Control and signaling units
Everything that goes into and out of the PLC passes through here: the pushbutton panel that a person operates, the sensors that watch the process, the signal conditioners that make the signal readable and the actuators that carry out the commands. It is the part of the automation that gets touched, and where most failures occur.
The operating principles of each type of sensor and actuator—inductive, capacitive, photoelectric, encoders, contactors, solenoid valves—are developed in sensors and actuators. Here we work on their connection to the PLC: conditioning, wiring, signaling and assembly.
01Standardized control and signaling
| Color | Pushbutton | Pilot light |
|---|---|---|
| Red | Stop, emergency stop. | Fault or dangerous condition: requires immediate action. |
| Green | Run, start. | Normal condition, machine in service. |
| Yellow | Intervention, suppression of an abnormal condition. | Warning: something is close to its limit. |
| Blue | Mandatory action, such as a reset. | Indication of a required action. |
| White or black | Functions with no specific meaning. | Confirmation of a command with no further meaning. |
- Mushroom head, red on a yellow background, slam-action, latching and released by twisting or by key.
- Positive-opening contacts: if the contacts weld, the mechanism opens them anyway. It is not an ordinary pushbutton painted red.
- Wired in series with the actuator power supply, through a safety relay. It is not just another PLC input.
- The PLC reads the state of the chain to report and log, but it is not what protects.
- Deliberate reset: releasing the mushroom button must never make the machine start by itself.
02Signal conditioning
| Situation | What is inserted |
|---|---|
| Sensor with a millivolt output (load cell, thermocouple) | Transmitter that amplifies and converts to 4-20 mA next to the sensor, not in the panel. |
| 0-10 V signal over a long cable | Voltage-to-current converter, or simply a 4-20 mA transmitter. |
| Grounds at different potentials | Galvanic loop isolator. Prevents stray currents and erratic readings. |
| Noise from drives and motors | Shielded cable with the shield grounded at one end only, and a filter at the input. |
| Mains voltage signal to a digital input | 220 V input module, or an interposing relay. Never 220 V to a 24 V input. |
| PLC output to a large load | Interposing relay or contactor. The output commands, it does not supply power. |
A 4-20 mA transmitter powered from 24 V, with 300 m of 0.5 mm² cable out and back, and a 250 Ω input resistance in the PLC.
- Cable resistance: 2 × 0.0172 × 300/0.5 = 20.6 Ω.
- At 20 mA, the drop in the cable is 0.020 × 20.6 = 0.41 V.
- That drop does not alter the measurement: the current is the same around the whole loop. You only need to check that the voltage is enough for the transmitter: 24 V − 0.41 V − 5 V at the input leaves plenty.
- With a 0-10 V signal and the same cable resistance, on the other hand, the drop would add directly to the error.
03Wiring and assembly
- Every signal to a terminal block: never a field cable straight to the module. The terminal block allows disconnecting, measuring and replacing without touching the PLC.
- Labeling at both ends of every conductor, with the same reference that appears on the drawing.
- Separate ducts for power, control and analog or data signals. If they have to cross, let it be at a right angle.
- Crimped ferrules on every flexible conductor, with the right die.
- Spare terminals and space: something is always added later.
- One light per relevant condition, not one per output.
- Steady for a stable state; flashing for something that requires attention. The frequency also informs: faster means more urgent.
- Lamp test: a pushbutton that lights all of them, to verify that none is burned out.
- Horn or beacon when the operator may not be looking at the panel.
During assembly there are always changes: a terminal that moves, a signal that is added. If they are not noted at the time on the drawing, they are lost. A panel whose drawing does not match reality is worse than one without a drawing, because it wastes time and causes errors during intervention.
04In the lab
Assemble a pushbutton panel with run, stop, emergency stop mushroom button and signaling, respecting standardized colors and with the stop wired as NC. Verify that cutting the stop cable makes the machine stop, and that cutting the run cable simply keeps it from starting.
Wire the emergency stop through a safety relay, in series with the supply of the outputs, and as an information input to the PLC. Check that the program cannot start the machine with the emergency stop pressed, and that the stop happens even if the output is forced from the programming terminal.
Build a 4-20 mA loop with a transmitter and measure the current with the multimeter in series at different points: it is the same at all of them. Add cable resistance on purpose and verify that the reading does not change. Then cut the loop and check the detection below 4 mA.
Assemble a complete panel with DIN rail, separate ducts, terminal block, labeling and drawing. Submit it to a cross-review by another group, which must be able to locate any signal in under a minute using only the drawing.
05Common mistakes
| Mistake | Consequence |
|---|---|
| Stop wired as a NO contact | If the cable is cut, the machine can no longer be stopped and nobody notices until it is needed. |
| Emergency stop only as a PLC input | If the CPU fails, there is no stop. It must cut power by hardware. |
| Automatic reset when the mushroom button is released | The machine starts by itself with people inside the danger zone. |
| Analog signals next to the power wiring | Noisy readings that jump with every motor start. |
| Shield grounded at both ends | Current flows through the shield and injects noise instead of removing it. |
| Field cable straight to the module | Any intervention forces you to handle the PLC, and an external fault takes it out. |
| Unlabeled conductors | Every diagnosis starts by tracing cables by hand. |
| Outdated drawing | Worse than having no drawing: it misleads whoever works on it afterwards. |
06Self-assessment
Why is the stop pushbutton wired normally closed?
So that a broken cable has the same effect as pressing it: the machine stops. Wired as NO, a cut cable leaves the machine with no possibility of stopping.
What colors correspond to run, stop and warning?
Green for run, red for stop and emergency, yellow for warning or intervention.
What does it mean for a contact to be positive-opening?
That the mechanism separates the contacts even if they are welded. It is a requirement for safety devices, and what distinguishes a real emergency stop button from a red pushbutton.
Where is the emergency stop connected?
In series with the actuator power supply, through a safety relay. It is brought to the PLC only as information, for signaling and logging.
Why doesn’t the voltage drop in the cable affect a 4-20 mA loop?
Because the information travels in the current, which is the same around the whole loop. The drop only has to be taken into account to verify that the transmitter receives enough voltage.
When is a loop isolator needed?
When there is a potential difference between the grounds at the two ends, which would make stray currents flow and produce erratic readings.
Why does every field signal go through a terminal block?
So that you can disconnect, measure and replace without touching the PLC, and so that an external fault does not reach the module directly.
How are the ducts laid out inside the panel?
Separating power, control and analog or data signals. If they must cross, at a right angle to minimize coupling.
What is the difference between a steady and a flashing light?
A steady light indicates a stable state; a flashing one, something that requires attention. The faster the flashing, the more urgent.
When is the panel drawing updated?
At the moment the change is made, during assembly. Afterwards it does not get noted, and a drawing that does not match reality misleads.