a 2 wire implementation

[ZZX]

HI all,

I was going to implement to wire communication link, and thought of RS-485 as an option but the common ground will add a 3rd wire in it. Can anybody suggest a pure 2 wire long (4000 ft or more) easy implementation.

Regards,
ZZX

[jbmiller]

You can use a version of what I've had for about 25 years. Single wire,using leased Bell lines, over 25 miles . Not as hard as you think. Simply put have the host pump data into a HV diff opamp feeding the 'commline'. Slaves listen for their address and data. If it's them, ground the commline with an HV transistor. It's up to you to create the format of the signalling.
In my case it's never been hacked,kinda hard when it's a 3 state,22bit data stream,interlaced,bidirectional system referenced to Mother Earth. Impervious to lightning,welders,etc. Using the second line from Bell gives a backup line and ability to isolate 'lost' remotes due to wires being cut.
jay

[dyeatman]

Why do you need a common ground? It's a differential signal and should not require a common ground.

I have more than 7300 feet of half duplex rs485 running here with just two wires...

[PCM programmer]

The RS-485 spec says in Figure 2 (on page 2):

[dyeatman]

Hi PCM,
I assume you're asking me that.

Yes, almost all my 19 transceiver locations have earth grounding, in part to help guard against static. These are variations of water pipe, electrical conduit or copper ground rod, power source ground etc. as required for the location.

I do have two locations without any type of true earth ground to speak of and, so far I have had no node comm failures on either of the two in the three years since installation. One is a node on the top of a 25 ft weather sensor mast where I expected nearby lightning strikes to be a problem for comms but none so far... To this node I am connecting four wires, two for DC and two for the rs485.

One other note, I use 9600 baud rather than a much higher frequency to minimize stray capacitance issues.

[ZZX]

So, you mean i "can" place my two transceivers on a 1 km twisted pair, with or without true individual grounds.

[dyeatman]

It should work. Make sure to have the proper termination resistors and wire twist.

I was very careful to get wire that met the RS485 "twists per foot" impedence requirements. I contacted my Belden distributor to get their recommendations on which wire to use. Telephone wire was definitely not acceptable but CAT 5 (what I am using) is an excellent choice.

Also, as PCM brought up, the earth ground for the circuits on either end, as indicated in the spec, will always help.

Figure 9 at the below link is the configuration I am using for my network.
http://www.maxim-ic.com/appnotes.cfm/appnote_number/763/

I am using the Maxim 1480/1490 15KV ESD rated, isolated drivers to protect my equipment.

All that being said, if you will need to run special wire for this link anyway, why are you limited to two wires? With four wires you can up the speed and use full duplex. Just curious...

[SherpaDoug]

The only reason you need a ground in an RS485 system is to make sure the differential signals are within the common mode range of the receivers. If a node is really hanging in mid air or somehow deprived of contact to an electrical ground you can put a resistive divider across the A & B lines and "ground" the node to the middle. When receiving the node ground will be within 2.5V of the transmitter ground so the spec is met. When transmitting the node drivers will overwhelm the resistor divider, so it does no harm. Make the divider resistance much higher than the line impedance (like 1K + 1K) so it does not cause impedance problems.
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