Flexible I/O Connections

The Iron Pi has 8 flexible I/O connections on the main terminal block. Each I/O point offers 3 operating modes:

  • Analog Input

  • Digital Input

  • Digital Output

This simplified schematic illustrates each I/O point's key features, including an open collector output switch and a switchable 5.1K pull-up to 5 volts.

The pull-up resistor for each input can be switched in or out via the DIP switches on the bottom of the unit:

The switch numbers correspond to the I/O number, e.g. switch 1 controls the pull-up for I/O 1. When the switch is up / on, the pull-up is enabled, and when the switch is down /off, the pull-up is disabled.

Digital and Analog Inputs

In most cases, the pull-up resistor should be disabled when using an Iron Pi I/O point as an input. To read an input value, connect the source's signal ground to the Iron Pi's signal ground, and connect the signal to the Iron Pi's I/O terminal.

In analog mode, each input can measure between 0 and 10 volts. The input can tolerate up to 24 volts, but the measurement range is only 0-10V.

In digital mode, the input will read low below 1V and read high above 2.5V, which makes the Iron Pi's IOs compatible with 3.3V and 5V digital logic outputs. As with analog mode, the inputs can tolerate up to 24 volts in digital input mode.

Never apply a voltage above 24 volts or below 0 volts to the Iron Pi's I/O points.

Dry Contact Inputs

The Iron Pi's inputs can be configured to read the status of a dry contact switch. Since the dry contact switch cannot supply any voltage of its own, we can use the Iron Pi's pull-up resistor as a voltage source.

To enable the pull-up, move the corresponding DIP switch to the up / on position. In this picture, the pull-up for IO #1 is enabled, and the pull-ups for all other IOs are disabled:

Digital Outputs

The Iron Pi features open-collector style digital outputs, which function like a switch that connects the I/O terminal to ground. The benefit of open collector outputs is that they can switch loads at a variety of DC voltages, since the high side voltage is supplied externally.

The example below shows how to connect a low-current load directly to an Iron Pi's IO point.

The Iron Pi can only switch DC loads between 0 and 24 volts, with a maximum current of 0.5 amp. If you need to switch higher voltage or higher current loads, see Controlling a Relay.

Controlling a Relay

If you need to switch high voltage or high current loads, you can use the Iron Pi's IOs to control the coil / control input of a relay:

If you're driving a mechanical relay coil, you need a flyback diode in the circuit to prevent large voltage spikes at the Iron Pi's IO terminal when the relay is de-energized. Check whether your relay includes a flyback diode, or whether you need to connect one externally. See the Wikipedia article on flyback diodes for more info.

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