RE: USB, 1394, PCI, ISA, IDE ??

[John Craddock <johncrad-at-xitech.com.au>]

Hi,
I have been doing a little digging into IEEE 1394 (FireWire). It looks very
good for what the list seems to want to do. Someone is already working on
porting the drivers to rtLinux. His post comparing 1394 and USB is in the
rtLinux mailing list archives. I have included the text below for
information. FireWire interface chips are getting pretty cheap. Might be
worth looking into: although it has scads of theoretical capacity. How about
10KHz servo update rates on 10 axes!!!
Regards
John Craddock

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List:     linux-rt
Subject:  Re: [rtl] 1394/USB Networking
From:     Kulwinder Atwal <kalatwal-at-home.com>
Date:     1999-11-28 22:34:58
[Download message RAW]

 > 1394 and USB allow for bandwidth  and time
 > slice reservations making real-time more real.   We could do the
same  > with
 > a NIC by software reservation of every K'th packet, but ...
 >
 > The other drawback of IPv6 is that nobody uses it.

 >
 > In any event,  RTLinux  networking drivers are strongly encouraged  >
and
 > I look forward to seeing people write them and contribute them.


  I did not want to make a contribution to RTL that would be passed
over.  I am strongly encouraged.  I have spent the last few days
reviewing 1394 and USB for suitability  as real-time protocols.  I have
chosen to port 1394 to RTL.  I have emailed the linux 1394 key people
(Emamuel Pirker, Andreas Bombe) of my intentions.
 
 I have chosen 1394 for the following reasons:
 
 - asynchronous messaging to start/stop a real time feed
 - isosynchronous messaging (with guaranteed worst case send time)
   to send a real time feed.
 - worst case send time allowed to change to accommodate signal delay
   as network grows/shrinks.
 - many cabling options: shielded twisted pair, plastic fiber,
glass       fiber, and infra-red.
 - many maximum cable lengths: 4.5m, 7m, 50m, 100m, and 500m.
 - mixing of high speeds on one line: 100Mbps, 200Mbps, 400Mbps,
   and 800Mbps (soon).
 - lower cost per Mbps than USB.
 - draft standard for low overhead protocol variant of 1394: IICP
   ( Instrumentation and Industrial Control Protocol ). Just what we
   need.
 - Audio/Video and instrument industries embracing 1394.
 - no host CPU required, peer to peer communications (a TV can talk to
   a VCR and a Camcorder without a PC).
 
 Drawbacks of USB:
 
 - maximum cable length of a few meters ( across the room).
 - fixed minimum response time of 26 ns (can't grow large factory wide
 network).
 - requires hub to expand beyond the two 'free' ports shipped with PC.
 - USB 1.0 and USB 2.0 not compatible on same wire without packet
   and speed conversion bridge (extra overhead).
 - low bandwidth USB 1.0 at 12 Mbps and USB 2.0 at 250 Mbps.
 - at least one year behind 1394 in development (1384 will be 800Mbps   
in next quarter).
 - USB 2.0 higher cost per Mbps.
 - requires CPU to act as host.
 
I will port to 1394 to RTL in the following order:

	- port raw 1394
	- port 1394/1394a
	- port IICP

I believe in minimalism.  I will not try to copy the 'NDDS publish and
subscribe' model as an other group is doing.  1394 API already uses a
very good model.  It uses the memory model where the network is viewed
as a 64 bit memory space.  Where writing to another node is like writing
to a local memory location.  I think programmers are capable of
understanding how to read and write to memory.  Anything more for hard
real time is extra overhead.  I will cooperate with the other group
where our efforts intersect.    

- Kal Atwal.
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