Re: EMC compatible computers

["ERIC KELLER" <eekeller-at-psu.edu>]

There are packages for hardwired networking for real-time linux, most notably
under RTAI.  You hook two cards directly together with a crossover cable.  

On the other hand, I think Jon's stepper board is a bargain, is available now,
and will work with the most powerful PCs available.  I guess we all like to
design new boards.  I know I do, at least in theory.
Eric


On Tue, 14 Jan 2003 01:59:22, Jon Elson wrote:

> 
> 
> 
> Dave Engvall wrote:
> 
> >Jon,
> >Did I miss  something. Isn't 10 Mb ethernet about as fast as the parallel
> >port?
> >  
> >
> Yes, it does 1.25 MegaBytes/second, not counting protocol overhead, 
> which can be quite substantial.
> The standard protocol requires gaps between messages and minimum packet 
> sizes to prevent two
> different nodes from starting a message at the same time, having the 
> messages pass through each
> other in the middle of the net, and both originators never realizing the 
> message had been garbled
> on some nodes, because they heard their own outgoing message clearly at 
> their location.
> 
> Most twisted pair nets are much shorter, and isolated into separate 
> segments by routers, so some
> of these restrictions may not have the same impact any more.  If you 
> were setting things up so
> that the CPU was the only 'master' node, and that the axis drives only 
> responded to messages
> from the master, then you could dispense with all the time overhead of 
> the CSMA/CD
> protocol.  This of course would mean writing your own net driver as well!
> 
> The IEEE-1284 protocol subset that I use doesn't incur all that 
> overhead, and doesn't packetize
> messages.  So, I do the absolute minimum of I/O to control the motors. 
>  First, there is an address
> transfer and a data write to latch the positions into the encoder 
> counter holding registers.  Then,
> there is one address transfer to select the encoder counters, and then 9 
> byte read cycles to get the
> 24-bit positions for 3 axes.  After the servo calculations, there is an 
> address transfer and 6
> byte writes to load the SERVO DACs for 3 axes, or 12 byte writes for the 
> step velocity
> counters.  Then, one more addr transfer, and a data write to make the 
> output changes take effect.
> So, that comes to 21 or 27 byte transfers for the complete servo cycle 
> for 3 axes!  There are other
> transfers to read and write digital I/O.  All this takes place in 50 uS 
> on a 333 MHz Pentium II,
> so you could run a 10 KHz servo update rate if you needed to.  I have 
> some doubt you could do
> it this fast on a 10 MB/s ethernet due to the protocol overhead.
> 
> Note that my PPMC and universal stepper boards have NO CPU at all!  So, 
> when a command is
> sent, it is acted upon before the acknowledge gets back to the CPU.
> 
> Jon
> 
> 
> 
>