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SN75175: Can the SN75175 Quadruple RS422 Differential Line Receiver also be used for a single ended TTL ?

Louis Carli
Prodigy 20 points
Part Number: SN75175
Other Parts Discussed in Thread: AM26C32, AM26LV32

Hi,

We are looking at the SN75175 for a product that will receive either or a RS422 or single ended TTL (5V.) We will need to handle either option on the same input port/pins. It would need to be for 3 differential encoder inputs.(A,B,Z) Would using the 1A,2A,3A be able to receive a single ended TTL? 

Is there another Receiver option that may be more suitable for this? 

Best regards

LC

  • 0
    Prodigy 20 points

    Just to add .... the TTL inputs will be delivered pretty quickly, sometimes at speeds of around 1Mhz. 

  • 0
    Guru 298810 points

    Yes, a differential receiver works like a comparator and can detect whether a single-inded signal is above or below a reference voltage.

    More modern CMOS devices like the AM26C32 or AM26LV32(E) use less power.

  • 0
    TI__Guru 63530 points

    Hi LC,

    Just so I understand, you are planning on using the differential pins to connect to TTL logic instead of RS422? 

    As long as the differential voltage on these pins meet the threshold (V+ and V- in datasheet), the device should be able to correctly output on the TTL output side (Y pins).

    I'm also assuming for the TTL pins to sit on the differential pins, you aren't doing cable transmission and the communication is mainly on board. 

    If you have two voltages that are the same on the differential pins (like 5V on A and 5V on B) the differential is about 0 so the device may not know what the output is and the output may be determined by noise on the system. Usually these type of circuits use comparators or some sort of digital logic instead of using an RS-422 transceiver. 

    -Bobby

  • 0 in reply to BOBBY
    Prodigy 20 points

    HI Bobby,

    Thanks for you reply. We need to make the DAQ board so that it will use an external encoder (and cable transmission) for either RS422 or TTL. The user will decide what encoder signal type they will use. We will have to work out a way of switching between the two sets of resistors for each option. I was really hoping that there was an IC for this with so many requirements for it nowadays.

  • +1 in reply to Louis Carli
    TI__Guru 63530 points

    Generally TTL on a cable can get distorted due to the loading of the cabling. Depending on how long the cable is and the speed you're trying to transmit, TTL may or may not work. If the slew rate of your TTL is too fast, it may also introduce ringing/oscillations on the cable itself and potentially damage the TTL outputs. This is the reason why transceiver devices like RS232 and RS485 exist. 

    I would say this device can support you using TTL on its differential lines (the TTL outputs are more likely to break than our device's pins) and we should be able to support this as long as the V+/V- specs are being met. 

    The only other device we have that switches between RS485 and another protocol (RS232) are our multi-protocol devices. I don't necessarily think this is what you are looking for though.

    https://www.ti.com/interface/rs-232-rs-485-multiprotocol-transceivers/overview.html

    -Bobby

  • +1 in reply to BOBBY
    Prodigy 20 points

    For sure TTL is fragile in a noisy environment compared to a differential but we still have to give people the option. We are going to have a real hard look into what options we have using the RS-232 & RS-485 multiprotocol transceivers.

    Thanks again.