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AM26C31: Two drivers on the same RS422 bus, if one will always be powered down

Matt Child
Prodigy 100 points
Part Number: AM26C31
Other Parts Discussed in Thread: THVD1410, THVD1450

I am working on an application where we need to have two controllers which can control a single RS422 device, at the moment we are considering using an AM26C31 to drive the RS422 lines.  One controller is there for redundancy and will never be switched on whilst the other is turned on.  If one controller is faulty, it will be powered down then the second will be powered up.

I have read SLLA070D but it is still unclear if it is possible to have two masters electrically connected on the bus if only one will ever be active, and if it is possible where the termination resistors should go?

  • 0
    TI__Guru 62920 points

    Hi Matt,

    I can't think of an issue with this.  The relevant specifications to reference would be the driver output current when unpowered, IO(off), and the driver output current when disabled, IOZ.  These are both low enough that they shouldn't substantially load the active driver or degrade the differential signaling on the bus.

    Regarding termination - ideally this would be at the furthest end of the cable opposite the transmitter.  If most masters are placed in a similar location on your network then termination placement should be straightforward (equivalent to a single-master case), but if they are on opposite ends then it becomes a little trickier.  You may need to evaluate different placements to determine which one gives the best signal integrity.  If the master nodes are not placed at network end points, something you could consider would be to terminate at both ends but to double the resistance value.  This provides equivalent resistive loading to the driver (so that the differential voltage amplitude remains within spec) and trades off a minor degradation in impedance matching at one end for a major improvement at the other end.  You could also consider keeping the resistance values the same but supporting double termination via usage of a stronger driver.  RS-485 drivers, for instance, are designed to support double termination (due to the multi-point nature of that standard) and the THVD1450/THVD1410 specifically should meet RS-422 amplitude requirements with double termination when operated at VCC= 5 V.

    Please let us know if you have any further questions.

    Regards,
    Max