App note from Maxim Integrated on their parasitically powered digital input interface chip which are used favorably on industrial applications. Link here
A digital input (DI) is a circuit designed to receive a binary signal transmitted from an industrial sensor and translate that input into a reliable logic signal for a programmable logic controller (PLC) or industrial controller. Common examples of industrial binary signals are pushbuttons and/or temperature or proximity threshold indicators. The MAX22191 parasitically powered DI circuit can monitor Type 1 and Type 3 sink and source binary input signals for PLC and industrial circuits.