Electronic – Opto-coupled RS-232 receive circuit


I am looking into a implementing a low-cost RS-232/422 opto-coupled receive circuit. The aim is surge protection and tolerance to DC offsets.

The challenge is that the inputs should be completely floating and work over a wide temperature range, say -40°C to 85°C. Speed up to 4800 baud. Logic null input 1.6V – 25V.

I considered simply chaining a 5kOhm with the diode input but the resulting forward current is too small to produce a useful current transfer ration with inexpensive opto-couplers. (Especially at high temperatures.)

How does this solution look to everybody? Am I overlooking something?


simulate this circuit – Schematic created using CircuitLab

My understanding is that this should cut in abruptly at (Vin1-Vin2)=Vf of the optocoupler with a rise and fall time of about 10µs and be fairly tolerant to CTS degradation and voltage surges on the Vin side. – Is this right?

Is it likely to be a problem, that the input impedance is 10kOhm at high input voltages rather than the standard 5kOhm?

Edit 1


simulate this circuit

Best Answer

A 4800 baud rate is probably not achievable with this circuit- the Tf is typically 100usec and Toff is approaching 200usec.


Maybe if you terminate it into a resistor of a few hundred ohms and use a comparator..