Re: Sensors / Switches / etc

["patricklidstone" <patrick@xxxxxxx>]

> You can use the same CAT5 cabling for serial communication. More,
you can
> have a single bus for all the devices, so no need for HUB/switches
or
> supplemental RJ45 sockets and the respective cable

A single bus is a Bad Thing imo. Do you remember the days of thin
ethernet cabling? A single, daisy chained bus, is a nightmare from
the point of view of discovering where a break occurs in the network.
Depending on the network design, it may also require that all devices
are active for the network to function properly. Then there is also
the issue that if you cannot add or remove a device from the network
with (albeit temporary) interruption to the network.

> >
> > - Fast. Time lag on a multi-device serial bus can be a big issue.
> If you consider let's say X10, where the time lag is sometimes very
big ,
> this is not an issue in a well designed serial bus (do not forget
10BaseT
> ethernet-10MHz on the same cable). I think that home automation
devices do
> not need a very fast connection (maybe with few exceptions..).

OK, at one level you could argue that ethernet is "just a very fast
serial network". But remember the motives for using a serial network -
it's cheap. High speed networks (say >19k2 baud) need fast
processors, and cost starts to climb. At the lower end of the scale,
a 9k6 or 1200 baud serial network is severely limited in terms of the
number of symbols you can push down the wire in a second. If you have
a large number of devices competing for the network infrastructure,
the time taken to transmit a command to/from a device is (a) going to
vary significantly depending on current network activity -->
perceived unpredictability from end users perspective and (b) may
take a significant amount of time (>750ms say).

> > Best compromise in terms of cost/performance:
> >
> > - Put an ethernet to serial bridge in each room, connected to the
> > cat5 TCP/IP infrastructure.
>
> For what reason to do that? You can have a single ethernet RJ45 in
that room
> and use 2 spare wires for the serial bus.... So you can hava a PC
and the
> serial bus on the same cable.

- It simplifies the infrastructure. There is no long daisy chain to
debug
- Performance is reasonably because there are a small number of
devices on the local serial network.
- It remains cost effective - you can use slow, low power processors
for the serial devices.

Patrick