On Tue, May 12, 2009 at 7:55 AM, Gerhard Fiedler
<lists@connectionbrazil.com> wrote:
> solarwind wrote:
>
>> I need a protocol driver for PICs or at least some information on how
>> to implement my own. I have a general idea on how to send data
>> between PICs but the largest concern is collision. I don't know how
>> to implement collision detection/avoidance.
>
> I don't remember who that was, but on one of the recent threads about
> this issue that you started, someone posted a list of 4 methods which
> seemed reasonably complete. Find that message before you continue...
>

This one from Harold Hallikainen?

On Sun, May 10, 2009 at 4:34 AM, Harold Hallikainen
<harold@hallikainen.org> wrote:
> 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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