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a little help needed about RS422 communications

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KTP:
Hi guys,

I need a way for the Atmel Xmega32 chips to communicate with each other on the RGB led matrix boards I am building.  There could be up to 100 of these boards, and I would like them all to be wired point to point in series with just a single transmitter for data.  The boards only need to listen to the transmitter, no ack or handshaking.  I think RS-422 only supports 10 devices hanging off a bus and RS-485 supports up to 32 devices (assuming this has to do with the capacity of the differential drivers).  Since I only need receive on each board, I was toying with the idea of using a transceiver chip like National DS8921 to act as a repeater by connecting the data out of the differiential receiver to the data in of the differiential transmitter, and then also running this data line to the Xmega32 UART.  I would have a termination resistor on each board.  Do you think this sounds crazy or would it work fairly well?  The chip specifies 12ns prop delay but that wouldn't really matter as I don't care if the 100th board gets the updated data 2uS after the first board.  Most of the RGB matrix data is going to be stored on a serial 8 pin flash chip on each board anyway.  Here is a link to the 8 pin transceiver I was considering:

http://www.national.com/ds/DS/DS8921.pdf


Thanks for any suggestions/help.

arcom:
There are RS485 driver ICs from Maxim which have 1/2, 1/4 and even 1/8 unit load which means that you can connect up to 64, 128 or 256 devices (respectively) on a single RS485 bus.

Some examples are: MAX487 (1/4 load, 250Kbps), MAX1483 (1/8 load, 250Kbps) MAX1487 (1/4 load, 2.5Mbps), MAX3061 (1/8 load, 500Kbps, fail-safe, hot-swappable), MAX13485 (1/4 load, 500Kbps, fail-safe, hot-swappable).


Your solution will also work but I would go with the RS-485 driver ICs unless you have long distances between RGB matrix boards.
I'm actually working on a similar system for garden lighting -> one master controller and a bunch of slave units communicating over an RS-485 network using MAX3061 ICs.

KTP:
Thanks.  I didn't know a 256 drop chip existed.  It is a bit more expensive than the other chip, but would give the option of communication back to the host (although I can't quite see why the rgb boards would need to)

Psi:
Could you perhaps raise the voltage of the transmitter above the typical -6V / +6V and then use two simple voltage dividers to get the voltage back within RS422 spec on each board.

Might require a bit of testing but it might allow you more devices.
As i understand it, too many devices load the line too much and drop the voltage too low. So a higher transmit voltage should make the voltage drop less critical.

Might end up being a bit to messy though.

arcom:
KTP> You could get temperature, current, voltage readings from the RGB boards (I know, overkill...just the way I like it ;D ) as well as their settings/memory data and then display faulty nodes on an LCD screen. On the other hand, you don't have to implement bidirectional communication...just leave it unidirectional.

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