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Original question:

SN75ALS1178: Receiver output voltage level

THVD1451: The feasibility of the driver's bus output high/low levels and the high/low thresholds of the bus destination

Keisuke IKEMORI
Prodigy 10 points
Part Number: THVD1451

Tool/software:

Hello,

I plan to supply 5V to the Vcc of this transceiver and input a +2V to +3V signal to the A/B Bus input.

The High/Low thresholds of the devices connected to the bus are defined as follows, but since the transceiver's |VOD| is MIN. 1.5V, is it correct to understand that these threshold conditions are fully met?

Requirement: Since RS-422 diff erential input A is positive, and B is negative, if (A-B) is +200mV ormore, the output is High, and if (A-B) is -200mV or less, the output is Low

Thank you,

  • 0
    Guru 262460 points

    Correct. The RS-485 specification requires drivers to have a much higher differential output voltage to compensate for the resistance of long cables and to have a large noise margin.

  • +1
    TI__Guru 54350 points

    Hi Keisuke,

    Keisuke IKEMORI said:
    The High/Low thresholds of the devices connected to the bus are defined as follows, but since the transceiver's |VOD| is MIN. 1.5V, is it correct to understand that these threshold conditions are fully met?

    VoD is in reference to the output driver in regard to a specific resistive load. For RS485 the requirement is 1.5V minimum across a 54 ohm load while RS422 requires 2V across a 100 ohm load.

    Keisuke IKEMORI said:
    Requirement: Since RS-422 diff erential input A is positive, and B is negative, if (A-B) is +200mV ormore, the output is High, and if (A-B) is -200mV or less, the output is Low

    This is correct. You can see this in the Vth+ and Vth- of the datasheet. This particular device uses an internal fail safe so the Vth+ is actually slightly below GND and the Vth- is -200mV max.

    So the VoD of 1.5V minimum meets the Vinput_thresholds. This is still system dependent though, if you have really long cabling you could start to see DC losses across the cable. If your datarate is too fast with a long cable, the VoD in an AC sense could get attenuated at the end node (AC losses).

    -Bobby

  • 0 in reply to BOBBY
    Prodigy 10 points

    Thank you for your detailed answer.
    Since I intend to use it as RS-422 and the bus is connected to the customer's device, I need to check the load parameters, but I have understood the necessary elements by referring to the datasheet and your answer.