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ISO1432: RS422 data over long cables | Termination

BOOM
Prodigy 155 points
Part Number: ISO1432


Tool/software:

Monitoring data on RS422 for longer cable lengths

Till 10 metres of length, RS422 works fine with and without termination till 921600 baud rate

With ISO3086DWR for 100 metres cable, with termination of 100 ohm, data till 460800 baud rate is fine. Higher baud rate is not seen. Without termination, data upto 921600 is seen fine.

With ISO1432BDW for 100 metres cable, with termination of 100 ohm, data till 115200 baud rate is fine. Higher baud rate is not seen. Without termination, data upto 921600 is seen fine.

With ISO3086DWR for 100 metres cable, with termination of 120 ohm, data till 115200 baud rate is fine. Higher baud rate is not seen.

Cable type is Li2YC11Y from Thermo Cables 26 AWG.

Device is connected to PC serial utility and data is monitored. Connected using USB to RS422 converter. Termination is only on Rx pins.

Why isn't it working when termination is turned on?

  • 0
    TI__Mastermind 31365 points

    Hello BOOM, 

    ISO3086DWR and ISO1432BDW are both full-duplex transceivers and should be capable of transmitting at this speed over 100m. However, data rate will be effected by the PCB layout.

    • Can you please share the schematic?
    • And please include the waveforms of R, and the A and B pins. 
    • What does "Higher baud rate is not seen" mean? Is the wave form distorted or does the RS485 bus completely stop? 
    • Please also share the impedance of the 100m of cable that is used? 

    Best,
    Andrew

  • 0 in reply to Andrew J
    Prodigy 155 points

    Will update on the waveforms and cable impedance soon.

  • 0 in reply to BOOM
    TI__Mastermind 31365 points

    The schmatic looks okay to start, however, I do not see any termination used on the differential lines. I will await your replies to my previous questions for further review. 

  • 0 in reply to Andrew J
    Prodigy 155 points

     By “higher baud rate is not seen”, I mean that when the baud rate is set at 921600, the receiver does not recover valid data. The bus is still electrically active, but the captured data appears as corrupted/garbage characters on the terminal (not a complete stop of transmission).

    Below shared are tested on 10 metre cable with ISO1432BDW

    Transceiver:

    Surge protection:


    Termination selection circuit


    Waveforms:

    1. R


    CH1 : Blue : RX_P,
    CH2 : Sky Blue : RX_N


    2. Termination ON at baud rate 921600, not working

    3. Termination OFF, working at 921600

    4. Termination OFF and externally 120 ohm resistor connected on RX_P and RX_N lines, working at 921600

  • 0 in reply to BOOM
    TI__Mastermind 31365 points

    Hello BOOM, 

    Thank you for the added details. The above waveforms show valid RS-485 waveforms. The minimum differential load resistance between either A/B or X/Y pairs is 54 ohms. An impedance less than 54 ohms can lead to communication errors. Please check the cable impedance and measure the differential load resistance of each differential line. 

    Please also try to connect a 100ohm resistor externally like you have done with the 120ohm resistor and see if communication is acceptable. I suspect there is more than one 100ohm termination turned on on each line and that is causing a differential impedance of 50ohms thus effecting communication.