I know, but as he appears not to want to help I'm just going to ignore it
however I think he believes all devices are of the same specification and therefore no matter what I do you could never achieve what I want, I did start this post by saying "The TTL rs232 bus is already used on another PCB" my device can be of any specification.
OK, there is an existing PCB with an existing working bus, using one line to TX/RX commands, this bus is "based" on the RS232 protocol in speed and timing only. the devices are connected to this bus using open collector transistors that I know for definite cannot withstand a pull up to 5v of just a few ma and I suspect (yet to check) a few tens on ma down to ground, I have tested the theory of what I want to do by simply placing the wire to +5v supply and GND in both cases all transmissions are blocked and after removing it comms continues without problem.
I want to mimic my manual placing to +5v supply and GND in time with my MCU pin thereby overriding any device talking on the bus and if a device is listening managing to get my command to it, after I issue my command I release the bus and listen for the reply.
Hope this makes things clearer.
P.S. I'll go look at these "data selectors" and thanks for continuing to try and help me