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AM26LS31: The difference between AM26LS31 and SN751178 in function

SHENG YANG
Expert 1230 points
Part Number: AM26LS31
Other Parts Discussed in Thread: SN751178

Tool/software:

HI,Team

There is a TIA/EIA-485-A difference between AM26LS31 and SN751178 in the first feature. What is the specific difference between TIA/EIA-422-B and TIA/EIA-485-A?
We currently have a demand to use SN751178. When we use it, we only use the output end and not the input end. So we want to use AM26LS31 instead of SN751178 to reduce costs Can AM26LS31 replace SN751178?

Thank you.

  • 0
    Guru 280000 points

    There are some small differences in the electrical characteristics, but for practical purposes, the main difference is that RS-422 uses unidirectional, point-to-point connections, while RS-485 uses a bidirectional, half-duplex bus.

    The AM26LS31 should work for your application.

  • 0 in reply to Clemens Ladisch
    Expert 1230 points

    HI. Ladisch

    As shown in the figure below, I only used the output channel of SN65HVD30DRG4, and the input is not used. Is it okay to use AM26LS31 instead?Regarding the first feature of SN751178, does it mean that SN751178 can be used in both 422 and 485 applications?

  • 0 in reply to SHENG YANG
    Guru 280000 points

    Yes. Yes.

  • 0
    TI__Guru 58860 points

    Hi Sheng,

    Clemens seemed to have answered the main differences between 485 and 422 but from a high level RS485 is for multi-point communication where it can flip between being a driver an receiver while RS422 is usually point to point with a dedicated driver and a dedicated receiver. The VoD (out put drive strength) of an RS485 driver is compatible with RS422 drivers (RS485 requires a minimum of 1.5V across a 54 ohm load while RS422 requires 2V across a 100 ohm load). The RS485 driver should be able to drive a 100 ohm load without issues. Rs485 can also support more devices on the bus (32 unit loads which can be more than 32 devices) while RS422 supports up to 10 receivers. 

    Generally, you can use an RS485 driver in place of a RS422 driver. 

    SHENG YANG said:
    Regarding the first feature of SN751178, does it mean that SN751178 can be used in both 422 and 485 applications?

    Correct (as Clemens has stated). the 1178 device can be used for either application.

    -Bobby