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SN65HVD75: RS422 and RS485 Difference

Kuan Wei Chen (Kevin Chen)
Genius 9710 points
Part Number: SN65HVD75
Other Parts Discussed in Thread: THVD1450

Dear Team,

Please help check if the design of the SN65HVD75 can be used for the RS-422 interface.

BTW, what is the difference between RS-422 and RS-485?

BR

Kevin

  • 0
    Guru 244270 points
    With RS-422, each differential signal pair is unidirectional; for full duplex, you'd need four lines.
    When you have more than one driver on the same bus, you have RS-485.

    This looks as if you copied figure 26 of the datasheet. If this circuit is at one of the ends of the cable, the termination resistor is missing (see figure 20).
  • 0
    TI__Mastermind 25705 points

    Kevin,

    Please refer to this app note for the comparison between RS-485 and RS-422. In the page 23, you can find a summary table (table 4). These two standards have minor difference. Like Clemens pointed out, RS-485 is a multiple-driver system, while RS-422 only allows one driver. Another obvious difference is the termination and Vod. RS-485 requires 1.5V voltage over 54Ohm load, while RS-422 needs 2V with 100Ohm. SN65HVD75 can meet this RS-422 standard with some conditions (Vcc>3.2V and Tj>0) (section 7.5 of datasheet). Even in case the device is short on this spec, it should still work in a RS-422 system. With this being said, if you want a RS-485 with a stronger driving capability, you can take a look at newly released THVD1450.

    Regards,

    Hao