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SN65HVD75: RS-485 Buffer circuits for half duplex

jom sabu
Intellectual 370 points
Part Number: SN65HVD75

Hello ,

We are looking for rs485 buffer circuit, please find the attached image. Our requirement is that if any short or open of RS-485 on sensor side should not affect the RS-485 on BUS side(Right side).

Required speed is up to 10mbps.Could you please suggest any buffer solution for avoiding the issue?.Is it possible with rs485 repeater?

Regards

Jom

  • 0
    TI__Guru 62910 points
    Hi Jom,

    You may want to take a look at this reference design to see if the same approach could be used:

    www.ti.com/.../TIDA-01365

    It implements a repeater function that "fans out" a bidirectional bus from one master node to four slave nodes. The circuit could be adapted for a single slave node (i.e., only one slave transceiver and no "and" gate) in order to work as a single-channel repeater.

    A transmission rate of 10 Mbps should be achievable as long as it is supported by the transceiver IC, but one thing to be careful of is the time required to switch directions of data flow using this type of scheme. This circuit only starts to drive an output when a start bit is detected on a corresponding input, and this process of detecting a received bit, generating an enable pulse, and turning on the corresponding output driver can take some time. You may find that you need to extend the length of the starting bit in order to accommodate this.

    Another option would be to use an MCU to receive a message and then retransmit it to the corresponding bus. This would be more complex and add latency, but would work with a more standard RS-485 set-up.

    Regards,
    Max
  • 0 in reply to Max Robertson
    Intellectual 370 points
    Hello Max,

    Thanks for the response!

    is there any issue on data transmission due to the delay?. Can we reduce the delay if the 555 has been changed to high frequency?

    Should it affect the RS-485 on master if any short or open of RS-485 on slave side ?

    Regards,
    Jom
  • 0 in reply to Max Robertson
    Intellectual 370 points
    Hello Max,

    Thanks for the response!

    is there any issue on data transmission due to the delay?. Can we reduce the delay if the 555 has been changed to high frequency?

    Should it affect the RS-485 on master if any short or open of RS-485 on slave side ?

    Regards,
    Jom
  • 0 in reply to jom sabu
    TI__Guru 62910 points
    Hi Jom,

    Just a delay wouldn't impact communication (RS-485 can work over long cables that introduce significant delays), but if the initial "start" bit is shortened in duration then it may not be recognized by the receiving system. You would need to check the delay from a falling edge on the D signal to a rising edge on the 555 output (which is then fed to the transceiver's DE input), then add that to the enable time specification of the transceiver to see how much the start bit gets shortened. Ideally it would be much less than one bit period.

    Shorts/opens on the slave side would not impact the master. It of course would not be possible to communicate with that slave if the bus had these kinds of hardware issues, but communication on the master-side bus would be able to operate normally.

    Max