>The messaging will be very specific to each station, so new nodes
>will require a firmware change at the master, since it will need
>to be setup based on the type of node.

I once did technical support for a small business computer system called
Qantel, which had a polled RS232 network (they ran an open collector driver
with RS232 levels). It had a protocol that went something like this.

Poll by master <ETX><address><ENQ>

answer by slave if no message <EOT>
answer by slave if message available <status><checksum><ETX> where <ETX>,
<EOT>, <ACK>, <NAK> are the corresponding ASCII characters.

if slave replied with a message available then the master would send a poll
requesting the message. The slave would send the message, and the master
would then reply <ACK> or <NAK> depending on the state of the message as
determined by a checksum calculation. A similar loop was used to send a
message from the master to the slave. The status byte was used to determine
if the slave was busy, had a message to send, or there were also a couple of
flag bits that you could set directly from the keyboard. Worked real well.

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