Electronic – Opto-coupled RS-232 receive circuit

opto-isolatorrs232surge-protection

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?

schematic

^{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

schematic

^{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..

Related Solutions

Electronic – RS-232 Buffer circuit

fwiw I've had success with the minimalist two-diode type circuit that you linked to. The only difference being that I used the TX pin of the 'sniffer' device as a handy source of -12V to use through a 47K pull-down to make sure the sniffer's own RX didn't float when neither of the snooped-upon devices was transmitting.

Unless you're driving tens of feet of wire, or are running at faster rates like 115.2kbps, the diode & resistor thing shouldn't affect the circuit too much.

If you really want to buffer the signals to TTL and back, there are of course the MAX chips, and even simple old line-driver/line-receiver type chips like the 1488 quad driver and 1489 quad receiver that would do the job.

Electronic – RS-232 switching

Perhaps the conceptually simplest approach to disconnect the TX line from the modem is with a phototransistor optoisolator:

            +-------------+
            |             |
DCD----R1-+-|-+      |/---|----TX of server  
          D | LED >> |    |
GND-------+-|-+      |\---|--+-RX of modem (often confusingly labeled "TX")
            |             |  |
            +-------------+  +--R2--pulled LO (idle)

There's a huge number of such devices available that would be more than adequate. A brief inspection of the selection charts at my favorite suppliers seems to indicate that the Isocom H11AA4X, Isocom TLP521-4, Avago 4N35-000E, Lite-On LTV-816, etc. all seem adequate.

There are many devices that may sound like what you want, but they won't work for your application:

optotriac or optoSCR won't work for you: these are designed to work with AC circuits, turning on when commanded to and then turning off at a later AC zero-crossing at the output. Many people try to use them in DC circuits, where they turn on just fine, but it is difficult to get them to turn off, so those people usually end up switching from SCRs to transistors.
RS232 optoisolators do something completely different.

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