The Picaxe "SerialPower" Network combines power delivery and bi-directional communications between processes (max. 255) on intelligent, microcontroller-based battery-less nodes. This can be physically implemented using just two interchangeable (preferably twisted-pair) wires. Simple diode-mixing networks with separate power and communication wires are supported as well. In this way low-speed distributed networks consisting of sensors and actuators can be applied in robotics, home monitoring, beacon networks, model railway automation and many other applications. In this case my favorite simple Picaxe microcontroller has been used for the implementation. The picture below shows the essential hardware concepts of the network:
A master node provides for power as well as "timeslots" - all supported through the network protocol - during which processes on nodes can exchange messages with each other using a standard format. In this way a multidrop, bi-directional (half-duplex) network is created. A message transfer is always initiated by the master node (for example to indicate which process is allowed to put a message on the network) and then the relevant process on a slave may use this timeslot to send its message, which can be picked up by any other node:
The slave nodes are powered from the network and only need a backup capacitor, but they may use their own power source as well (for example in case of an actuator node). Both master and slave nodes can be implemented using any available Picaxe type and this implementation can be easily ported to other simple 8-bit microcontrollers.
At power-up the network software at the master node automatically registers all processes on slave nodes that wish to send messages on the network. In this way, timeslots are automatically generated by the master node for all of them (and additional timeslots for different processes can be generated on the fly as well).
In the simplest first version a message format with two databytes is supported, for use with very simple applications. A second version allows any message between 1 - 32 bytes to be transmitted. In that case, power provision to the nodes is managed in between the transmitted bytes (using short pulses) as part of the protocol as well, still allowing very small backup capacitors in the slave nodes to be used in any application. An impression of the second message format is shown here:
Processes may be distributed over several nodes. In the example below, temperature sensing process #11 at the middle slave node calls display process #2 located at both another slave node and the master node to display its measurement data. Given the particular node's resources (e.g. lcd display, sound), display process #2 has local implementation variants:
I created a separate website with all the information required to understand the concepts and to implement applications. It can be found here: SerialPower Website. The related discussions on the Picaxe Forum can be found here: SerialPower on Picaxe Forum