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From: Francois Retief
Subject: Re: Understanding the IEEE 1394 (FireWire) Physical Layer
Date: Thu, 03 Oct 2002 20:35:26 +0200
Organization: Universiteit van Stellenbosch
NNTP-Posting-Date: 3 Oct 2002 18:35:29 GMT
Kevin Buchs wrote:
> 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?)
The strobe is use to retreive the clock that is used to synchronize the
data. Actually 1394 and 1394a is half duplex (1394b is full duplex, but
that is another story)
The data and the clock is encode onto TPA and TPB in such a way that
the clock can be recovered with XOr function. (TPA xor TPB = clock).
> 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?
No, there can only be one transmitter of data on the whole bus. Each node
is just a (unbuffered) repeater of the data. There is a arbitration
process where a transmitter is granted access to the bus. Thus if 
transmit to , every body else gets the data.
> If my understanding expressed above is correct, the next question would
> can a given node be a transmitter and receiver at the same time? I
> 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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