The way I use that transceiver is to allow only ONE master to issue
commands, and
everyone on the  network listens. Then the INTENDED slave (ONLY)
answers. So sometimes
a slave may not be interrogated for hours.

My communications protocol is simple: the first byte is the
DESTINATION (intended slave) ,
the next byte is the COMMAND, and data pertaining to that data
follows. To make sure the
slaves received the command correctly, there is a CHECKSUM byte (or
bytes) sent that represent a simple sum of all characters in the
message (except the checksum chars themselves). All slaves monitor the
checksum, and if they receive a bad checksum, they must IGNORE the
message because its data was corrupted. It might sound difficult, but
in fact it is very simple to implement. Even a better checksum scheme,
such as CRC8 or CRC16 can be used to improve reliability; with CRC16
you can send data on telephone lines for 1000 meters or more with very
few repeats(if the slave fails to respond, you REPEAT the command).

--Bob A




On Mon, May 11, 2009 at 2:12 PM, solarwind <x.solarwind.x@gmail.com> wrote:
> Hey all. I just got 10 samples of MAX485 (half duplex RS485
> transceiver) today. I want to try and make a simple multi master
> network using RS485 and PICs with hardware UARTs. Can anyone recommend
> where I can get started in terms of the protocol?
>
> 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. How do you check if the line
> is busy anyway?
>
> Also, the MAX485 has a receiver enable and driver enable pins. This is
> what the data sheet says:
>
> ---
> RE: Receiver Output Enable. RO [sends data INTO the PIC] is enabled
> when RE is low; RO is
> high impedance when RE is high.
>
> DE: Driver Output Enable [sends data OUT from the PIC]. The driver
> outputs, Y and Z, are enabled
> by bringing DE high. They are high impedance when DE is low. If
> the driver outputs are enabled, the parts function as line drivers.
> While they are high impedance, they function as line receivers if
> RE is low.
> ---
>
> So does this mean that the receiver and the driver should be kept in
> high impedance when I'm not doing anything? If it does, then how do I
> know when I should enable the receiver?
>
> -- [ solarwind ] -- http://solar-blogg.blogspot.com/
> --
> http://www.piclist.com PIC/SX FAQ & list archive
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