Broadcasting one UART to many

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r_y_a_n
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Posted by r_y_a_n: Mon. Oct 13, 2008 - 08:04 PM
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Hello all,
I have one AVR sending data on Tx, and several AVRs listening for data coming in (attached Rx). Essentially I have a system that is broadcasting data to many processors, in one direction only. 

 ___________          ___________           ___________
|           |        |           |         |           |
|  Main  Tx |   *----| Rx  Child |    *----| Rx  Child |
|___________|   |    |___________|    |    |___________|
       |________|_____________________|__________________ _ _ _

This works great for a few processors, but the data gets lost once ther are many on the line. I would like to broadcast to several hundred. What do I need to put on the line between the Main and the Child to make sure the signal is strong enough to get to all of the Children?

(Standard caveats apply, i'm a newbie with signal processing :)
Thanks in advance!
Ryan

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AVR Microcontrollers, megaAVR and tinyAVR
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Bronze Markbobgardner
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Posted by bobgardner: Mon. Oct 13, 2008 - 08:06 PM
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Using +-12V RS232 levels, or 0-5V TTL levels? (seems like either should work). Have a good ground to all of them? If the input is .1 standard load, or .1 ma, and the output pin is 40ma, then you should be able to drive 400 of them.

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r_y_a_n
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Posted by r_y_a_n: Mon. Oct 13, 2008 - 08:09 PM
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Yep, ground is good. The TTL is 3.3 (and also 5v tolerant, i think).

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ka7ehk
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Posted by ka7ehk: Mon. Oct 13, 2008 - 08:41 PM
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What baud rate?

Jim

 

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r_y_a_n
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Posted by r_y_a_n: Mon. Oct 13, 2008 - 09:20 PM
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Thanks for the fast responses guys!
For safety of testing we were going with a low baud of 9600, and were hoping to speed it up later.

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rstahlhu
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Posted by rstahlhu: Mon. Oct 13, 2008 - 09:55 PM
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I would use RS-485 drivers for long distances and noise immunity.

Randy

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ka7ehk
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Posted by ka7ehk: Mon. Oct 13, 2008 - 11:08 PM
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Standard RS232 receivers are NOT good for this application as they have a built-in input resistor. So, as you add more and more, the resistance that the transmitter has to drive goes down. RS422 (not RS485 since it is one-way) is an excellent choice. There are 3.3V drivers.

"TTL" or CMOS is NOT built to drive long cables. Period. It is intended to drive other logic. You are going to see a LOT of load capacitance with length and number of receivers. I'd bet that is what is killing you. That said, there are CMOS buffer chips that can source OR sink 25ma. If cost is a critical issue, I would try one of these at the transmitter, configured non-inverting.

Jim

 

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DocJC
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Posted by DocJC: Mon. Oct 13, 2008 - 11:45 PM
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Could also put another driver, or two, along the chain, to boost the signal to the next leg.

JC

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CirMicro
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Posted by CirMicro: Tue. Oct 14, 2008 - 12:03 AM
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Quote:
I would like to broadcast to several hundred.

Wow, may I ask what kind of application this is for?

What kind of distance will be involved?

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jgmdesign
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Posted by jgmdesign: Tue. Oct 14, 2008 - 01:27 AM
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I have to agree with the RS485 method. Just use the slaves in listen mode and a 120ohm resistor at each end of the link. Yu can go up to 4000 feet and up to over 1.5 megabaud with the right driver ic.

Jim

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Guillem Planisi
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Posted by Guillem Planisi: Tue. Oct 14, 2008 - 06:53 AM
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I also support (as many other here) the RS-485 approach, but I will aware you that if you have several hundreds, then you will need some kind of 'buffer' or 'repeater' in the path, since usual RS-485 drivers allow to drive up to 128 (some only up to 32) devices.

Guillem.
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r_y_a_n
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Posted by r_y_a_n: Sat. Oct 25, 2008 - 01:01 PM
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@ CirMicro
the distance involved is only about a meter and a half.

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leon_heller
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Posted by leon_heller: Sat. Oct 25, 2008 - 01:16 PM
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At 1.5m, there is a much better way to do that than lots of AVRs! Have a look at the XMOS chip:

http://www.xmos.com

One or two of those will outperform dozens of AVRs, at a fraction of the cost.

Leon

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CirMicro
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Posted by CirMicro: Sat. Oct 25, 2008 - 01:39 PM
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r_y_a_n wrote:
@ CirMicro
the distance involved is only about a meter and a half.

For such a small distance I would say a simple transistor buffer would suffice.

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