Mixing RS232 signals (Rx and Tx)

[oy1987]

Multiplexing RS232 signal

Hello,

I have a system with two RS232 ports.
I wish to have the option to multiplex each of these signals.
When my system is ON, I want my signal to go through a microprocessor.
When my system is OFF, I want the signal to go form one end to another.
Which means I need sort of a relay that would hold one state when the system is on and another when it's off.
A colleague suggested a solid state relay.
Any suggestions?

Thanks

 

[zuisti]

Re: Multiplexing RS232 signal

Only an idea...

Apply a 5v DIP reed relay with morse contacts (n.c. and n.o, eg a HAMLIN type), powered with your system's 5v.

The solid-state relay is not appropriate in this case because of its high load capacity (for example).

 

[srizbf]

Re: Multiplexing RS232 signal

What is the state of power available , when you say "When my system is OFF".

Give a skeleton digram of your requirement for further forum members suggestions.

 

[oy1987]

Re: Multiplexing RS232 signal

Hi,
This is the basic idea I had in mind


My main supply is 5V, and when it is off I want S1 to close so the RS232 interfaces could still communicate.
In other words - when this unit is on the data should go through the microprocessor and when it is off it shouldn't effect the communication.

 

[betwixt]

Simplest is two 2-pole change over relays (or one 4-pole c/o relay). An unpowered RS232 driver might load the signals so it would be necessary to isolate it completely if the MCU was powered down. Connect the relay coil across the MCU supply so it operates the change over when power is applied or removed.

You can do it electronically but switching RS232 is more complicated than first appears because of the polarity reversal and relatively high voltages.

An alternative, which may be applicable (diagram isn't clear) is to leave the RS232 drivers connected to the DB9 ports so you always convert the data to logic levels then use logic gates to 'steer' the signal path. That will only work if the RS232 drivers are always powered and you do not show if they share the power source with the MCU or not.

Brian.

 

[oy1987]

I missed that in my diagram - the drivers and the MCU are part of the same unit and share their power supply. Looking at the unit from outside, one shouldn't care if it's on or off in terms of data flow. The data would flow from port A to port B either through the MCU or directly through S1.

 

[betwixt]

Ok, forget the logic idea, use small signalling relays, they can switch very rapidly and use little coil current. If you can confirm a powered down receiver (and maybe the transmitter) do not load the TX and RX wires when powered down, you can simply use the relay to link the two DB9 sockets together. If you do that, make sure you use normally closed contacts so they open when the power is applied.

Brian.