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LVDS and RS-422 translation for low speed signal

Timothy McComish
Prodigy 10 points

Other Parts Discussed in Thread: MLVD20XEVM, SN65MLVD201, THVD1450, SN65LVDS050

Hi TI,

Newer engineer here, I have a 1PPS signal that will be provided via LVDS signal standard but is required to be sent to the receiver as a RS-422 compliant signal. Does TI have a part that would receive LVDS and redrive it as a RS-422 signal? 

I had originally thought about using a MLVD20XEVM and replacing the SN65MLVD201 with a THVD1450. They both appear to operate from a 3.3 Vdc bus.

Is there an alternative solution that already exists, any advice would be appreciated. 

For reference the signal parameters are extremely slow, 1 Pulse per Second with a total pulse duration of 25 us. As such the data conversion from LVDS to RS422 is purely a electrical parameter question.

  • 0
    TI__Guru* 81231 points

    Hi Timothy,

    Are you sending the signal in one direction? 

    Timothy McComish said:
    Does TI have a part that would receive LVDS and redrive it as a RS-422 signal? 

    RS-422/RS-485 and LVDS aren't usually paired together. Their common mode outputs aren't the same and the output magnitudes don't really mesh well together. 

    You could take an LVDS driver and then send it to an LVDS receiver then take the TTL output of the receiver and send it into an RS422/485 driver.

    Timothy McComish said:
    Is there an alternative solution that already exists, any advice would be appreciated. 

    Since your signal is going very slow, we probably don't need to worry about reflection differences with termination differences and data rate limitations wouldn't be a problem. 

    The output swing of an RS422/RS485 device on the differential line could swing upwards to 4.5V (2.5V common mode and +/- 2V swing) and LVDS devices generally use 3.3V FETs/BJTs which could break at 4V (example is SN65LVDS050 absolute max 4V on differential pins and same for the 201 device you referenced). So going from RS 422 driver into an LVDS receiver may damage the LVDS receiver. For that reason, I would suggest not trying to transmit from RS 422 into LVDS using normal termination....

    If you are going from an MLVDS (instead of LVDS for wider output swing) then if we assume 1.1V for common mode and 650mV peak to peak differential output then the peak would be 1.425V. For a device like THVD1450 the receiver threshold is -20mV max for noninverting and -200mV min for inverting. So you should be able to overcome these thresholds with the MLVDS driver. So going from MLVDS driver into RS485 receiver could be done. 

    Let me know if you have additional questions.

    -Bobby