Signal + power wiring

Estragon · June 2019

I'm wiring some sensors to a controller ~100ft distant. The sensors each have a ground/negative, signal, and +5v power wire, which are wired to three local (3 feet +/-) busses. From the three local busses, I plan to run the 100' or so to the controller using cat 5 ethernet cable. What I'm not sure of is how to use the twisted pairs.

With 4 pairs, it makes sense to parallel wire for less voltage drop. The varying twists in pairs minimize data corruption. The power part is minimal (milliamp), to maintain state in sensors.

I'm thinking pair 1 could be sig/neg, pair 2 sig/neg, pair 3 power/neg, pair 4 power/neg, with sig, power, and neg recombined at the controller.

Does this make sense? Is there a better way?

BB. · June 2019

Should be OK... Using a voltage drop calculator for 100 feet, 24 AWG and 2 milliamp, you get 0.01 volt drop.

What is the signalling? RS422 or RS485?

Cat5 with 100 Ohm termination? One or both ends (RS422 vs RS485)?

I always like to bury a PVC or ABS pipe of (relatively) large diameter so I can pull more/different cable later without having to rebury the new pull.

Running a single ended signal on twisted pair runs the risk of coupling noise on your "ground". Any lightning/other electrical noise (I had ~60 volts @ 60 Hz from a saltwater pumping system between two ground rods ~100 feet apart).

-Bill

Estragon · June 2019

The signal is 1-wire;
https://en.m.wikipedia.org/wiki/1-Wire

Bit rate is low, but timing on buss could be an issue?

Lightning could be an issue. Sensors will be in a (freshwater) lake. Power will be from batteries ~150' wire run away.

12vdc to the controller (converted to 5v at the controller) might run ~1/2 that distance parallel to, but about 4' distant from a 240vac pump circuit.

BB. · June 2019

100 feet should not be a timing problem... Having slew rate limited driver to prevent "too fast" of pulse (and causing transmission line ringing without proper termination... 100 Ohm termination is "too" low of resistance for 1-wire bus--typically 1-5kOhm--I think).
Here is a design guide for longer distances (and more complex topologies):
https://www.maximintegrated.com/en/app-notes/index.mvp/id/148
One question is--if there is a failure of the bus/sensors, will everything "fail gracefully"?
-Bill

Estragon · June 2019

Thanks Bill. The sensors aren't part of any sort of critical system, really more of an experiment which may or may not be used later in something more important. For now, it's just to measure lake water temperatures at various depths.

It's a long weekend here, and have a herd of folk visiting. Hopefully next week I can wire it up and see what happens

Signal + power wiring

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