I think that I'd try to put the heavy brainy side of TCP/IP maybe combined
with a small WWW and WAP server on a SBC running Linux with ether and/or
PPP capability and four serial ports and I'd implement TTL level serial
networks using MODBUS or something like that to the actual devices. Maybe
even I2C. Such a SBC starts at $150 or so and probably less in some qty.

The embedded web server would allow bidirectional control (read status and
interact) even using a cellular phone with WAP from pretty much anywhere,
and could implement enough security to keep any hackers at bay (even some
govt. agencies probably).

Peter

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