using two pairs of CAT6 for power

Discussion in 'Power Electronics' started by bug13, Jun 27, 2016.

  1. bug13

    Thread Starter Well-Known Member

    Feb 13, 2012
    1,208
    38
    Hi guys

    I want to use two pairs of CAT6 for power, the other pairs will be RS-485, distance will be 4000 foot (1200 meters) max. which way of the following is better? I am thinking method A, because they are tightly couple, but I am not sure.

    Would love to hear your opinion.
    IMG_1106.JPG
     
  2. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,507
    2,367
    My pick would definitely be A.
    Twisted pairs of this nature have self cancelling properties as to noise etc.
    Max.
     
    cmartinez likes this.
  3. bertus

    Administrator

    Apr 5, 2008
    15,645
    2,344
  4. bug13

    Thread Starter Well-Known Member

    Feb 13, 2012
    1,208
    38
    Interesting, the Alternative B of IEEE802.3af standard suggesting to use method B in my drawing. Anyone know any reason why?
     
  5. Papabravo

    Expert

    Feb 24, 2006
    10,136
    1,786
    What supply voltage will be available to anode at the extreme? You must computer the IR drop of 8000 feet of cable to get a handle on this number. Depending on the current you require it might be a great deal less than you are expecting.
     
  6. bug13

    Thread Starter Well-Known Member

    Feb 13, 2012
    1,208
    38
    That's a good question, I was paying too much attention until now. I don't know yet. I am thinking 24V or 48V. From what I can understand, there is no point using higher voltage if I am using leaner regulator at my end devices. As it won't reduce my current draw on the power lines.

    But if I use DC/DC converter, my end device will be more expensive. That's definitely something I need to thing about.

    Thanks for pointing this out Papabravo.

    PS: 1000 meters loop resistance is about 100ohms, if I am correct.
     
  7. eetech00

    Active Member

    Jun 8, 2013
    647
    110
    Cat6 is only guarantee'd 1000mb up to 100 meters. Probably lose signal integrity at 1200 meters..
     
  8. bug13

    Thread Starter Well-Known Member

    Feb 13, 2012
    1,208
    38
    I am planing to use RS-485 on a cat6 at 200kbps. I don't need a lot of speed, even 100kbps is a lot for my application.
     
  9. cmartinez

    AAC Fanatic!

    Jan 17, 2007
    3,555
    2,513
    What chipset will you be using? For example, the MAX490CPA is capable of up to 2.5Mbps, but the MAX3082CPA is capable of only 115Kbps, and I wouldn't be using either at more than 50% their stated speed at the length you've just described.
     
  10. bug13

    Thread Starter Well-Known Member

    Feb 13, 2012
    1,208
    38
    I am just researching at the moment, haven't look at any chip set yet. We only need about 2.4kbps max (in theory), so half of 115Kbps is still a lot. I am happy with anything that is more than 2.4Kbps.

    A cat6 have a impedance of about 100ohm, but RS-485 is designed for 120ohm, that's what I am looking at for now. And I will be looking into what protocol I can use next. In term of protocol, CSMA/CD is something I would like to focus on.

    What chip would you use if you were me? Reliable is very important.
     
  11. cmartinez

    AAC Fanatic!

    Jan 17, 2007
    3,555
    2,513
    Max knows a lot more than me about that question. But I'd probably use one of the fastest chips available, and then use it at the very slow speed you've just described. That ought to be reliable enough for your application. As for protocol, I'd include a CRC byte (preferably of the type X8 + X5 + X4 + 1) after each set of characters have been sent, plus timeout detection and error recovery routines.
     
  12. bug13

    Thread Starter Well-Known Member

    Feb 13, 2012
    1,208
    38
    Thanks
     
  13. cmartinez

    AAC Fanatic!

    Jan 17, 2007
    3,555
    2,513
    What are you going to use the line for? For control purposes, or only for monitoring?
     
  14. eetech00

    Active Member

    Jun 8, 2013
    647
    110
    Cat6 isn't designed to be used for such long runs. Twists will loosen and cause signal distortion..
     
  15. bug13

    Thread Starter Well-Known Member

    Feb 13, 2012
    1,208
    38
    I think it's a bit of both.

    • There are some buttons on each end devices/slaves, when one button is pressed, the device send a packet to the master. And the master do something with the data.
    • Or the master can send a command packet to an end device/slaves, the end device will flash some LEDs and/or make some sound.
    • There are different type end devices/slaves
    • All end deivces/slaves can only send data to the master, end device can not send data to another device
    • Master can send data to any end devices/slaves
     
  16. bug13

    Thread Starter Well-Known Member

    Feb 13, 2012
    1,208
    38
    All my devices will be daisy chain together, will that work? The bus is not a single lone wire of 1000m. But the total length of the bus will be 1000m max.
     
  17. cmartinez

    AAC Fanatic!

    Jan 17, 2007
    3,555
    2,513
    daisy chain should work, just make sure you use the right topology for RS485 when wiring, with proper termination resistors and all. Now I see that developing your own protocol will be a challenge. What I suggest, is have a single master continuously monitor the status of each device sequentially, and then have it transmit a single line of instructions that would be read and interpreted by the devices.
    Considering that, CRC is a must for your application.
     
    bug13 likes this.
  18. eetech00

    Active Member

    Jun 8, 2013
    647
    110
    Cat5 Cable designed for long runs is solid conductor and requires plugs or jacks designed for same. It can be used reliably up to 100 meters.
     
    bug13 likes this.
  19. bug13

    Thread Starter Well-Known Member

    Feb 13, 2012
    1,208
    38
    Using a master to poll each device sequentially is not a good idea for me. As my slave will be added and remove. I believe I still need a CSMA/CD protocol. I will properly don't want to develop my own protocol, but instead I will adapt to something we already have. I am sure someone have done this long before me, I just need to find it.

    And I use CRC16 in all my com, even in my TTL serial to my computer.
     
  20. Sensacell

    Well-Known Member

    Jun 19, 2012
    1,127
    266
    RS-485 at lower data rates can go much farther.
    Proper network topology and line terminations are key.


    Let's not confuse ethernet ratings with slow RS-485

    Use a chipset rated for the speed you are sending, the edge slew rates are set to minimize EMI. Going faster doesn't help.
     
    Last edited: Jun 28, 2016
Loading...