A few comments on matters mentioned previously in different replies on
serial communication to 10 systems. I had an interest in a similar project
about twelve months ago. I did a lot of research on the topic, but the
project was subsequently abandoned due to circumstances beyond my control,
however the following information may be helpful to others  in their design.

My project  involved 10 systems interconnected on a multi-drop system
operating over a cable of total length 1000 metres. All systems had to be
capable of sending several bytes of data (including 10 address bits),  to be
received by all other systems, but only those address bit was set in the
data stream would respond. As all systems received all data sent, they could
keep track of  line busy or clear status. By imposing different timeout
period for each system  after a message was received, before transmitting,
the risk of collisions was reduced. I chose to use a 1200 baud modem and
connect all systems in parallel on a two wire circuit. The modems which I
chose had an automatic gain adjustment, which I expected would compensate
for any loss of signal caused by the line being loaded by 10 systems

My research also led me to RS485 as a possible alternative to RS232. It is
similar to RS232 except that it is a differential (i.e. balanced
line )system, operating at lower line voltage than RS232 and is designed
specifically for multi-drop applications, with up to 256 transceivers
connected across the line, and can operate in full duplex. When using RS232
with a PIC, the UART in the PIC is usually connected to a RS232 level
shifting chip such as MAX232, but by using a different level shifting chip,
it becomes a RS485 system. There is a wealth of applications information on
the MAXIM  (Maxim Integrated Products) website. The following application
notes are of particular interest ;-
APP723 ,APP763, APP367, APP736, APP123 and    APP1063 - address recognition
in RS485 systems.

Another  information source is WWW.hw.cz/english/docs/rs485/rs485.html
and for a PIC RS232 to RS485 converter see rat_ring@yahoogroups.com (I don't
have the actual url but it is there) You may also find other useful
information on the rat_ring site, It's for people interested in various
serial communication projects

In reply to the comments about oscillator frequency, if the master system
and all the other systems could be made to operate at the same frequency,
then it would not matter if they were all a little bit off the correct
frequency, as it is the actual clock frequency which determines the data
timing.

 Regards
   Harry

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