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Establishing Cat 5 standard for sensors January 23, 2011

Posted by rik94566 in aquaponics, DIY aquaponics, Hacks, sensor, Standards, Temperature Probe, Uncategorized.
Tags: , ,

In order to be consistent in reading sensors a standard must be established for the Cat 5 cable that I will be using.  As it turns out some work has been done in this area.  I just need to pull it together to make sense in my projects and be able to consistently achieve results across all sensors and match up with what is standard industry practice.


As it turns out PoE (power-over-ethernet) is the established standard for this.

Early PoE systems were created by various companies with no particular standard in mind, so unfortunately there are a number of systems around that are not compatible with each other. Different companies used different voltages, different polarities, and different pin assignments – sometimes even varying between products manufactured by the same company!

Thankfully a standard has now been defined and modern PoE systems have converged to the point that they are mostly compatible. You can read about the standard, designated “802.3af” (2003) and the newer “802.3at” (2009), on Wikipedia. But even with the 802.3 standard it’s not all clear sailing, because the standard itself is quite painful to implement. It stipulates the use of up to 48V (more than typical components such as voltage regulators can handle) and a signalling scheme to allow the PD to tell the injector how much power it will require. Implementing a full 802.3 standards-compliant powered device is therefore far more complex than it really should be.

Many hobbyists therefore implement a simplified system that uses the same connections as the 802.3 standard, but runs at a lower voltage and doesn’t use a signalling scheme.

The most common PoE pin assignment typically used with 10/100base-T Ethernet (which only uses 2 of the 4 pairs in the cable for data) is as follows:

Pin Use Description
1 RX+ Receive data +
2 RX- Receive data –
3 TX+ Transmit data +
4 DC+ Power-over-Ethernet +
5 DC+ Power-over-Ethernet +
6 TX- Transmit data –
7 DC- Power-over-Ethernet –
DC- Power-over-Ethernet –

Reference of above material



TIA/EIA-568-B.1-2001 T568B Wiring[1]
Pin Pair Wire Color
1 2 1 Pair 2 Wire 1 white/orange
2 2 2 Pair 2 Wire 2 orange
3 3 1 Pair 3 Wire 1 white/green
4 1 2 Pair 1 Wire 2 blue
5 1 1 Pair 1 Wire 1 white/blue
6 3 2 Pair 3 Wire 2 green
7 4 1 Pair 4 Wire 1 white/brown
8 4 2 Pair 4 Wire 2 brown


1 – RX (+) —  Receive Data (+)  —————-WHITE/ORANGE ——none
2 – RX (-) —   Receive Data (-)  —————–ORANGE     ————–YELLOW
3 – TX (+) —  Transmit Data (+) —————-WHITE/GREEN ——–none
4 – DC (+) —  Power-over-Eithernet (+)  —– BLUE ——————–RED
5 – DC (+) —  Power-over-Eithernet (+) ——WHITE/BLUE———-none
6 – TX (-) —   Transmit Data (-) —————–GREEN ——————none
7– DC (-) —  Power-over-Eithernet (-) ——–WHITE/BROWN ——none
8 –DC(-) —   Power-over-Eithernet (-) ——–BROWN ————— BLACK

This is the configuration I will be using going forward on all sensor conections.  Should I need to double-up on a Cat5 cable I can use the WHITE/COLOR wires to accomplish this task.



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