a cable, which is to be tagged at both ends. This is to ensure that engineers and sub-contractors identify this particular cable in a standardized way. The example below shows the parts of a tagging number, and Fig. 1 shows the way the tagging number should be laid on the cable in the drawing. Drawings also contain note explanations on the side of the title block (Fig. 2)

Example of the tagging number

Example of the tagging number


Tagging number on Cable

Fig. 1 Tagging number on Cable

Figure 2 - note explanation of tagging number

Fig. 2 Note Explanation of Tagging Number


3. Tag wiring connection


Whenever you connect one component to another, make sure that the tag wiring connection connects to the component with the correct tag. The connection also indicates the colour and wire size e.g. 1.0mm² Grey.

Layout of Wiring Connection Tab

Fig. 3 Layout of Wiring Connection Tab

Example of Wiring Connection Tag

Example of Wiring Connection Tag

4. Components connecting Field Termination Assembly (FTA) power supply


Some pages in the FTA power supply schematic diagram consist of parts from I/S Isolator and Relay when connecting. It is important that I/S Isolator and Relay are present to power up the FTA power supply.


Relay Connecting to the FTA Power Supply

Fig. 4 Relay Connecting to the FTA Power Supply


IS Connecting to FTA power Supply

Fig. 5 I/S Isolator Connecting to FTA power Supply


5. Termination Block Wire Connection


The Termination block connection has a different wiring system. Red represents the 6mm² RD (RED) wire towards the Power Distribution system. Cyan represents the 2.5mm² BK (BLACK) cable towards 0 Volt.


Termination Block Diagram

Fig. 6 Termination Block Diagram


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