Re: 1394 Performance

On Sat, 2004-02-07 at 14:00, Stefan Richter wrote:
> On 7 Feb, Mark Knecht wrote:
> > On Sat, 2004-02-07 at 07:57, Stefan Richter wrote:
> >> Can [gap_count] be set identical to the maximum number of cable
> >> hops? [...]
> > I'm at home and don't have a copy of the spec here, but the spec does
> > cover (in either the code examples or tables somewhere) how the gap
> > timing equation is effected by the value of the gap count.
>
> I have literature here that shows these two equations:
> subaction gap: detected by (27...29)+(gap_count*16)/base_rate
> arb.reset gap: detected by (51...53)+(gap_count*32)/base_rate
> [base_rate = 98.304/s]
> But what I don't know from this is what minimal gaps (and hence
> what minimum of gap_count) are needed for a given bus topology.
> I will dig deeper and learn about how the gap timing relates to
> the topology; though I'd certainly welcome any insight you could
> provide on occasion.

The optimal gap_count is based off the maximal hop count between any 2
nodes. There is a table and an equation (different for what you list
above) in the IEEE 1394a-2000 spec. which I don't have at home.

Because the gap_count is based off the maximal number of hops, for
performance, it makes sense to use a hub with lots of ports and
everything plugged into the hub rather than daisy chaining the devices.

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Received on Sat Feb 7 23:35:41 2004