Questions on an RS-232 line receiver circuit

I have an application needing to optically isolate an Arduino from an RS-232 transmit line. I’ve found an Avago application note that recommends the circuit shown in the attachment as an RS-232 line receiver.

I’m curious about two aspects of the circuit:

  1. The circuit suggests that an antiparallel diode across the input is only necessary if the reverse voltage exceeds 15V. Yet every datasheet for the 6N139 lists 5V as the maximum permissible reverse voltage across the input. Would it really be ok to omit the diode if the line voltage only goes to -13V?

  2. The circuit includes two resistors described as optionally providing hysteresis. Do I want hysteresis? Is there some drawback to including hysteresis in the circuit?

The line I’ll be receiving from should go from +13V to -13V, 9600 baud.

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AFAIK most LEDs don't like reverse voltages >5volt.
I would say 15volt is wrong. Use the diode.
Leo..

I would say it is right because the 3K3 series resistor limits the current and hence the heating. I am a great believer in data sheets. However a diode can't do any harm and is less then a penny.

Do I want hysteresis?

Handy to have, it stops oscillation on slow edges.

Is there some drawback to including hysteresis in the circuit?

No.

The diode would increase the current when the line is idle, correct?

This is an application that hangs multiple receivers on a single RS-232 line (Lionel Trains TMCC devices), so it may be desirable to keep the load lower. But there isn't any specification for the transmit line I can rely on.

This is an application that hangs multiple receivers on a single RS-232 line (Lionel Trains TMCC devices), so it may be desirable to keep the load lower.

True, but there is no reason to use RS-232 for multiple devices, in violation of the RS-232 specifications.

RS-485 is intended for multiple devices.

Grumpy_Mike:
I would say it is right because the 3K3 series resistor limits the current and hence the heating. I am a great believer in data sheets. However a diode can't do any harm and is less then a penny.

Show me a datasheet where they specify the safe reverse breakdown current of a LED.
I have an old night light where there was AC on the LED through a CL resistor.
The LED failed (dim) every time after a year or so.
Since I have added a diode (and a new LED), the light is still working after ~5years (24/7).
I would say add that diode.
Leo..

I don't know about "safe reverse breakdown current", but reverse voltage is often specified, as 5V in here:
https://www.superbrightleds.com/moreinfo/through-hole/5mm-red-led-30-degree-viewing-angle-8000-mcd/281/1208/#/tab/Specifications

jremington:
True, but there is no reason to use RS-232 for multiple devices, in violation of the RS-232 specifications.

RS-485 is intended for multiple devices.

Alas, the Lionel engineers did not share your views.... :relaxed:

The additional current draw probably won't be a problem, but I'm still interested in learning about whether the protection is needed.

Show me a datasheet where they specify the safe reverse breakdown current of a LED.

On the note in the diagram that was posted. If it says it is safe then it is. The data sheet is almost like a legal contract, what is in it is what the manafacturer promises to deliver. If that note says it is true then it is. Simple as.

Sure you can add the diode and it will do no harm, might even do some good, but you don't need it.

Perfectly acceptable to have multiple RS-232 receivers and ONE transmit on a line. The TX will hold the line low unless transmitting. RX don't care. They don't have any way to activate the line.

The problems occur as soon as you try to put more than one transmitter on a line. Hard to switch the line to + when the other guys are holding it -.

Paul