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DS89C21: Can I use this RS422 with tidu0033 (RS485) repeaters?

Yi Zen Lim
Prodigy 20 points
Part Number: DS89C21
Other Parts Discussed in Thread: TIDA-00333, THVD1510

Hi all,

I am designing an RS422 point-to-point link ring network (connecting STM32 uControllers) with DS89C21 Transceivers. The distance between the 2 nodes will be 2 km and I will be using UTP twisted pair. May I know if tidu0033 repeaters will work with the transceiver ? 

I am fairly new to comms, can anyone guide me on the bandwidth I should be looking at ? the data which I am going to transfer won't have anything to do with imaging so I suppose the data rate shouldn't be high. How can I determine that? 


Yi Zen

  • 0
    TI__Guru 62920 points

    Hello Yi Zen,

    TIDA-00333 uses extra circuitry for properly controlling the "enable" inputs of a half-duplex transceiver. (These transceivers use the same differential pair for transmitting and receiving data, so care needs to be taken in ensuring the transmitter portion of the transceiver is disabled when data needs to be received.) For a full-duplex transceiver like DS89C21 (with separate lines for transmitting and receiving), this extra circuitry is not needed and two transceivers can be directly connected (with the receiver output of one connected to the transmitter input of the other and vice-versa) to achieve a repeater.

    By the way, you may want to consider a more modern device like THVD1510. (Note that most RS-485 transceivers will support the RS-422 standard as well since the physical layer definitions are similar.)

    You can get a better idea what distances and data rates are achievable via this app note:

    Regards,

    Max

  • 0 in reply to Miguel Robertson
    Prodigy 20 points

    Hi Max,

    Thank you so much for your input ! :D
    I had a look at RS485 devices and I personally like them more than the RS422 devices since most of them have ESD protection and Fail-safing and not many RS422 devices have those features.

    From what I understand is that RS422 doesn't have DE RE pins, therefore reducing the complexity of implementation. My application was to implement a CDDI, so I imagine I can get  4 RS485 transceivers to imitate RS422, whereby 2 of them with always enabled Driver and two of them with always enabled Receiver. Or maybe just 2 full duplex transceivers, one for each ring.
    The problem is my application was just point-to-point links between nodes and I was asked the question, why not consider RS422? May I know if there is a particular benefit of using RS485 over RS422 in point-to-point link ? If not, could you recommend any good repeaters in RS422?
    Best Regards,
    Yi Zen

    P.S. The system diagram is as follows:
  • 0 in reply to Yi Zen Lim
    TI__Guru 62920 points
    Hi Yi Zen,

    For a point-to-point application, there probably isn't an advantage to using the RS-485 physical layer versus the RS-422 physical layer; either one should work. I just wanted to point out that RS-485 transceivers were an option as well. Since RS-485 is mostly a superset of RS-422 (it is similar but extended to cover multipoint applications with larger node counts), most newer devices are developed with RS-485 in mind but will support RS-422 as well due to similarities between the standards. In this case you should be able to pick a device based on whatever other characteristics you like (e.g., presence/absence of enable controls, internal ESD protection, etc.).

    Best regards,
    Max