The signals with the switching frequency below 10Hz generally select relay output.Typical field instruments include: valve position switch, liquid level, flowmeter low speed unit pulse(one pulse per cubic), etc. The advantage of the relay output is non-polarity, which can drive a large current (eg. 2A). The disadvantage is that it is only suitable for low speed.

The signals with the switching frequency between 10Hz and 50Hz select transistor output.

Typical field instruments include: rotating speed switch, encoder, flowmeter

high-speed instantaneous pulse, etc.; 

Transistor output has the advantage is that it is suitable for high speed signals without mechanical life limits. The disadvantage of polarity, can only drive small current (less than 40mA);