Bidirectional 3.3v to RS232 level translator

[xuio]

Hi fellow forum members,

I want to build an USB to RS232 converter with variable pinout. It will be part of a "smart" serial terminal server, which can adapt to any pinout and doesn't need any adapters.

To do this, I need a bidirectional level shifter to translate 0/3.3V to +12/-12V, which are needed for RS232.

I can't use a MAX232 with the TX and RX pins connected together (see attachment), because its output doesn't have a high impedance mode, and as a result always forces a state.

I also can't use pullup/pulldown resistors, because the converter has to be able to work with high baud rates and long cables.

A direction signal is available for each pin.

Do you have any idea, how I can solve this problem?
I can't wrap my head around this right now 

Thanks for your help,
Moritz

[oPossum]

Use a DG413 to switch between the RS232 transmitter and receiver.

http://www.intersil.com/content/dam/Intersil/documents/dg41/dg411-12-13.pdf

[xuio]

Use a DG413 to switch between the RS232 transmitter and receiver.

http://www.intersil.com/content/dam/Intersil/documents/dg41/dg411-12-13.pdf

Thanks! That should do the trick. 

[LukeW]

output doesn't have a high impedance mode, and as a result always forces a state.

MAX242 / ADM242 / equivalent may also be an option to address this.