Wouldn't easy and cheap be just using the RS232 existing?  Tie PC TX to all
instr's RX thru resistors, and similar with PC RX; then just do the typical
"don't speak unless spoken to" protocol you sketched.  Works fine for
simple things.

>A slight deviation from the original topic...
>
>I have about a dozen instruments with RS232C/D interfaces and one PC
>with one serial port. Is there an easy and cheap network that can be
>inserted between the PC and instruments to allow addressing and
>multidrop configuration. If not, i plan to implement a PIC based
>packet network to connect all instruments, configuration as follows:
>
>
>                        [RS484 Half or Full Duplex Backbone]
>PCSERIAL---NODEMASTER---------------------------------------
>           (ADDR  00)     |          |          |          |
>                        SLAVE      SLAVE      SLAVE      SLAVE
>                        (01)        (02)      (03)        (nn)
>                        RS232      RS232      RS232      RS232
>                      [INSRUM]    [INSTRUM]  [INSTRUM]  [INSTRUM]
>
>Rather than reinvent the wheel so to speak, can any one suggest a
>solution.
>
>Nino Benci
>
>
>******************************************************
>* Antonio (Nino) Benci                               *
>* Electronics & Computing Services Manager           *
>* Monash University - Dept of Physics                *
>* Wellington Rd, Clayton. 3168                       *
>* Victoria, Australia.                               *
>* TEL - 61 3 9905 3649, FAX - 61 3 9905 3637         *
>* EMAIL - nino.benci@sci.monash.edu.au               *
>* WWW - www.physics.monash.edu.au/services/ews.html   *
>******************************************************
>
>


Bruce Cannon
Style Management Systems
Remember: Electronics is changing your world...for good!