I/O (inputs and outputs)

I/O (inputs/outputs) are the signals a controller uses to communicate with devices in an installation.

• Inputs are information that reaches the controller: “the temperature is 72°C”, “the tank is full”, “the valve is open”, “someone has pressed the emergency stop button”.
• Outputs are commands the controller sends: “open the valve”, “start the pump”, “set the agitator speed to 50%”.

Signals are further divided into:

• digital (discrete) signals, which have only two values: on/off, open/closed,
• analogue signals, which carry a continuous value, such as temperature or pressure.

Every sensor and every actuator accounts for at least one signal, and often several. A medium-sized process installation has hundreds of them, a large one thousands. The I/O count is one of the basic parameters used to assess the size and complexity of an automation system.

Example: Take a single pump feeding sugar syrup in a beverage plant. It may have:

• an output “start pump”,
• an input “pump running” (confirmation that it has actually started),
• an input “pump fault”,
• an analogue output “set speed” (a signal to the variable frequency drive),
• an analogue input “actual speed”.

That is already five signals for one device, and on top of that come the pressure sensors upstream and downstream of the pump. If an installation has 30 pumps, 150 valves and 100 sensors, it is easy to see where the hundreds and thousands of signals come from.