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From: Kevin Buchs
Subject: Understanding the IEEE 1394 (FireWire) Physical Layer
Date: 27 Sep 2002 15:23:10 GMT
Organization: Mayo Foundation
NNTP-Posting-Date: 27 Sep 2002 15:23:10 GMT
I would like to get independent confirmation of my understanding of the
physical layer (electrical signal level) of the IEEE 1394 (aka FireWire)
Each cable has 4 signal conductors in 2 differential pairs. The 2 pairs
swap position on the connectors on each end of the cable so there is a
cross over going from device to device. Twisted Pair B (TPB) at a device
can transmit data and receive a strobe signal. Twisted Pair A (TPA) can
receive data and transmit a strobe. (What is the function of the strobe?)
For a given 1394 network, multiple devices can be transmitting at a given
time and sharing these single-direction communication links. If the
network looks like the following, where numbers in brackets represent nodes
and parallel lines represent the two twisted pair channels:
then, for example, 1 can be transmitting to 6 at the same time that 3 is
transmitting to 2 and 5 is transmitting to 4. Is this a correct
understanding? Does this mean you could reach the maximum of 400 Mbps in
one direction from a node, but the network could sustain much higher rate
if the topology supported it?
If my understanding expressed above is correct, the next question would be
can a given node be a transmitter and receiver at the same time? I suppose
the answer may depend upon the interface chip/circuit used. Also, in a
daisy-chained network, as drawn above, does one use two independent
transcievers and route signals through the link layer, or is the daisy
chain accomplished another way?
Thank you for your help.
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