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RS485 Why the limit of 32?

Discussion in 'Electronic Basics' started by Anthony Fremont, May 11, 2007.

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  1. I've implemented a few different serial protocols, but never set up an RS485
    network. I have several home automation do-dads that need to talk to each
    other from different parts of the house. I'm thinking of implementing a
    half-duplex 485 bus on an unused pair of the telephone cabling. Any
    non-obvious gotchas?

    I've noticed that many transceivers specify a maximum of 32 nodes on the
    network. Since they are tri-stateable, why is there a limit? Is it really
    just a somewhat arbitrary number based upon bus-loading? Is there some
    overlying comm protocol that I'm supposed to implement that can only address
    32 hosts? FWICT, it looks like I need only to fiddle with the enable lines
    and send/receive ordinary serial data (of my own format) to the transceiver.
    Am I missing something important here?
     
  2. Doug Miller

    Doug Miller Guest

    Just guessing, but perhaps the address bus is only five bits wide?
     
  3. Bus load is a part of the answer. As receivers should have an impedance
    no load to the bus as they are either sending or tri-stated. There's much
    more about RS485. Just Google for "RS485 specifications".

    petrus bitbyter
     
  4. James Beck

    James Beck Guest

    It is due to the reciever bus loading.
    There are fraction load parts that allow more recievers on the bus.
    Maxim, National, Sipex, and others have 1/8 load recievers allowing 256
    nodes.

    Jim
     
  5. James Beck

    James Beck Guest

    RS485 is an electrical spec, not a network spec.
    There is no address there that is part of the receiver/transmitter
    hardware.

    Jim
     
  6. Jamie

    Jamie Guest

    32 is just a round number derived from existing equipment that due to
    load on the pairs and driving abilities of the receivers pulling up the
    load..

    In may devices you get that operate on 485 for computer interface
    normally use some form of ASCII or base line fixed length data streams
    that uses the first part as the device ID.

    Modbus comes to mind.

    The Device ID's is just part of the message/data that the device
    operates on. ALl device's actually see all data that is taking place
    on the bus.
     
  7. Ok, thanks for the reply. :) I've found some info on implementng MODBUS.
    Doesn't seem terribly hard, but I don't think I'll really need even that
    much sophistication. I'll likely have a master that simply polls the remote
    slaves. Collisions should be quite infrequent if slaves only speak when
    spoken to. Thanks again. :)
     
  8. I saw some transceivers out there that allowed for man more than 32 nodes.
    Do you have any favorite cheapy parts?

    Thanks :)
     
  9. These page may help answer some of your questions;
    http://www.interfacebus.com/Design_Connector_RS485.html

    Yes you can increase the devices above 32.....

    http://www.interfacebus.com/
     
  10. That's what I was figuring.
    I probably won't have a need to connect anything that already implements a
    protocol, but it might be a good idea (and the experience wouldn't hurt) to
    go ahead and implement something like Modbus.

    Yep, that certainly confirms my suspicions. Seems too easy actually. ;-)
     
  11. James Beck

    James Beck Guest

    My favorite (read the most rugged) is the Maxim stuff.
    We use a full duplex system, so the MAX3080 series is what I like.
    The down side is that Maxim is notoriously slow getting production parts
    in the channel. I have had 12 weeks lead times (many times). We just
    buy enough at one whack to make it to the next production run, or we use
    a Linear Tech part that works pretty good too. A lot of times you can
    find a distributor that carries the Maxim parts in stock, but you will
    pay their mark up too.

    Jim
     
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