RE: Your Opinions please

["John B" <home-automation@xxxxxxx>]

Hi Dave,

> Excuse me for being a bit cynical, but 160 bytes just to switch on a
light
> is a bit heavy going. Having not really looked at xAP in too much
detail,
> mainly because I am working on my own CAN bus based system, I find it
hard
> to believe it takes so many bytes. With my protocol I use a single 29
bit
> identifier for the sender, receiver and ID with 2 bytes in the message
to
> indicate what to do with the output. Same for the reply.

This is one of the main reasons why the xPL protocol was created.  xPL
offers similar functionality to XAP (and a few things that XAP doesn't
support fully, such as targetted messages) and does so using a much smaller
message footprint.

In your example of turning on an X10 device, an xPL message to do this
would require roughly 86 bytes.
Whilst this is still far from being the most efficient protocol in the
world, it is approximately half the size of it's XAP counterpart.

> My main reason for asking is how you accommodate such a large telegram
with
> small microcontrollers? Some of the devices I use only have 2K bytes
of
> programme space. Maybe Ian Bird could enlighten me to how he has been
> getting on with this?

Ian Lowe has been doing some embedded xPL development, and many of the
features of the xPL spec were specifically aimed at making it easier to
write xPL-enabled embedded software.

For example, if your device is only interested in command messages (and is
therefore not interested in status or trigger messages) it only needs to
read the first 5 bytes of the xPL message to determine whether to discard
it or continue processing it.

In contrast, each XAP message begins with "xap-header", which
doesn't tell you anything about the message, and IMO is a waste of 10 bytes
which could have been put to a better use.

> Maybe I should look at it again and see if I can come up with a xAP to
CAN
> bus gateway as this would allow me to use some of the excellent
hardware you
> guys have been building/creating. In fact I tried to find the xAP
website on
> sourceforge just now but it was not there... Has this been moved
elsewhere?

If you are interested in seeing what xPL has to offer, check out the
official project site at http://www.xplproject.org.uk/
There is also the UKHA_XPL discussion group on Yahoo.

Both XAP and xPL are excellent initiatives which have the potential to
vastly improve integration in the home automation world, and both protocols
have their strengths and weaknesses.
The best advice is to look at both protocols and decide for yourself which
one best fits your needs.

Regards,

John


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