Using a 485 bus, there are a variety of options for device to device
communications without going through a master device. Here are some
possibilities.

1. Use Aloha protocol where each device just transmits when it has
something to send. If the overall traffic is low, most of the time there
will be no collision. If missed data is not critical (the next sample is
fine), only error checking on the receive side is required. No retransmit
is required.

2. A master grants permission to each device to transmit in turn. This
avoids collisions, but fails when the master dies.

3. Slotted network. Each node is granted a time slot when it is allowed to
transmit. Each node has a timer that keeps track of which slot we're on.
Received data (including that destined elsewhere) synchronizes the slot
counter.

4. Mini-Slotted network. A sequence of short time slots (shorter than
required for a message, but long enough for a node to start transmitting),
are allocated, one to each node. As in slotted, each node has a timer and
a mini-slot counter. When the mini-slot counter matches the node number,
this node is allowed to start transmitting and does start transmitting if
it has anything to say. On detecting data, all other nodes stop their
mini-slot timers and hold off advancing the mini-slot counter until
mini-slot time after the last byte is received. Then, they all start
advancing the mini-slot counters again. As with a slot system, each node
synchronizes its mini-slot counter based on received traffic. I've also
called this an "absence of data token passing system." A lack of data
transmission passes the permission to transmit on to the next node. This
is the system I used using Bell 202 modems on voice grade wire and radio
circuits in the DRC190
(http://louise.hallikainen.org/BroadcastHistory/index.php/HallikainenAndFriends
)

Harold


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