What about using (perhaps in modified form) a protocol such as
MODBUS RTU (maybe with relaxed timing specs, and different packet
definitions), with CRC's for the communication between the PIC and
the gateway? That can be implemented on RS-485 or other physical
layer. Keep the part in the PIC to *well* under 1K and very little
RAM (and a UART). Keep the buffering in the gateway.

Echelon had a quite well thought out system, could something be
"borrowed" from that?  Particularly the variable types and units
definitions for physical parameters?

Just some random thoughts, I am not deeply into this right now,
except for MODBUS.

Best regards,

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Spehro Pefhany --"it's the network..."            "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com
Contributions invited->The AVR-gcc FAQ is at: http://www.bluecollarlinux.com
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