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THVD8000: TO increase the range of communication

TEMC
Intellectual 955 points
Part Number: THVD8000
Other Parts Discussed in Thread: THS6222, THVD1500

Tool/software:

Dear Sir,

I have Tested THVD8000-EVM's in lab for 1.2Km and found to be ok, in our case we use this for Communication only we don't want to transfer any power in this in our application, So in our prototype we are not connecting any L,C values and termination resistance is 120ohms but when are testing in site it's not working more than 300 in site and the cable resistance is 6.2ohm for 100meter but it's not working what are the hardware changes that we need to do in order work 1.5km and is there any need of THS6222IRGTR(line driver ) in order to increase range.



  • 0
    TI__Guru 74252 points

    TEMC,

    Please be aware that most of our experts are out of office for US holidays. Response will be delayed, and people will begin returning the week after next. All of that being said, we will try to get back to you by next week.

    Regards,

    Eric Hackett 

  • 0
    TI__Guru 52192 points

    Hi TEMC,

    I really apologize for the delay here. 

    What is your data rate and modulation frequency setting? 

    Also what cable is being used at site?

    Is there any jitter level that you need to be under in the application? 

    Do you have a scope shot of the receiver differential signal? If the system has a lot of noise present the THVD8000 could have some issues as it has a very sensitive receiver. 

    Generally speaking the THVD8000 can transmit about 1.5km (with up to 20% jitter) at modulation frequencies of 125KHz to 300KHz when using  20-AWG UL2464 power cables which I believe has a higher resistance of about 8.86 ohms per 100m - but since you are not using power signals standard RS-485 cabling is going to have about 1.82 ohms per 100 m (3105a Belden cable - ideal cable for RS-485 applications) which would be better because at large distances it usually is the DC resistance that is the limiting factor. 

    Please let me know - because depending on application setup 1500m may not be possible without some further modification - the modifications could be either a THVD8000 repeater (which is essentially just 2 THVD8000 connected together with some logic circuitry in between the THVD8000's on the repeater) or possibly using the THS6222 - but for the line driver you would most likely have to increase gain to get much farther distances - and we have only really tested applications at unity gain because we just needed a larger current output - but in this case you would most likely run into similar issues at unity gain.  

    Best,

    Parker Dodson

  • 0 in reply to Parker Dodson
    Intellectual 955 points

    Dear sir,

    Please find the below responses for your questions.

    What is your data rate and modulation frequency setting? 
    >> Data rate : 75kbps , modulation frequency - 750 KHz (R F_SET =  12.5 k ohm) --(for testing purpose)
    >> but, in our application there is a chance to set the baud rate to maximum limit (i.e;500 kbps)
     
    what cable is being used at site?
    >> Signalling multi core 1.5 sq. mm MAX CAB cable.
     
    Is there any jitter level that you need to be under in the application?
    >> In our application, the data transfer has highest priority (we prefer below 20-30 msec).
     
    Do you have a scope shot of the receiver differential signal?
    >> At present we didn't have such data.
     
    Yes, we want to use THS6222 Line driver to transfer the data for long distance.
  • 0 in reply to Parker Dodson
    Guru 316780 points

    To copy my answer from the other question: THS6222: Range extension for RS485:

    A common limit quoted for RS-485/RS-422 is 1200 m. This assumes 24AWG conductors and a maximum signal loss of 66 % with a 100 Ω termination.

    If you use full duplex signals (i.e., two differential signal pairs, one for each direction), you need a termination resistor only at the receiver. And at very low speeds, you do not need a termination resistor.

    If you use cables with a lower resistance (e.g., 22 or 20 AWG or even lower), you should be able to extend the distance. But note that it is unlikely that you will be able to reach speeds above 75 kbps at high distances.

    All this assumes normal RS-485 transceivers (e.g., THVD1500). Other devices are unlikely to help because what limits you are the distortions caused by the cable resistance/capacitance.

  • 0 in reply to TEMC
    TI__Guru 52192 points

    Hi TEMC,

    Thanks for the additional information - so with that being said - you will have some issue at that speed hitting that distance (even if you are limited to 500kHz) - I don't think you are really going to get much benefit from adding the THS6222. 

    You are using lower impedance cable than typical RS-485 cable, which does help mitigate DC losses and can increase distance - but with the modulation frequency you would also be hitting significant AC losses as well. 

    So with that in mind the best way to move forward hitting that distance is with a Power bus repeater  - we did some preliminary testing on the concept last year - and I have attached our findings to this thread. We did use the highest modulation frequency for testing as higher speeds were our concern - I don't see too much concern at 750kHz or 500kHz using something similar - but the timing elements will need to be modified for your specific system. 

    As a sneak peak - this is basically what you are looking at for a repeater - two THVD8000's and a control circuit in-between them to control the logic of the devices.

    Essentially the repeater control circuit is two circuits - one to control the mode of the THVD8000 and one to delay the data by one bit. 

    Since you are not using Power Line Communication features of this device you can ignore the power coupling bypass. 

    What happens is that both sides of the repeater are in RX mode until they receive a data frame at which time the opposite side of the repeater is changed to TX mode and then the frame is transmitted to the rest of the bus. The data delay is there so that bits are not dropped during the mode transition. 

    This solution does have down sides:

    1. It adds some propagation delay to system - so more repeaters more propagation delay is added - which should be taken into account for any control system. For example if one device in sub-system A and one device in sub-system B turn transmit at the same time - you may not see a collision right away and it could take longer than standard before the collision is seen. 

    2. You need to use a Frame based communication protocol - something like UART as the timing elements are based off the frame length - it doesn't have to be UART - but every transmission packet needs to be the same length and duration to properly size repeater - you could use an MCU instead of the logic circuit to increase flexibility - but it will also increase complexity

    3. It isn't as simple as a straight forward single bus - but at the speed and distance you are looking at it may be necessary. 

    PowerbusRepeater.docx

    Please let me know if you have any other questions!

    Best,

    Parker Dodson

  • 0 in reply to Clemens Ladisch
    TI__Guru 52192 points

    In general you are correct - but the THVD8000 doesn't have a standard RS-485 receiver (it is more sensitive generally and the THVD8000 isn't fully compliant with classical RS-485) and in many THVD8000 apps you can actually push the 1200m and termination isn't as necessary. In this specific case the 1.2km doesn't seem tenable.