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RS-485 arrangements

Discussion in 'Electronic Design' started by Richard Henry, Sep 17, 2007.

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  1. I have long been familiar with RS-485 wired as a single differential
    pair connected to all the devices and termination at the both ends,
    however "end" may be defined. I have recently been looking into a 4-
    wire "full-duplex" implementation. However, it seems to be full-
    duplex only if there are only two devices attached. Otherwise, there
    appears to be a need to designate one device as the master (the device
    to whose receiver all the other drivers are connected) and the rest as
    slaves.

    Unless there is some clever arrangement of which I am ignorant. Any
    comments?

    For reference:

    http://datasheets.maxim-ic.com/en/ds/MAX3483-MAX3491.pdf
    http://www.bb-elec.com/tech_articles/rs422_485_app_note/system_configuration.asp
     
  2. Steve

    Steve Guest

    Full duplex (FDX) RS 485 can be wired as point-to-point (typical use) or as
    point-to-multipoint (unusual but possible). If you use the
    point-to-multipoint configuration, you are correct that it looks effectively
    like the half duplex configuration, because you still need a master and
    slave(s) relationship. Replies from the various slaves have to be
    non-overlapping, and their output transceivers need to be controllable just
    like the 2 wire HDX configuration. So its hard to see any benefit to the
    extra wire pair in point-to-multipoint.

    A point-to-point FDX configuration is usually used when you need high
    throughput, or when you don't have access to the 485 transmitter enable
    signal. If you are sending command-reply type messages, FDX doesn't increase
    throughput because the two devices are alternating their messages anyway.
    Most FDX 485 configurations with which I am familiar are really FDX hardware
    with HDX software implementations.

    Steve
     
  3. David Starr

    David Starr Guest

    Basic rule for any kinda bus. Only ONE device can drive the bus at the
    same time. If two (or more) devices attempt to drive the same bus at
    the same time it's called a bus fight. The bus may go high or low or
    halfway inbetween, depending upon the strength of the bus drivers doing
    the fighting. Does not matter what kinda bus, you cannot allow two bus
    drivers to drive at the same time. The only common solution to bus
    sharing is a strict master-slave arrangement. If other solutions exist
    they ain't common.

    David Starr
     
  4. TT_Man

    TT_Man Guest

    Full duplex fully, overlapped is totally possible, but all the slaves have
    to have a 'system address'.
    A tip, design your system for the master to send, say, x bytes per packet,
    and all slaves to return, x-1 bytes per packet..
     
  5. Jamie

    Jamie Guest

    slave devices don't talk unless spoken to.
    this means, You need a protocol.
    MODBUS is a good example.. Look it up.
     
  6. TT_Man

    TT_Man Guest

    Far too complex...... just use for a packet,
    address/address/command/parameter/parameter/crc. The biggest task is
    defining the command/parameter set that will satisfy your application..
     
  7. Donald

    Donald Guest

    Re-inventing the master/slave protocol is far more complex.

    There are code examples of MODBUS out on the net that that OP can find
    and use.

    Maybe even use whatever MODBUS code thats out there and modify it to
    make it simpler for his application.

    Code reuse is a good thing.

    don
     
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