Serial communication + power over two wires

[andyturk]

I'm looking for a way to power a small mcu at the end of a long wire (e.g., 10 feet) and also give it a way to communicate with the device on the other end. A few tens of mA at 5V DC for power and say 9600 baud for data bandwidth (bidirectional).

Any recommendations?

[free_electron]

voltage current modulation.

make a supply source that can be modulated. lets say 5 volt and 6 volt.
the slave board has a local regulator that makes 3.3 to power the local cpu. a comparator then picks off the incoming voltage.
te return data you put a mosfet with a resistor across the supply. a current sensor in the source system measures consumption.
pulsing the load creates for example a 50mA delta current.

Cerberus ( fire alarm systems) have been using this mechanism for years.

[philpem]

Look at some of the industrial control protocols -- HART is a good one. That uses capacitive coupling to modulate a 1200 or 2400Hz tone onto the incoming signal/power line. The Analog Devices AD5700 HART modem datasheet might give you some ideas.

[andyturk]

@free_electron
Very clever. Sounds like a nice "jellybean" solution.

@philpem
I'm reading the AD5700 datasheet with my coffee this morning.

Thanks guys!

[dfmischler]

I don't know how much power you need, but the Dallas 1-wire stuff can be powered and communicate over a single pair as well.  Their setup depends on keeping the signal line high a lot and robbing power from it (they refer to it as "parasitic" power).  I don't know if this technique would actually work for your application, but it is pretty simple.

[andyturk]

I don't know how much power you need, but the Dallas 1-wire stuff can be powered and communicate over a single pair as well.  Their setup depends on keeping the signal line high a lot and robbing power from it (they refer to it as "parasitic" power).  I don't know if this technique would actually work for your application, but it is pretty simple.

The 1-wire stuff looks good, for a prototype at least. The DS2408S+ interface chip is pretty pricey though ($6/ea quantity 1).

[daveatol]

You don't need an interface IC for 1-wire; you can just bit-bang with the microcontroller, or even use the UART (http://www.maximintegrated.com/app-notes/index.mvp/id/214).

[codeboy2k]

There was at least one fire alarm control panel that daisy chained all the fire warning strobes and just did short duration pulses on the powerline to address them and signal them to turn on, turn off, what tone to make, etc... they relied simply upon a capacitor at the other end to hold up the voltage during the off time of the pulses.  This technique would only allow low speed communications.

[IanB]

Does it have to be two wires? There are many multicore cable solutions cheaply off the shelf, e.g. network cable, telephone cable, or even low voltage solenoid power cable for irrigation systems. You can use two conductors for power and have other conductors left over for communications. Mechanical solutions are often simpler than electronic solutions.

[codeboy2k]

Does it have to be two wires? There are many multicore cable solutions cheaply off the shelf, e.g. network cable, telephone cable, or even low voltage solenoid power cable for irrigation systems. You can use two conductors for power and have other conductors left over for communications. Mechanical solutions are often simpler than electronic solutions.

The fire panels that did this were designed within the constraints of an existing two-wire installation, and for retro-fitting newer, addressable and synchonizable alert receivers instead of just a ringing bell . Some of these wall mounted alerts even had digitized voices for emergency messages, pre-recorded and stored locally at the wall mounted alert, and some models had the capability of a live operator voice originating from a control panel. There was a sync pulse on the 2 wires as well, so that during an alert all the units on the wall would flash in unison, or go whoooooooooOOOOOOOOOOPPPP in unison. This was all into existing wiring, which was usually just 24V bell wire. (i.e. zip cord, low voltage, 2 wires, 18-22 awg)

In new installations I suspect they use multi-core wire now.