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TIDA-00333: Isolated Master and Isolated Slave using RS485

Part Number: TIDA-00333
Other Parts Discussed in Thread: ISO1176, , ISO3082, ISO1410

Hello, 

Will below block diagram work? I needed to connect the Slave PSU's using RS485 to the Master MCU, but it both PSU's/Slaves and MCU/Master need to be isolated from each other.

I'm already communicating with one of TI's FA but unfortunately he is on leave today and was advise to check in this forum. I needed an answer as soon as possible so I can start working on the finalizing my schematics. Let me know if there are other options to achieve fully isolated master/slave configuration.

Note: I needed to use HV opto in this case as part of the requirement.

Below are some questions:

  1. Will there be any issue using below RS485 repeater combination?
  2. Do I still need to control RE/DE separately from the our ECAT/MCU  board?
  3. Or I can just enable both RE and DE permanently since the ECAT/MCU  board is already aware that it in receive or transmit mode?
  4. We prefer not to have another RE/DE control for cost reduction purposes but wanted to confirm if there will be any complications in taking this it out.

  • Hi Norman,

    I see the block diagram is missing in your post but I have already received the block diagram from the TI FAE you are referring to. I see that you are trying to use an isolated RS-485 repeater to connect between PSU slaves and the ECAT MCU. Any specific reason why you want to use the repeater and not just an isolated RS-485 device like ISO1176 that you have used before?

    I believe you intend to achieve isolation not just between PSU and ECAT but also between the PSUs as well. While achieving this you probably do not want to control have any RE/DE and/or PV control and still allow connecting PSUs in parallel and then connected to ECAT MCU. If this is your need then an isolated RS-485 repeater is probably put to best use. If not then it could be an overkill which otherwise could have been achieved using your previous design with ISO1176 where RE/DE can be permanently enabled but PV needs to be controlled to allow only one slave connected to ECAT.

    Both of your ISO1176 based and TIDA-00333 based approaches should work fine. In case of TIDA-00333, you need to take extra care to make sure the additional external components are properly designed according to TIDA-00333 and the max datarate of the design is not exceeded.

    Could you please confirm what is the maximum datarate that you need to support on the RS-485 bus? Thanks.

    Regards,
    Koteshwar Rao

  • Hello,

    Sorry for that I thought the attachment went through, anyway please see below.

  • Please see reply in your questions:

    I see the block diagram is missing in your post but I have already received the block diagram from the TI FAE you are referring to. I see that you are trying to use an isolated RS-485 repeater to connect between PSU slaves and the ECAT MCU. Any specific reason why you want to use the repeater and not just an isolated RS-485 device like ISO1176 that you have used before?

    Norman: Our main intention to to tie all AB together but unfortunately we cannot share thesame gnd's for the isolated PSU's.That isolation  requirement is preventing us to easily tie all A/B together. Using repeaters, we can tie all AB together while maintaining the isolation between PSU's and ECAT

    I believe you intend to achieve isolation not just between PSU and ECAT but also between the PSUs as well.

    Norman: Yes that is our objective, maintain isolation between PSU's and ECAT.

    While achieving this you probably do not want to control have any RE/DE and/or PV control and still allow connecting PSUs in parallel and then connected to ECAT MCU. If this is your need then an isolated RS-485 repeater is probably put to best use.

    If not then it could be an overkill which otherwise could have been achieved using your previous design with ISO1176 where RE/DE can be permanently enabled but PV needs to be controlled to allow only one slave connected to ECAT.

    Norman: Just to be clarify  if my understanding is correct, if we use ISO1176, we will just need to control the PV pin/Chip enable pin which can serve as Mux control signal and we don't need to control the RE/DE to direct the Tx/Rx signals going to the slave device? 

    Both of your ISO1176 based and TIDA-00333 based approaches should work fine. In case of TIDA-00333, you need to take extra care to make sure the additional external components are properly designed according to TIDA-00333 and the max datarate of the design is not exceeded.

    Norman: Understood, we can follow the recommended ckt except for the bias ckt which will be different in our case.

     

    Could you please confirm what is the maximum datarate that you need to support on the RS-485 bus?

    Norman: According to our FW lady, we will use 115200bps in this case.

    Thanks.

  • Hi Norman,

    Thank you for your detailed response to all the questions I have listed out and also for re-sharing the block diagram, appreciate it.

    Norman Oliva said:
    Norman: Just to be clarify  if my understanding is correct, if we use ISO1176, we will just need to control the PV pin/Chip enable pin which can serve as Mux control signal and we don't need to control the RE/DE to direct the Tx/Rx signals going to the slave device? 

    Yes, your understanding is correct. Both and RE & DE can be left permanently enabled and control only the PV like a MUX to individually enable only one ISO1176 to establish communication between PSU and ECAT MCU. Enabling both RE and DE permanently is not a problem, this only makes 'R' receive the same data that is being sent from 'D' but it is not going to be a problem as MCU will monitor 'R' only after it completes sending data through 'D' and can ignore all the data that is being received on 'R' while transmitting data through 'D'.

    Norman Oliva said:
    Norman: According to our FW lady, we will use 115200bps in this case.

    The reference design TIDA-00333 supports datarate upto 100kbps as stated in its User Guide and also shown below hence, I believe this also meets your requirements.

    Regards,
    Koteshwar Rao

  • Hi Koteshwar,

    We are planning to use it for 115.2kbps, will this be an issue for a 100kbps repeater per TIDA-00333? seems like it is already beyond the recommended data rate?

    For the same repeater configuration, do I still need to control RE/DE separately from the our ECAT/MCU  board? I also sent some more schematics details to the FAE guy, I guess he will also inform you regarding that one.

    Related question:

    If we change  one PSU slave to be non-isolated, will I just take off its corresponding TIDA-00333 repeater and just tap the slaves A/B to the common bus of the remaining repeaters? do you see any issue in this config?

    Please advise.

    rgds,

    Norman

  • Hi Norman,

    Please see below my inputs, thanks.

    Norman Oliva said:
    We are planning to use it for 115.2kbps, will this be an issue for a 100kbps repeater per TIDA-00333? seems like it is already beyond the recommended data rate?

    I believe there should already be margins considered and 115.2kbps should be fine. If required you can slight adjust the RC filter at the inputs of two Schmitt triggers to further increase the speed upto 200kbps. I would also recommend using ISO1410 instead of ISO3082 as ISO1410 is our latest RS-485 isolator and offers better performance. It is also pin compatible to ISO3082 as well.

    Norman Oliva said:
    For the same repeater configuration, do I still need to control RE/DE separately from the our ECAT/MCU  board? I also sent some more schematics details to the FAE guy, I guess he will also inform you regarding that one.

    RE/DE are already hardwired and are inner side of the repeater hence, you do not have access to them to control. You only have access to A/B pins on both side of the repeater.. The repeater is designed to support data transmission in both direction with auto-direction control.

    Norman Oliva said:
    If we change  one PSU slave to be non-isolated, will I just take off its corresponding TIDA-00333 repeater and just tap the slaves A/B to the common bus of the remaining repeaters? do you see any issue in this config?

    No issue doing this; this should be absolutely fine. Not just one, you can take-off as many repeaters as possible and it would still work as all the A/B terminals are coming from valid RS-485 devices. The only difference would be that you will be losing isolation which you are already aware of.

    Regards,
    Koteshwar Rao

  • Hi Kateshwar, 

    Just wanted to clarify that the RE/DE i'm referring is from the master and not from the repeater, see below mark-up.

    Does the master still need to control the RE/DE pin of ISO3082 to direct the traffic within the ISO?

    Or I can permanently hard-wire RE/DE to be always enabled, will there be any issue?

  • Hi Norman,

    Thanks for the clarification. Controlling RE/DE pins is only for enabling or disabling the driver or receiver blocks and are not for changing direction. These are done to save power consumption when the device is not being used for data transmission. Hence, hard-wiring RE/DE pins to enable permanently doesn't have any impact on device operation.

    Keeping the driver enabled all the time does increase device power consumption significantly. Please refer to driver enabled (& bus loaded) and driver disabled (bus loaded or unloaded) condition current consumption in below table from ISO1410 datasheet. Load here refers to the terminations resistor RL = 54Ω which will be present on the RS-485 bus. Thanks.

    Regards,
    Koteshwar Rao