Re: silly ravenscar question

[jan.de.kruyf@gmail.com]

On Wednesday, February 25, 2015 at 12:45:53 PM UTC+2, Dmitry A. Kazakov wrote:

> The high end is 8 analogue channels 100盜 over the network + hundreds of
> lower speed channels. Data volumes is not a big problem. E.g. we can sample
> 8 100盜 channels and distribute them over the network without data loss,
> doing 10ms oversampling.
> 

What is '100盜 ' ?  '0xc79b9c' was nowhere to be found.
what do you mean to say by 10msec oversampling? Average in the terminal and send only every 10 ms?

in any case to focus the thinking, I did some quick sums:
A packet with a payload of 50 bytes, according to the theory should be able to do the roundtrip (100 m cable, 1 switchbox, no packet contention) in 25 usecs through an stm34 board, with time to spare. that includes DMA and offloading and onloading of data.
So that makes 40 packets per msec. Leave some space for syncframes, arp frames and data loss, then 20 roundtrip packets /msec should be doable, provided the jitter is strictly controlled. 
An individual terminal has all the time in the world to handle the data between commframes, but I bet that, depending on what you want out of the system, the pc might feel the strain at that rate. (and the rt scheme must be up to scratch in the pc)
So the bottleneck according to me is in the pc  in most cases.

In any case so according to your specs we would need some kind of multiplexing scheme. I did not need that, the project is too small; but it is good in this case to do a little bit of thinking up front, before too much code is written.
 


> . . . which may be not an issue for single-core
> board with non-preemptive scheduling.

Thats how we did it before Noahs flood, and boy did it run fast.

cheers,

j.