# 6. Inputs, outputs and E-stop

## The isolated I/O

Two inputs and two outputs on optocouplers, with their own supply rails **HV_SEL** and **GND_SEL** on the terminal
blocks. You decide what feeds those rails:

=== "Isolated supply"
    An external supply's negative to GND_SEL, positive to HV_SEL. The I/O then runs at that voltage, 24 V nominal
    and at least 12 V for the inputs, and a short or a surge on a sensor cannot reach the robot's electronics.

    ![Isolated supply wiring](https://docs.source-robotics.com/PAROL-docs/1.1/assets/Ext_supply.webp){ width="650" }

=== "The robot's 24 V"
    Link the board's 24 V and GND terminals to HV_SEL and GND_SEL. The I/O shares the robot's supply, with a 2 A
    fuse protecting the outputs. Simpler, and enough for a pneumatic valve.

    ![Non-isolated wiring](https://docs.source-robotics.com/PAROL-docs/1.1/assets/Internal_24V.webp){ width="650" }

The E-stop is on the robot's own supply in both cases.

### Inputs

Between HV_SEL and IN1 or IN2: a switch, a push button, a relay contact, or the signal of a sensor fed from HV_SEL
and GND_SEL. 12 to 50 V. The firmware reads them every cycle and reports them in the I/O byte of the packet.

### Outputs

Between GND_SEL and OUT1 or OUT2: a relay coil, a valve, a lamp. 0.5 A continuous, 1 A peak, 48 V maximum, with
a flyback diode across any coil. The firmware sets them from the I/O byte of the PC's packet.

## The E-stop

Two connectors on the same input, for a normally closed contact. The firmware reads the input every cycle and
reports it in the I/O byte; the PC software stops the robot when it sees the contact open ([Emergency stop](https://docs.source-robotics.com/PAROL-docs/1.1/02-safety/emergency-stop/)).

![E-stop wiring](https://docs.source-robotics.com/PAROL-docs/1.1/assets/ESTOP_CON.webp){ width="800" }

- The contact must be **normally closed**: an open circuit means stop, so an unplugged button or a cut wire stops
  the robot too. A normally open button does not work.
- Both connectors share one microcontroller pin: a second button goes in series on the other connector.
- Any NC switch works for a bench setup. The BOM's 19 mm E-stop and the printed housing go on the base.

## The power button

A momentary push button with an LED, on the JST PH connector. A press switches the 24 V stage on; a press while
on switches it off. The LED is fed from the button connector. The firmware reads the button on every pass of
its main loop.