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EIA-485 driver and receiver enables

Marty McConnell
Prodigy 20 points
Other Parts Discussed in Thread: SN751177

Hi,

I've been looking through on-line material for an application note that ties the driver enable and receiver enable (negative active) together (for say a DS3695AT) which is then controlled by a uProcessor for half duplex interfaces, but have not found one.  I'd like to assume I can, but is there any timing constraint in doing so? I'm also assuming that for an idle condition one would leave the driver disabled and the receiver enabled.  Or for a full duplex device, such as a SN751177, can I tie them directly to VCC and GND and leave them enabled permanently?

Thanks.
Marty

  • 0
    TI__Mastermind 28265 points

    Marty, there can be time constraints if you stop transmission during a low level on the bus.

    Typically with RS-485 a UART protocol which starts with a low pulse (the START bit), followed by 8 data bits, and ending with one or two high bits (the STOP bits). In this case, when transmission stops, the bus previous output condition was high which is also present at the inputs of the disabled receiver. If the driver then becomes high-impedance, the bus voltage at the A/B lines droops down to 0 vols. The enabled receiver however has a much faster response time than the drooping signal and hence will also present a high at the receiver output R.

    If you would stop transmission during a low signal on the bus, then when the driver becomes high-impedance, the bus signal slowly returns to 0V. But during this rising to 0V, the fast response time of the receiver would be able to detect a short negative voltage with the receiver output turning low for a short time. Once the bus voltage has returned to 0V, then the receiver output goes high again.

    With regards to full-duplex transceivers, connecting the enabling pins to the supply rails is possible in a point-to-point connection only. In a multi-point connection however, you have one master driver connecting to many slave receivers, and many slave drivers connecting to one master receiver. Here all but one slave driver must be disabled to avoid bus contension.

    I hope this helps. Best regards, Thomas

  • 0 in reply to Thomas Kugelstadt
    Prodigy 20 points

    Thanks Thomas.

    So it can be done with the risk that switching to receiving prior to the transmission of the stop bit could cause a false start bit on the receiving side due to the fast enable time of the receiver.

    These interfaces are point-to-point and the data may well be intermittent, which makes this case easier.

    Thanks for the response.

    marty

  • 0 in reply to Marty McConnell
    TI__Mastermind 28265 points

    Marty,

    to avoid switching directions prior to the stop bit occurance, dedicated UART functions are used, either as spearate devices, or as integrated functions in the peripheral interface blocks of standard micro controllers.

    Regards, Thomas