Explanation: 1394a VS 1394b device speed

[BruteForce]

OK, after dealing with various customers and even our own engineers it
is clear to me that lots of people don't get one of the crucial
distinctions between 1394a and 1394b.

The problematic, but widespread, understanding is that:

Hey, 1394a is 400Mbps while 1394b is 800Mbps, right?

Well, almost... There are many differences between 1394a and 1394b,
like bandwidth, signalling and connectors, but I want to clarify a
specific aspect.

Let us consider the question: What is the max speed of a 1394 device?

For 1394a devices the answer is simple: 400Mbps, or 200Mbps, or
100Mbps.

For 1394b devices the question is void. VOID. NULL.
For 1394b devices the question is instead: What is the max speed of
each PORT of this device?

In 1394b devices each port may have its own maximum speed.
A 1394b device may easily have one port whose max speed is 800Mbps,
another with 400Mbps and one with 100Mbps. Or have all ports with max
speed S100 (for whatever reason this is deemed necessary). There is
nothing that forces 1394b devices to operate at S800. So don't act
surprised the next time you hear about a 1394b 400Mbps device.

When all the ports have the same max speed then we may talk about "the
max speed of the device", but in general this is an incorrect line of
thinking so get rid of it.
When thinking about 1394b devices, refrain from thinking about the
"max speed of the device", think ports instead.

Hope this helps clarify the confusion :-)

Dimitris Staikos
1394 R&D Manager
Unibrain