On Wed, Feb 16, 2011 at 04:22:21PM +1000, John L. Craddock wrote:
> If someone with the know-how could port the slave functions of RTnet
> to something like a NXP LPC1768 see
> http://www.littlebirdelectronics.com/products/mbed-%252d-LPC1768-Development-Board.html
> then it is possible that a 200us cycle time could be achieved on 128
> bits of IO on a fast ethernet connection pretty cheaply with the bonus
> of 300 feet transmission limit without a hub. Altenatively, porting to
> Ralph Stirling's setup might also be possible. Just my thoughts after
> a bit of research.
>From here it begins to look like a potentially difficult way to achieve
relatively simple ends. At 10 Mb/s, we can poke 128 bits + 32 bits
overhead [1] over RS485 in 16 us, for much much less effort, I think.
Handled by interrupts at the receiving end, the transmission time is not
lost to the slave, so at 200 uS interval, An ATxmega, for example, would
have 200 * 40 = 8000 instruction cycles. That's not a lot, but then it's
not doing motion control, so that might be fine.
But then I'm a KISS enthusiast from way back.
Erik
[1] For 16 bytes sent asynchronously.
May be reduced with e.g. HDLC.
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