Learn Automation with MINATA #31: The Connection Diagram and the Terminal Schedule — Turning a Drawing into a Correct Panel

The Connection Diagram and the Terminal Schedule: Turning a Drawing into a Correct Panel

The control circuit diagram tells you how the logic works. The connection diagram and the terminal schedule answer a more practical question: which wire runs from where to where, what signal it carries, and at which point the installer is supposed to check it. This is the bridge between the design drawing, the panel, the machine out on the floor and the maintenance team.

A panel can run perfectly on the day it is installed and still become a burden if the wiring follows no rule. When a sensor is replaced, the maintenance technician should not have to trace each wire by hand and guess. The aim of a good connection diagram is simple: remove a device, put it back on the right point, verify it by the wire number, and break nothing else in the process.

What you will learn

The difference between the schematic, the connection diagram and the terminal schedule.
How to follow one line from a field device back into the panel.
How to build a terminal schedule that makes installation and acceptance verifiable rather than assumed.

Three layers of information that must not be mixed

The schematic explains the switching conditions — the Start–Stop–KM1 arrangement from #30, for example.
The connection diagram identifies both ends of a wire: the wire number, the cross-section or cable, and the terminal it passes through.
The terminal schedule lists the terminals in order, with their purpose, signal, location and the matching point out in the field.

A schematic can show that sensor B1 feeds input X0 on the PLC. The connection diagram adds the rest: B1 brown to +24V, blue to 0V, black to TB2-07, and from TB2-07 to X0. The terminal schedule turns TB2-07 into a line that can be checked during installation.

How to trace one signal properly

Pick a device and read in one consistent direction: device → cable → terminal → wire inside the panel → I/O or switchgear. For a three-wire PNP sensor you have to check the supply as well as the output, not only the signal wire:

Confirm the device tag on the machine — B1, for instance.
Check the supply type and the pin colours against the datasheet of that specific device.
Follow the cable through the connector or terminal; record the core number and the terminal number exactly.
Follow it from the terminal to the PLC module; confirm that the COM of that input group has the same polarity.
Test continuity with the power off, then measure voltage with power on, following the safety procedure.

The parts most often forgotten are the common 0V, the shield and the cable-gland earth. A signal that looks "already wired to X0" but has no 0V reference will never behave stably.

What a terminal schedule needs to contain

Each line should carry at least: the terminal tag, the pole number, the signal label, the signal type or voltage, the device outside the panel, the point inside the panel, and an installation note. For example:

Terminal	Signal	Outside the panel	Inside the panel	Note
TB2-07	B1_OUT	Sensor B1, black wire	PLC X0	24VDC input
TB2-08	B1_0V	Sensor B1, blue wire	0VDC	Common supply
TB2-09	B1_24V	Sensor B1, brown wire	+24VDC	Through the group protection

Do not use the terminal schedule to paper over a vague schematic. The device tag, the signal name and the terminal number have to agree across every drawing. Changing one name in one place creates a fault for the next person who maintains the machine.

A worked engineering situation

For a conveyor station with a photoelectric sensor, a solenoid valve and a motor, the tidy approach is to separate the terminals into groups: 24VDC supply, inputs, outputs, and shield/PE. The wires leaving for the machine pass through terminals, while the short wires inside the panel run to the PLC, the interposing relays or the power supply. When the sensor has to be replaced, the technician works only within the right TB group and never touches the logic side of the panel.

This is a reference principle. Pole counts, cross-sections, wire colour standards, protection ratings and supply-separation rules must be fixed against the machine documentation, the applicable standards and the datasheets of the actual devices.

Common mistakes

Writing only "sensor 1" instead of a consistent device and signal tag.
Running field wires straight into the PLC without a terminal to test and replace at.
Sharing a terminal block between power wiring and control signals.
Leaving COM, 0V or the shield out of the terminal schedule.
Numbering the wires after installation, so the drawing and the panel no longer agree.

Checklist before the panel is closed

[ ] Every field wire has two ends that can be identified on the drawing.
[ ] Wire number, terminal, device and PLC tag all match.
[ ] Supply, 0V and PE/shield are shown separately and follow a clear rule.
[ ] I/O, supply and power groups are separated so work can be done safely.
[ ] Continuity, polarity and wire labels are checked before commissioning.

A good connection diagram does not make the machine any smarter, but it makes installation, fault finding and expansion far less risky. That is part of what MINATA means by engineering quality: the person who comes after you can still read it, check it and hand it over.

Read more automation knowledge at MINATA: https://minatavn.com/en/blog/industrial-automation

Previous — #30: The control circuit diagram: reading and building the switching logic: https://minatavn.com/en/blog/automation-30-control-circuit-diagram

Next — #32: PLC basics and the scan cycle: https://minatavn.com/en/blog/automation-32-plc-basics-scan-cycle

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