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THVD1420: RS485 transmission and reception wiring

Kevin
Prodigy 60 points
Part Number: THVD1420
Other Parts Discussed in Thread: THVD1451

Tool/software:

Hello.

I have a question about the wiring of THVD1420.

I want to use THDV1420 to send and receive signals from RS485.

Receive differential signals from RS485
↓
THDV1420
↓
Microcontroller USART RX IN
↓
Microcontroller USART TX OUT
↓
THDV1420
↓
Send differential signals to RS485

I tried drawing a wiring diagram, but I don't know if this wiring diagram will work properly. This time I will use two THDV1420 for receiving and transmitting, but is it okay to leave the D of the receiving THDV1420 floating? Also, I will not use the R of the transmitting THDV1420, but is it necessary to connect the 3.3V and R25 10K resistors?

I look forward to your reply.

  • +1
    Guru 303830 points

    Unuses outputs can be left open.

    The D pin has an internal pull-up resistor and can be left open.

    Using two unidirectional connections is typically called "RS-422". You are not showing any termination resistors; for RS-422, you would need one at the receiver.

    You can  combine both transceivers into a full-duplex one like the THVD1451.

  • 0 in reply to Clemens Ladisch
    Prodigy 60 points

    Thank you for your reply.

    So if I use the THDV1451, I can get by with just one transceiver. I'm not planning on switching between transmitting and receiving this time, so the THDV1451 would reduce the number of parts and might be suitable for my design. Thank you very much for your kind advice.

  • 0 in reply to Kevin
    TI__Guru 65130 points

    Hi Kevin,

    Everything Clemen's said is correct. 

    Be sure to include termination resistors on the receiver node (usually 100 ohms for your type of set up but it depends on the cable you use).

    THVD1451 is a faster datarate support device. So on the driver side, you may want to add a differential cap on the Y/Z pins to slow down the edges if you need it. (Atleast have it there as do not populate just in case).

    -Bobby