REPLY from CHENZHU
If the signal with the switch frequency between 10Hz and 50KHz, it is generally selected as transistor or TTL output.
Typical field instruments include: rotating speed switch, encoder, flowmeter high-speed instantaneous pulse, etc.;
Advantage of Transistor or TTL output: it is suitable for high speed signals without mechanical life limits.
Disadvantage of Transistor or TTL output: it’ll occur polarity with small drive current (less than 40mA);
If the signal with the switch frequency below 10Hz, it is generally selected as the relay output.
Typical field instruments include: valve position switch, liquid level, Flowmeter low speed unit pulse(one pulse per cubic), etc.
Advantage of the relay output is non-polarity, which can drive a large current (2-5A).
Disadvantage of the relay output is that it is only suitable for low speed.