RS-232 Signal Loss Detection

[oy1987]

Hello,

I want to build a device that would tap an RS-232 line between two machines, and would signal me if one of them is disconnected.

The first thing that came to mind was to use the RS-232 DCD (Data Carrier Detect), but I wonder how common is the use of this signal, since I have seen that it does not exist in every design.
I thought perhaps I could sample the voltage on the Rx and Tx lines and detect an invalid logic level (0V for example).

Are these a reliable sources for this purpose?
If not, any other ideas?

Thanks,
Or

 

[BradtheRad]

I don't know for sure, but I imagine a disconnected RS232 wire picks up mains hum. That is, unless the wire is connected to an output of a machine. The output is sinking or sourcing current.

The inputs, OTOH, are high impedance. They do not sink or source current. Suppose you monitor the wires for 60 cycle hum? Or their ability to source or sink current?

 

[oy1987]

THanks, but I'm afraid I did not understand the idea behind your solution.
Could you please elaborate?

 

[KlausST]

Hi,

DSR and DTR are used for this. (depending on what end of RS232 line)

"Carrier detect" is/was the carrier frequency of the modem/telephone connection. CD is mature somehow. RS232 is mature somehow... and still used also.

Klaus

 

[betwixt]

A far more reliable method it to use two monostables. Re-trigger one from each of TX and RX lines and set the timing period to be longer than the longest gap in normal data transmission. If one or the other 'time-out' it means no data passed along the wire in that direction so either the sending end is dead or is disconnected.

Brian.

 

[BradtheRad]

THanks, but I'm afraid I did not understand the idea behind your solution.
Could you please elaborate?

Sorry, I guess I don't have a working solution. I speculated that you could apply a signal to discover a wire's impedance, but this could confuse a genuine transmission unless done very carefully, and at the proper times.

As another option, there are devices called 'breakout boxes' which you can install in the cable. They have an led indicator for each wire. I have not used a breakout box but I think the most useful type should have different color led's to tell you the wire's state. By watching the led's I think you should be able to find out what happens when the cable is connected and disconnected.

 

[oy1987]

After looking at some different devices' specs, it seems almost none of them are using DSR and DTR (industrial end point devices, sensors and such).
So I think sampling the Rx and Tx lines is a good idea.
I'll try the timeout method.
Thanks