Devices and Inputs: Dry Contact / Open Collector / Open Drain

Digital Matter devices commonly have digital inputs. These are inputs are designed to read on or off. This article discusses connecting the DM inputs to external devices with dry contact style outputs.

Dry contact, open collector, and open drain outputs all refer to similar interfaces, although the implementation on the external device will differ. Effectively there are 2 states: high impedance (disconnected), and driven (connected, usually driven low).

In the example of a magnetic reed switch, consider this a dry contact. When off it is open circuit, and when on it is a closed circuit.

The easiest way to connect these inputs is to use the DM Bias Resistors - explained here.

Consider the example of the reed switch. Set the input on the DM device to "pull up" with the bias resistor. Then connect the reed switch between ground and the input. When open circuit, the pull up will read 1, and the logic can be inverted on the device or on the server to be "off". Then when the switch is closed, the input is pulled to ground and will read 0. Again, invert the logic on the device or the server to be "on".

Consider an open drain output from a PLC. Ensure the DM device and the PLC have common grounds. Set the DM input to pull up. When the PLC output is off (high impedance), the DM input will read 1. When the PLC input is on (driving low), the DM input will read 0.

To read about "active level" and getting an ON when the output is driving low - read this.

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