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?
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?