# Controlling pins: from pretend devices to Raspberry Pi

[Programming guides](README.md)

**GPIO** means *General Purpose Input/Output*: connections, called **pins**, that can read or send electrical signals. **Input** reads a signal; **output** sets one. **BCM numbering** names the GPIO signal, not the pin's position in the connector. Always check the board's [GPIO diagram](https://www.raspberrypi.com/documentation/computers/raspberry-pi.html#gpio).

In Pliro's interpreter and browser, pins are **virtual**: pretend connections with values remembered inside that run. They do not operate real wires. An explicit native Raspberry Pi build can operate physical GPIO. **Configure** means choose a pin's job before using it. **Logical state** is the program's true-or-false view of that pin; it is not a voltage you type as a Number.

GPIO is provided by built-in functions, so learning it does not require a different version of Pliro's grammar.

Pins are whole BCM GPIO numbers from 0 through 27, not physical header
positions. A pin must be configured before use. Reconfiguring a virtual pin,
or configuring a native output, resets its logical output state to `false`:

- `pinOutput(pin)` configures output mode and initializes a physical native
  output safely low;
- `pinInput(pin)` configures input mode;
- `writePin(pin, value)` requires output mode and a Boolean value;
- `readPin(pin)` returns the last successfully written logical value for a
  configured output, or samples the current level of a configured physical
  input.

The interpreter and browser use isolated virtual pin state. Virtual inputs
default to `false`. Reading a virtual output returns its last written value. A
Raspberry Pi ARM64 or ARMv7 native build uses Linux GPIO character-device access and BCM
numbering with the same program semantics.

```pliro
# language: en
# BCM 17, not physical header pin 17.
let ledPin = 17

pinOutput(ledPin)

for blink in [1, 2, 3]:
    writePin(ledPin, true)
    wait(0.25)
    writePin(ledPin, false)
    wait(0.25)

writePin(ledPin, false)
```

Follow the board manufacturer's electrical guidance and use appropriate
resistors, pull-ups or pull-downs, and safe voltage levels.

## Words used with real circuits

A **resistor** limits electric current. A **pull-up** or **pull-down** helps give an input a clear high or low state when nothing else drives it. An uncertain input is called **floating**. Follow the [manufacturer's circuit guidance](https://www.raspberrypi.com/documentation/computers/raspberry-pi.html#gpio) when moving from the virtual workbench to real components.

## Related entries

[Syntax and values](../syntax/README.md) · [Built-ins](../builtins/README.md) · [Programming guides](README.md)