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SN55LBC180: Can SN55LBC180 be used for RS422 application

Mickey Zhang
Genius 12080 points
Part Number: SN55LBC180
Other Parts Discussed in Thread: SN65HVD1791, SN65HVD76

Hi team

Q1: Can SN55LBC180 be used for RS422 application?

Q2: How to verify if a RS485 transceiver can be compatible with  RS422? 

The attach is the screenshot for the SN65HVD1791 datasheet. My understanding is if the RS485 transceiver can support  the full duplex

communication, it will be support the RS422. If the RS485 transceiver can support  the half duplex communication,

it will be not support the RS422. Is my understanding correct? 

Q3. 

From this document, it shows the max signal rate is 10Mbps in the table 4.

The max signal rate  of SN65HVD76 is 50Mbps. If my above understanding is correct, SN65HVD76 will support RS422.

If SN65HVD76 is used for the RS422 application, then the max signal rate can only  be reached to10Mbps, correct?

If SN65HVD76 is used for the RS485 application, then the max signal rate can  be reached to 50Mbps, correct?

Best Wishes,

Mickey Zhang

Asia Customer Support Center

Texas Instruments


  • 0
    TI__Intellectual 1410 points

    Hi Mickey,

    thanks for reaching out. The RS485 standard is written to be electrically compatible with RS422 so to use the SN55LBC180 is usable in a RS422 application. A full duplex RS485 is usable as full dublex RS422 and a half duplex RS485 in half douplex Rs422. 

    /cfs-file/__key/communityserver-discussions-components-files/138/6180.slla070d.pdf

    Since The RS422 has the same limitations on data rate vs cable length as the RS485 standard the maximum speed is 10 Mbps for both. The RS485 is used broader and the demand for higher speed it is more common so manufactures start to increase the speeds. There are more RS458 devices than RS422 with speed beyond the standards definitions but it is possible with the RS422 as well.

    The data rate of a device is determined by tr(OD), tf(OD)Differential output rise and fall times. Shown at chapter 7.10 Switching Characteristics — 50 Mbps of the SN65HVD76 datasheet.

    A rule of thumb is take the max rise / fall time of the driver and divide by 0.3. You then invert this to get the data rate. For example for the SN65HVD76 transceiver has a maximum rise and fall time of 6 ns. The calculation would be 1/(6 ns/0.3 s). This result corresponds to 50 Mbps.

    https://e2e.ti.com/support/interface/industrial_interface/w/industrial_interface/2512.rs485-communication-speed-vs-bus-length

    By the way, you may want to consider a more modern device like THVD1510

    Let me know if this is hlping you out, don't hesitate to ask further questions

    Kind regards

    Dierk

  • 0 in reply to Dierk Bronner
    TI__Genius 12080 points
    Hi Dierk,

    Thanks for your reply. For the data rate question, I also want to verify it.

    E.g. SN65HVD76 can support 50 Mbps data rate. This mean SN65HVD76 can support 50 Mbps data rate for both

    Rs485 and Rs422 applictaion, correct?