Re: New C-Bus DMX Gateway Module Launched

[Colin Bradford <cjbradford@xxxxxxxxx>]

On 3 April 2010 16:56, Kevin Hawkins <yahoogroupskh@xxxxxxx>
wrote:
> I don't know much about DMX really , I believe it needs constant
> maintenance of the channel levels or they all turn off. =A0I think
maybe
> you can only have one controller on a given circuit as well which
means
> if you have one already that is handling scenes or colour mixes (as I
> think you do Paul) =A0then you can't pop an additional one on ? =A0
=A0 T=
hat
> is maybe why that C-Bus gateway is single directional as all control
> messages could then only originate from itself ?

That's all correct - DMX is sent from a single transmitter to multiple
receivers, and there can be only one (active) transmitter at a time.
Up to 512 channels are sent up to 44 times a second - the standard
says that a reciever must be able to cope with a gap of up to one
second.  The original standard is unidirectional, though there is a
new update called RDM that allows management messages to be sent
bidirectionally.

The signal itself is RS422 - 5v or so over a balanced pair.  It's a
serial protocol, running at 250kbit/sec.  I've had no problems running
it over 150 metres, with multiple receivers on the line.  In rare
cases I've had to terminate the line - I normally do it because it's
good practice, rather that needing to.  It's easily within a small
processors range to transmit or receive DMX - I've used 8051s clocked
at 16MHz.

There are some cheap DMX dimmers available - NJD do a 12 channel
version that CPC normally have for less than =A3100.  I've not tried it
yet.

Cheers,

Colin.


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