Re: Phantom power on RS485 network


John B wrote:
martin griffith scrobe on the papyrus:

On 5 Jun 2006 18:50:59 GMT, in sci.electronics.design "John B"
<spamj_baraclough@xxxxxxxxxxxxxxxxxxx> wrote:

I am going to build a stage signalling system for our local amateur
dramatics group and will be using an RS485 network. For the
network, I will use standard 3 pin XLR microphone cables as we have
lots of them. I would prefer that the remote boxes had no mains
connection and will probably end up using wall-wart power supplies.

It occurred to me that I might be able to feed phantom power down
the RS485 signal lines to the remote boxes, but I don't know how. I
'Googled' on various combinations of 'RS485' and 'phantom power' but
came up with nothing useful.

Has anyone out there done such a thing and would be willing to share
their experience.

Thanks for reading.
You haven't said how much data you are sending, this might be useful,
and are you looping through, box to box, and is it bidirectional? it
might be possible to FM a 40KHz signal for data and use transformers
for phantom powering. How much power do you need, anyway? Do you need
to put baseband audio into the system as well?



martin

There will be no baseband audio and the data rate will be very low. The
idea is that there will be several boxes around the stage. Each will
have two buttons and two LEDs (red & green). The master station will
monitor all boxes for button presses and flash the appropriate LED on
the box where the button was pressed. All LEDs can be cancelled from
the master station which will have clear buttons and an LCD display to
indicate which box was activated. I am planning on using ATmega8's in
all the boxes and a serial protocol using ASCII messages. The protocol
will require two-way communication on the network and the network will
probably be around 75 metres in total length. I am thinking of RS485 as
it doesn't radiate emissions as badly as an unbalanced system and there
are microphone cables running nearby.

However you have given me food for thought and I could use a system
similar to 1-wire. A pull-up at the master station and all USARTs could
connect their TX pins to the bus through open-collector drivers. RX
pins could simply hang on to the wire. I don't know if this would
system would be capable of 75 metres, but I'll kludge something
together and see what happens.


Sounds like a reasonable plan, but split the pullups at both ends.
Driven with an NPN transistor to ground, or the equivelent ic output, I
would go with about 200 ohms pull ups, for a total of 100 ohms. Then a
slow data rate - say 1200 baud, because you are not sending much data
nor needing especially fast response times. I double that this will
induce much noise into nearby, ballanced, shielded, microphone cables.

Good luck,
Luhan

.

Loading