Digital Outputs Of Instruments

Common discrete ("digital") outputs on industrial instruments include these types:

* RELAY: useful for switching either AC or DC signals, and even analog signals (e.g. 4-20 mA) if needed. Disadvantages include contact "bounce", sparking, and limited cycle life due to the presence of moving parts.

* TRANSISTOR: useful for switching DC signals only. Very fast switching and no "bounce" effect or potential for sparking as with relay contacts. If the load being driven by a transistor output is inductive, a commutating diode or other type of "snubbing" circuit needs to be connected in parallel with the inductive load to absorb the energy released by that load when it is de-energized. If this protective measure is not in place, the transistor may be destroyed by transient voltages. It is also very important to note what type of transistor is being used (NPN vs. PNP, N-channel vs. P-channel) in order to correctly connect the DC source to the transistor output channel.

* TRIAC: useful for switching AC signals only. No sparking as with relay contacts, and it also has the very nice feature of zero-crossing turn-off which means the TRIAC doesn't turn off until the AC sine wave current has naturally gone to zero.

If the discrete output type on an instrument doesn't suit the application (e.g. a TRIAC output which needs to control power to a DC load), you will need to install an "interposing" relay between the instrument and the load.

On occasion, one will find a 4-20mA signal used as a discrete output, with only two states available, 4mA or 20mA, no other mA values in-between. It's not a modulating output, it is discrete output.

An example is Siemens' CLS100 point level capacitance switch with dual discrete outputs, one a solid state switch output, the other a 'discrete' loop powered 4-20mA output:

It fits that somewhat rare circumstance where an AI is available but DI is not.