Interfacing RS-232 and RS-485 on the same line ?

[makif]

Hi,

I have only single port due to heavy pin number constraints and i have to place both RS-485 and RS-232 in the same pins. UART of my CPU will be connected a multiprotocol transceiver.

I plan to use Linear's LTC2870 multiprotocol transceiver. It allows switching between RS-232 and RS-485 modes. However, switching is done externally so that i should somehow sense whether incoming message is RS-485 or RS-232 message. Do you have any idea how could i sense whether incoming message is RS-485 or RS-232 message ?

My processor is TI's DM3730 Multi-Media Processor.

Regards,

Makif

 

[barry]

I SUPPOSE you could make an external circuit that would determine if an incoming message was RS-232 or 485, but by the time you've made that determination, you will have lost some data.

But maybe you can look at the idle state. Since RS-485 uses a minimum differential voltage of 1.5 volts and RS-232 uses single-ended +/-3V minimum, you can look at the input when it's idle and determine what's connected.

 

[makif]

Thanks for your great idea.

RS-485 will have following pins : TX+ TX- RX+, RX- and GND
RS-232 will have following pins : TX, RX, GND

Only TX, RX and GND will be common for both type of connections. So in RS-232 case TX- pin will be floating. Do you think it would be possible to use signal level between -TX and GND as operation mode controller ?

What i mean is that, if RS-485 produces idle stable voltage level on -TX pin, then i could use an optocoupler to produce necessary logic. When RS-232 is connected, then there wouldn't be any voltage on -TX and operation mode would have switch to RS-232.

 

[barry]

I would put a fail-safe termination on the RS-485 input, that way, the RX- will never be floating. The problem is that with RS-485 a transmitter is allowed to be tristated, so you might not be able to tell the difference between an RS-232 connection with nothing on RX-, and an RS-485 connection with the remote transmitter tristated.

However, if your system is more like RS-422 (where transmitters are ALWAYS active) this might work.