How to mock 20km of 22 AWG twisted pair?

Discussion in 'Wireless & RF Design' started by evbro, Oct 6, 2014.

  1. evbro

    Thread Starter New Member

    Oct 6, 2014
    Hey everybody - So let me explain... (skip to the last 3 sentences for the question)

    I am designing on a SCADA system for a rural township water authority. There is a fresh water intake structure located down the road from the main plant, where the water from a creek is filtered and pumped to a 3m gallon tank for the town to drink etc. At the intake structure there will be a small PLC that will control the opening and closing of a motorized sluice gate based on the turbidity of the water from the creek. For example, if a bank falls into the creek and the water gets muddy, the PLC will read a high turbidity level and open the gate, which will reject the muddy water back in to the creek. When the water clears up the turbidity will drop and the gate will close, allowing the water back into the system for filtering and distribution. (If a gasoline tanker crashes in to the creek... let's not talk about that:eek:)

    The small rural telco will install a line to the remote site (intake structure) for a nominal fee. The length of the telephone line from the plant to the remote site is about 3000 m. Usually in these cases I would specify a 900 MHz ethernet radio to connect the remote PLC with the main system for purposes of monitoring and remote control, but there is a mountain blocking the radio path. The integrator (the contractor that will be building and programming the control system) wants to use 56k dial up modems at the remote site and at the main plant to communicate to the remote PLC (Allen Bradley Radkits). This will require the authority to pay for two phone lines, and just be lame. I have found an ethernet extender (Phoenix Contact 2313643) that will (supposedly) extend the network up to 20 km over a pair of 22 AWG. Its basically a private DSL network. This will work too since the telco is willing to lease the authority a "dry pair".

    The kicker is my boss (and the integrator) doesn't like to try new things and doesn't believe me. (And going through the cloud is a no no in this case.)

    Sooo... just for fun, I have two of the phoenix contact modems in my office, connected with paper clips, the LAN on one side and my PC on the other. It works great, I'm not experiencing any delays, downloading 5Mps. But I want to mock 20 km of 22 AWG by putting 1.1 k ohms of resistance in the lines. Will this adequately mimic 20 km of 22 (assuming 16.46 ohms/1000 feet, (20km = 65,167 t))? or do I need to also have some reactance on the lines too, to make a short length of wire "act" like 20km?
  2. Papabravo


    Feb 24, 2006
    Resistance is not the only thing you need to model. You need to be concerned about delay, signal distortion, and reflection. There is no good way to model 20 km of wire that I know of except 20 km of wire. I don't think 5 Mbps is reasonable under those conditions, but 56K with a V.35 differential transceiver might be. I would ask the telco for the use of the "dry pair" for a short time so you could do some BER and signal integrity testing.
    absf likes this.
  3. BR-549

    Distinguished Member

    Sep 22, 2013
    evbro, I don't think its a case of your boss not believing you. His decision is proper, fit, and with good engineering practice. You are not paying for it. You should back him on it.
    Your boss has a boss........the utility. The utility requires standard, well known systems that THEY want and understand.
    Your proposed link is valid also.
    However.....with any remote and control system....we need at least two dissimilar links, for redundancy.
    Especially with utilities.
    You should lobby for that rf link.

    A few suggestions.
    Install rf link.
    Install creek level indicator on remote plc.
    Install solar panel and battery for remote plc, control circuits, and radio. If the power goes out or if phone line can still monitor creek level and gate position.

    I have installed dozens of remote solar sites with a phone line, line of site radio and a satellite link.

    With packet radio........the receiver can put plc data on a web page for monitoring and control.
  4. alfacliff

    Well-Known Member

    Dec 13, 2013
    the specs for the wire should contain what you need. it should say the capacitance per meter, the resistance per meter, and other info to wourk out the model for the wire.
  5. KL7AJ

    AAC Fanatic!

    Nov 4, 2008

    Here ya go; no need to reinvent the wheel!
    absf likes this.
  6. Papabravo


    Feb 24, 2006
    When we were doing network development we actually went and bought a couple of 1000' spools of network cable. It was 5 conductor, with a data pair, a power pair, and a shield. We learned what we needed to from this and we never even took the cable off the wooden spools during the investigation. When we were done we sold it to the initial customers in any custom length they wanted until it was gone.