Use of Ethernet Cable for Power and RS485

Discussion in 'General Electronics Chat' started by HallMark, Sep 29, 2013.

  1. HallMark

    Thread Starter Member

    Apr 3, 2011
    89
    5
    Hello Friends,

    I want to use Ethernet CAT 5e cable for Power transmission and RS485 transmission.
    But I am not sure this suits to my application or not.
    I have 5 different load which requires 20mA normally and 1A instantaneous.
    I need to transmit 5V via Ethernet Cable as load accepts 5V. If not possible to use 5V then I will go with 12V.
    Distance I need is 100meter. Ethernet Cable is ok for RS485 transmission over this range?

    CAT 5e seems better solution for this case or not?

    Thanks,
    HallMark
     
  2. panic mode

    Senior Member

    Oct 10, 2011
    1,320
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    i think it is a bad idea and judging by the number of other replies you got, nobody else is willing to encourage you on this. you need to do your own tests and prove that this works for your application.

    why not use proper cable? are you sure you need 485 and 422 would not work? with 422 you you can't have full duplex but you only need one twisted pair. you coulld also use cable such as DeviceNet (thin or thick) because it gives you twisted pair and power wires but it is only good for low baud rates (500kbps).

    R485 is a bus, it requires permanent end to end circuit (cable) with terminators at the bus ends. all connections need to be solid. if you break bus anywhere, all communication stops (even among devices that are still physically on a same bus segment). key advantage of bus is simple topology, simple hardware (just cable, no hub/switch), high speed (up to some 10-20Mbps) and reasonably long range (one meter to one or two km). with longer distance, baud rate must be reduced (long cables are large capacitive loads).

    cat5e cable is meant for point to point connection, short distance only (less than 100m). if disconnected, you loose one node but rest of the network still operates. this is possible because of additional hardware (hubs/switches) and different typologies. if you have loose connection (intermittent problems) everything will likely still work because ethernet is built around colission detection and retries - nothing stops you for implementing your own alghorythms to do the same on whatever network you use but have you even considered this?

    cat5e can be used up to 100m ethernet link. it has 4 twisted pairs so technically you should be able to use it for RS485 or RS422. if using 485, you need two twisted pairs so remaining 4 wires can be used for power. 485 cable should be shielded but twisted pairs of cat5e may work for some environments and baud rates. you can of check industrial ethernet cables (shielded) or POE cables, with much more secure connections than flimsy RJ45 but they are only 4-wire.

    also it is very important to use correct wires or you will get nowhere even on a short segment (you must observe how twisted pairs connect to RJ45!).

    devices requiring low voltage (5V included) require tight tolerance on supply voltage. considering that you have multiple nodes and large fluctuations in load, your only hope is to use higher DC voltage and step it down and regulate at each node. of course you should parallel power wires to allow more current since small crosssection of wires in ethernet cables.

    problem with linear regulators is that they only work well if voltage drop is close to optimal or if load current is sufficiently low (otherwise you need to consider bulky cooling options). if your loads require high current only briefly and rarely (low duty cycle), you can add capacitors to bridge the high demand periods. othrwise your only option would be to use higher DC supply and SMPS regulators.
     
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  3. HallMark

    Thread Starter Member

    Apr 3, 2011
    89
    5
    Hi Panic Mode,

    Thanks for detailed information. I will try to power load with two pair of CAT5E cable so it will decrease cable resistance little-bit.
     
  4. evilclem

    Member

    Dec 20, 2011
    118
    16
    We use 2 pair telephone cable for RS485 communication up to 1200 metres.

    You need to check the resistance of the cat 5 cable but the suggestion to use capacitors at each load and regulate remotely is a must (you'll lose a couple of volts in the cable). We trickle charge a battery pack at the end of our RS485 run so that peak loads are not a problem and the 25mA we put through the cable is sufficient on average to power our load.
     
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  5. HallMark

    Thread Starter Member

    Apr 3, 2011
    89
    5
    Hi Evilclem,

    1200meter is really long distance. You have used battery to each load so need not to transmit power via cable right? Or you have transmitted power via cable and also kept provision for battery?
     
  6. evilclem

    Member

    Dec 20, 2011
    118
    16
    The battery is a Ni-MH so we provided a trickle charge power through the cable. The battery is mainly used as just a large capacitor.
     
  7. HallMark

    Thread Starter Member

    Apr 3, 2011
    89
    5
    Used total 4 Telephone cable?
     
  8. THE_RB

    AAC Fanatic!

    Feb 11, 2008
    5,435
    1,305
    You can run power and coms over 100 m of Cat5 cable with no problems, provided your datarate is not needing to be too high.

    What datarate do you need, and is it single direction or in two directions?

    Power transmission of 20mA average is no problem either, and you can use a local cap on the receiving device to provide the 1A peak power, if it is short pulses.
     
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  9. HallMark

    Thread Starter Member

    Apr 3, 2011
    89
    5
    Baudrate will be 9600, Yes it will be in two direction as I will send command from mail PCB to device and they will reply based on command.
     
  10. colinb

    Active Member

    Jun 15, 2011
    351
    35
    Actually you have it backwards, RS-485 is one pair, half duplex. 422 is Two pair full duplex.

    Also it is rarely necessary to shield twisted pair RS-485 cable since common mode noise is canceled due to differential signaling.

    Cat5e cable is OK, you probably can get your desired amps out of a few pairs.

    However the characteristics impedance of cat5 is not the same as official 485 standard specifies. At shorter distances/baud rates you are probably OK. Note that you can lower baud rate to achieve longer distance.
     
  11. HallMark

    Thread Starter Member

    Apr 3, 2011
    89
    5
    I was checking about maximum RS485 device I can connect and there were confusion somewhere I see maximum 32 device is allowed on one bus while somewhere it says RS485 can support address maximum upto 255 to maximum 255 device can be connected.

    which one is correct figure?
     
  12. MrChips

    Moderator

    Oct 2, 2009
    12,432
    3,360
    Depends on which RS-485 device. All devices do not have the same properties.
     
  13. HallMark

    Thread Starter Member

    Apr 3, 2011
    89
    5
    I am using MAX485 chip and Multidrop connection.
     
  14. panic mode

    Senior Member

    Oct 10, 2011
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    read the datasheet, that's what it is for.
    if 32 nodes is not enough, replace MAX485 with MAX1487 for example. it still supports same baud rate but you can have 128 devices on same bus segment. using repeaters you can connect more segments.
     
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  15. HallMark

    Thread Starter Member

    Apr 3, 2011
    89
    5
    Thanks Panic mode,
    So this node limitation is depend upon chip.
     
  16. panic mode

    Senior Member

    Oct 10, 2011
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    that is correct, some support more nodes (256 for example). ultimately it comes down to how little power is used, how much load is on the bus etc.
    cable has capacitance, the longer the cable, the worse you are off (so for long cable you may have to lower the baud rate). the higher the baud rate, the more pronounced effects of capacitance and inductance of the cable. to charge capacitor faster, we need more current, but the transceiver ICs are low power and some also have limited slew rate (and that limits max baud rate). that is why some chips are good for 500kbps, others for 2.5mbps while some go to 12mpbs
     
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