Kim asked:
Hi DavidOn the Xwire scheme is it possible to have a master and a slave and if the master drops out, the slave becomes the new master ? Like a redundancy type setup.
Also, based on the redundancy scheme, if 2 separate power supplies are used to power each board, what special considerations do I need to take ?
Glad you asked!
On redundancy: Essentially you can craft any scheme you like on top of Xwire. For instance, to detect a loss of comms simply increment a byte once every 100mS and at the other end check that it is changing. You can use a similar ploy to ensure that a set of data is only actioned once. Another use would be to flag that a data set has all been written. Say you have 5 bytes of data that all add up to some action to be taken at the slave, and it is vital that all 5 bytes are received before they get actioned. Use a 6th byte as a message counter and make sure it is the last thing you increment. You can probably work out several other scenarios yourself.
This is what fascinates me about this Xwire idea. The basic concept is so simple, swapping a few bytes of data, but the applications are made numerous by simply using the bytes in creative ways.
Regarding power supplies: The testing we did between the lab and my office was with separate power supplies on different mains circuits. The key to it is that the power supplies are floating. The only earth connection between boards is the earth in the Xwire connection. We used shielded wire in deference to the 21m distance. BTW, we just happened to have 21m of cable on the wire. On the 'scope the signals were looking very clean.
Just bear in mind that these are signals directly off the chip pins. On the CC16 and MS120 we've placed a zener across the line to give a measure of protection against static, but is otherwise a "naked" chip. That means you would be taking a risk if you allowed users or untrained installers to do any wiring, so we really intend this for "inside the box" use.
