Lpc2000

Some answers

--- In lpc2000@yahoogroups.com, "Lasse Madsen" <lasse.madsen@e...
> I have a project, where my customer demands CAN bus interaction.
> Unfortunately I have never worked with CAN only RS485 etc. so it's 
all new
> to me - sorry for the "dumb" questions.
> 
> I'm planning to use an LPC device as I need some processing power, 
but
> frankly I'm a bit afraid to use CAN with all the talking about bad 
CAN
> modules. 
> 
> 1.
> Is there a LPC with an "OK" CAN module? 
They all use the same module as far as I can make out.

It depends what you mean by OK. There are bugs in the CAN module, but 
they can all be worked around one way or another. I have successfully 
used it fine.

> 
> 
> 2.
> The project consists of about 700 units communicating with each 
other.
> I'm planning to use twisted pair cable, wiring all the 700 
controllers in
> series in a large 9 story building so there's a lot of cable! 

CAN will not work like that. You will need to split the system into 
segments with a maximum of perhaps 20-50 nodes or so per section and 
each section with a limited length bus.

> 
> 3.
> Does one need CAN bus "amplifiers" when there are so many 
controllers and so
> many meters of cable? 

Yes, you'd use "bridges" that retransmit packets from one network to 
another.

> 
> 4.
> I've heard rumors about CAN being fault tolerant, what happens if 
two
> controllers try to "talk" at the same time? - same as RS485 where 
the
> communication is messed up?

CAN handles this very well. A 0 is dominant and a 1 is recessive (ie. 
if two nodes drive the bus, one with a 0 and one with 1, then the 0 
will override the 1). As a can node talks it also listens. If it does 
not see what it is sending then it backs off and trries gain in the 
next messaage slot. This means that collisions do not ever destroy 
packets or result in lost bandwidth (as they would on, say, RS485). 
This also means that 100% (or really close) utilisation is possible on 
a CAN network.

The way the dominant/recessive stuff works limits the capacitance and 
bus length. This is the major challenge you face with a 700 node 
system.

> 
> 5.
> On the LPC's there are a CAN "uart" what kind of "transceiver" do I 
need to
> convert the 3.3V signals to what ever it is CAN is using?

Yes Typically the 82C250/251 are used in automotive. You can use 
alternative drivers (basically anything that allows a 0 and 1 to be 
sent in a way that allows the 0 to override the 1 and allows you to 
monitor the bus.

> 
> 
> Sorry for the newbie questions.

We've all been there.