2-Wire Network Recommendations

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Thread Starter

nuatn

I am working on a project to upgrade several (100s) of existing 4-20mA system. All these systems are at different locations but use a similar topology:

- Star topology

- Up to 28 nodes (most have 16 or less)

- Each node is connected via 24AWG unshielded twisted pairs (just two wires)

- Wiring length to each node is a maximum of 120ft away from central connection hub

The project requirement forces us to use the existing wiring. We cannot pull new wires to the new devices. We cannot use a wireless interface.

Standard mains power (120 volt) is available at each node. There are only two conductors available for communication. There is no shield or ground wire -- just the two twisted wires connected to a central hub 120ft or less away.

The new devices provide bi-directional measurement and control. Each node will transmit and receive up to 4kb (4 kilobytes) of data at a time. We'd rather not have a master-slave system but allow each node to send data when it's available (asynchronous). Obviously that means there would be collision issues. Fast speed and multi-drop would be ideal [Ethernet on two conductors ?!?].

The new nodes are comprised of sensors, actuators, a single board computer and whatever communication interface we select.

We are looking at HART but afraid of the low speed and polling issues for fast response.

Remembering that there are only two wires (existing 4-20mA analog wiring) used for communication what other existing network system could you recommend?

Thank you.
 
That sounds like a mis-designed RS-485 network with 120ft stubs to a central point in a star topology. Google RS-485 hub and you'll find hardware solutions for RS-485 with long stub runs like this. Presumably whatever protocol you're going to use has some device addressing and can run on 485.
 
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Hi Nuatn,

For point to point half duplex RS-485 using unshielded twisted pair cable with 120 Ft length (36.57). it is still be able to get data rate of 4Kbps, but the problem is RS-485 is not designing to be working with star topology. Other challenge is you need peer to peer communication methods that your remote will initiate communication once get new data, that looks like can't be achieved by Modbus RTU or other protocol that run on half duplex/2-wire RS-485 physical layer. As long as I know there is no peer to peer communication protocol run over 2 wire RS-485 line, please correct me if I'm wrong.

If you want to retain existing 2 wire UTP cable, your best chance is to use master-slave based communication protocol like Modbus RTU or DNP. then since you need star topology, you should use RS-485 hub with 28 ports. but I have not been found Modbus RTU hub with port quantity up to 28, one example of Modbus RTU hub is below:

http://cel-mar.pl/en/hub_rs485rs422_m4044h.htm

Other methods is to use ModbusRTU to ModbusTCP converter for each individual 2 wire RS-485 line and then consolidate them into one standard Ethernet switch:

http://www.moxa.com/product/MGate_5105-MB-EIP_Series.htm
 
Thank you very much for your response.

>That sounds like a mis-designed RS-485 network with 120ft
>stubs to a central point in a star topology. Google RS-485
>hub and you'll find hardware solutions for RS-485 with long
>stub runs like this. Presumably whatever protocol you're
>going to use has some device addressing and can run on 485.

It is not a mis-designed RS485 network. It was a 4-20mA current loop network.

Yes, the new controller could run RS485 or whatever we decide.
However, there is only 2 wires going to each stub. Not two wires plus ground (three wires) -- just two wires is all we have to work with.
 
Hello,

Thank you for the information and links. Doesn't 2-wire rs485 require a ground wire also? Effectively, it requires 3-conductors going to each point.

Unfortunately, we only have 2-wires to work with.
 
> Unfortunately, we only have 2-wires to work with.

I believe RS485 compares the Voltage level between the A & B wires
If A > B its one state
If A < B it's the other state

Of course it's nice to have a ground and shielding but if it's not a noisy environment you may get away with it.

Perhaps study a project using RS485 to communicate to a number of electrical meters such as in large electrical switchboard. I have seen those wired daisy chain communicating with Modbus.
 
>10 Mbaud ethernet only uses 1 twisted pair IIRC.

Not true. 10BaseT Ethernet uses an 8-conductor CAT5 cable, and uses 4 of these conductors for transmit and receive. There is a tentative IEEE project to extend Ethernet for the industrial environment that is being planned to use a single twisted pair cable. I am a member of this study group.

To the original question, Foundation Fieldbus H1 was designed to replace 4-20 Ma on any old twisted pair cable and run to 1600 meters at 31.25 Kbps. The topology may be star or trunk and spur multidrop. Power is supplied on the same twisted pair when needed or may use locally powered devices. I am the chair of the ISA committee that created the Foundation Fieldbus standard.
 
Yes, RS-485 is technically 3 wire, the 3rd wire a signal common. It usually works OK on the bench with 1m cabling and no common mode between nodes, but 485 is not isolated like Ethernet is - NIC Ethernet cards used do transformer isolation with that little black 'magnetic' cube.

RS-485 hub ports might offer isolation on each port, which might make a 2 wire network viable.
 
Thank you for your response.

>To the original question, Foundation Fieldbus H1 was
>designed to replace 4-20 Ma on any old twisted pair cable
>and run to 1600 meters at 31.25 Kbps.

The equipment I have seen for Foundation Fieldbus H1 have the following connection [S, -, + ] is an optional Shield/Ground connection?

As we will be using Single Board Computers, they will need converters from Fieldbus to USB/Etherner/RS232, etc. However, I am having a difficult time finding those. Are these available?
 
For short cable runs, Foundation Fieldbus will work with unshielded twisted pair cable. You won't find that in the specs, but it was designed to run on "junk" cables.

There may be a commercial converter for H1 Fieldbus available from FINT in Norway: see this website - http://fint.no/

Also, a number of companies make a gateway device called a Foundation Fieldbus HSE Linking Device. It takes 2-4 Fieldbus H1 segments and makes them accessible on 100BaseT Ethernet. You can find Foundation Fieldbus Registered Linking Devices on this website:

http://www.fieldbus.org/index.php?o...Registered&ffbmfg=&ffbcategory=Linking+Device

DickCaro
Certified Automation Professional
 
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