Two Wire Power and Coms - Page 1
Forum section: electronics / projects

author: ludzinc
author_rank: Unknown
timestamp: October 22, 2013, 12:59:35 am
content: I'm interested in rolling my own DIY home automation, and have been investigating distributing power and coms.I keep oscillating between 'poor man's power over ethernet' (using some CAT5 with dedicated pairs for power and coms) and running separate power (beefy figure 8 cable to reduce voltage drop issues) and coms cables.Then I stumbled upon this:http://www.touchplate.com/2wnptf.php2 wires that daisy chain between modules, polarity independent (easy hook up) and carries power and data. Cool.If you look at the product manual there's some basic pictures of the button plate, and through squinting and wishful thinking I don't think they implement a power over ethernet approach (e.g. no transformers). But for the life of me I cannot fathom how they do this.Any suggestions?(And I fixed the heading typo!!)

author: NiHaoMike
author_rank: Super Contributor
timestamp: October 22, 2013, 02:27:11 am
content: Easiest way is to have the senders modulate the current they draw so as to implement a serial bus. Somewhat higher in complexity is to put a high frequency carrier on the lines.BTW, that is sometimes done with higher end HVAC systems. Back when central A/C became common, the outdoor unit just had a relay to turn it on and off. (Most newer units work exactly the same way!) They routed a single pair of wire for the controls, which was good enough for the intended use. The problem came when some higher end units started showing up with sophisticated inverter controls or even a little feature like a maintenance needed indicator or outdoor temperature sensor. They initially used a multi core cable, which proved problematic when installing a new unit on an existing building. Nowadays, there's a trend towards doing it all with the one pair, using serial communication to make that work.

author: Whuffo
author_rank: Regular Contributor
timestamp: October 25, 2013, 07:41:29 pm
content: The method used depends upon the reliability required. Simple modulation of the current draw will work, but will suffer from a high error rate due to noise and coupled signals. Much more reliable is to use the data to apply an on / off "high" frequency signal to the line. Often, the embedded UART in a MCU is used to encode and send / receive the signal; all that's needed beyond that is a signal source at the transmitter and a simple tuned circuit and detector at the receiver end feeding a similar MCU. Even that method is subject to error.By using a "tell me three times" (or more times) system you can reduce the error rate on the HF system to an acceptable level. Use LC circuits at each end to separate the data from the power. Yes, it is worth the extra effort to do the HF system - getting data across an unconditioned pair reliably isn't as easy as you might think.

author: Marco
author_rank: Super Contributor
timestamp: October 25, 2013, 09:23:29 pm
content: This guycloned the UPB hardware (using pulse position modulation).What this kind of wideband noise will do to the throughput of something like a homeplug network I don't know ...

author: NiHaoMike
author_rank: Super Contributor
timestamp: October 25, 2013, 11:02:47 pm
content: Repetition codes are easy to implement, but way outdated when there are far better ECC algorithms like Reed Solomon. At low bitrates, a common microcontroller can easily handle it. And to be really sure, add a CRC to each packet.