There is a standard for doing almost exactly what you are doing.  Check out
http://nwaprs.org/hsp.htm it uses the DTR line to switch between the GPS
and TNC data.  If you only need to read data from the GPS and not send
commands to the GPS you may be able to use this design.  You can always use
a PIC to control the switching.

-Dale


At 07:21 PM 4/28/2003 -0500, you wrote:
>Hi all,
>
>Here again with aquestion about how to read 2 devices with a single RS232
>Port on my laptop.
>The idea is to read data from a GPS unit (DB-9 "A" connector) and keep
>conected a TNC unit (DB-9 "B" connector) via the RS232 COM port of my laptop
>(DB-9 "C" connector).
>I4m not sure if a couple of CD4066 Quad bilateral switch ( one for the "A"
>and one for the "B" connectors ) will be enough in speed to switch the
>incoming and outgoing data across the COM port.
>Using a PIC16F84A-20 I can sense the DCD from the GPS "A" connector and
>enable the the CD4066 to switch on the communication between the PC and the
>GPS, then keep off both CD4066 when no DCD is detected, same procedure with
>the DCD from the COM port when the outgoing signals must flow to the TNC and
>no to the GPS.
>
>Any idea or suggestions are wellcome.
>
>Regards,
>
>Augusto, OA4CVT
>
>
>******                                       ******
>* GPS*                                       *TNC*
>******                                       ******
>      *                                                *
>      *                                                *
>      *         **********                   *
>      ****** SWITCH ************
>                 *    BOX    *
>                 **********
>                          *
>                          *
>                          *
>                   *********
>                   * COM    *
>                   * PORT   *
>                   *********
>
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