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Looking for a Quad (or 2x Dual) Line Receiver...

Darren (FAE)
Expert 5483 points
Other Parts Discussed in Thread: SN75ALS193, DS34C86T, AM26C32, AM26LV32

I have really struggling searching through all the devices. 

I am in need of a Line Receiver (quad, or 2x dual) to handle receiving a total of 4x differential inputs, and output a single-ended H/L signal for each differential input.
(Basically 4x differential input line receivers)

The use-case will have 120Ω termination resistors. TI has a lot of devices that state "failsafe when outputs are open; Output = HIGH" but they are just 100k or 300k pullup/pulldown resistors on the input lines.
So when 120Ω termination is used, the VID is around 1~2mV for 5V supply voltage. But the devices usually needs VID >200mV in order to set the output HIGH.

The goal is to find a device that supports differential inputs where VID can be up to ±5V, and termination resistors can be used, while the device still allows for an open-condition at the inputs to result in the output being either HIGH or LOW.

It looks like one way to do this is to add 330Ω-or-so pullup/pulldown resistors to A and B, but the goal is not to have these external resistors on the lines.

Ignoring package size for now, are there any quad/dual devices that have failsafe outputs when input is "open", that still work when 120Ω termination resistors are used?

EDIT:

I was reviewing slyt514 and snla031 and it looks like the standard use-case is to have the low-Ω Fail-Safe biasing resistors on the Master/Host side (Tx), so the Rx side only needs the termination resistor.
I am understanding devices like SN75ALS193 have integrated 300kΩ biasing resistors for failsafe, which work when there is no termination resistor between A/B. But when there is termination, the resistors are too large, and we don't achieve >200mV needed to ensure output is HIGH. Correct?

Is there any literature on using failsafe biasing for both Tx (Host) and Rx (Device)?

Regards,

Darren 

Darren

  • 0
    Guru 316440 points

    Some datasheets (e.g., DS34C86T) admit that the failsafe is for "open, not terminated".

    However, there are devices with failsafe also for an idle or shorted bus. The AM26C32 datasheet says:

    A loss of input signal can be caused by an pen circuit caused by a wire break or the unintentional disconnection of a transceiver from the bus. The AM26C32 has an internal circuit that ensures functionality during an idle bus.

    The AM26LV32 datasheet says:

    The AM26LV32 device has an internal fail-safe circuitry which prevents the device from putting an unknown voltage signal at the receiver outputs. In the following three cases, a high-state is produced at the respective output:

    1. Open fail-safe: Unused input pins are left open. […]
    2. 100-Ω terminated fail-safe: Disconnected cables, drivers in high-impedance state, or powered-down drivers does not cause the AM26LV32 to malfunction. […]
    3. Shorted fail-safe: Fault conditions that short the differential input pairs together does not cause incorrect data at the outputs. […]
  • 0
    TI__Guru 74252 points

    Darren,

    I've notified an applications engineer of this thread and they will respond accordingly. Thank you for your patience.

    Regards,

    Eric Hackett

  • 0
    TI__Guru 53515 points

    Hi Darren,

    Many of the devices you reference feature internal fail-safe biasing which allows the receiver to maintain a known state even when the (terminated or unterminated) bus is left idle or floating. This is done by including series resistances on the input lines before the internal biasing paths to bring the voltage levels seen by the comparator outside of the indeterminant range. This allows the receiver itself to be fail-safe biased from an idle bus while presenting minimal common-mode loading to the bus. However, the resulting fail-safe only exists for this receiver, so the bus itself is not fail-safe biased and any node without this feature remains unbiased. This solution is ideal for point-to-point systems or in systems which only implement internally fail-safe biased transceivers. If the system is to contain non-internally fail-safe biased devices, an external fail-safe network can be included to bias the bus and subsequently all nodes. The below app note has some good figures and equations for this.
    RS-485 failsafe biasing: Old versus new transceivers

    The driver of an RS422 system does not need to be biased as it will either actively be driving the bus to a known state or be off (high impedance) and will thus not be affected by the bus state. 

    Let me know if this is clear and if you have any more questions.

    Regards,
    Eric Schott