The Optomux protocol avoids collisions by using a master that talks to 1
slave at a time or global broadcasts to all slaves, like time sync.  No
slave can take the bus unless the master tells it to.  If the slave fails to
respond within 200ms, that permission is revoked and the master will give it
to the next slave.

Several devices use variations of this protocol.  Johnson Controls' N2 bus,
for example.

http://www.opto22.com/Datasheets/0203_Optomux_Protocol_Manual.pdf


Barry

----- Original Message -----
From: "Edson Brusque" <ebrusque@TERRA.COM.BR>
To: <PICLIST@MITVMA.MIT.EDU>
Sent: Wednesday, February 13, 2002 9:02 PM
Subject: [PIC]: Simple (but not perfect) data collision avoidance in RS485


> Hello PICple,
>
>     as I'm doing communication between various devices in an half-duplex
> RS485 network, I'm thinking in a way to avoid data collision.
>
>     As I'm using the hardware USART of the PICs, the big danger is that
> while the PIC is receiving a data on the serial input, the software pull
> down the DRS (drive/receive select) line on the 75176 (driver) and put
> something on TXREG. It will do this because there's no way to know that
> there's an incoming byte on the RX pin until the byte is completely
> received. The RCIF Flag is set only when the byte have completely arrived
on
> the RX pin.
>
>     So, I think that I could tie the RX pin to the RB0/INT pin, so that
when
> an start bit arrives at the RX pin, pulling it LOW, an interrupt is
> generated. In this interrupt I set a bit variable (BUS_BUSY) to inhibit
> serial output. When I receive an RCIF interrupt, I reset BUS_BUSY, so the
> serial output routine could know it can send the data.
>
>     If the RB0 pin isn't available, we would pull the RX pin for about the
> duration of 10 bits to have confidence that we could start sending data.
>
>     Is this an standard way to implement data collision? Would this be
> reliable? What do you think.
>
>     Best regards,
>
>     Brusque
>
> -----------------------------------
> Edson Brusque
> Research and Development
> C.I.Tronics Lighting Designers Ltda
> Blumenau  -  SC  -  Brazil
> www.citronics.com.br
> Say NO to HTML mail
> -----------------------------------
>
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>

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