TSB14C01

5-V IEEE 1394-1995 BACKPLANE TRANSCEIVER/ARBITER

SLLS231B – MARCH 1996 – REVISED MAY 1997

10

POST OFFICE BOX 655303

During arbitration, each node that is arbitrating for the bus drives its code then its node number out onto the

bus. During each bit period, each node reads back what has been placed on the bus. If it reads back the

same data it was sending, the arbitrating node stays in contention for winning the bus. If it reads something

different then what it was driving, the arbitrating node loses the bus and drops out of contention. This is the

reason for requiring Wired-OR operation during arbitration. As long as each node is still sending 0s onto the

bus during arbitration, all nodes are still contending to win the bus. The node with the highest priority (or if all

priorities were zero then the highest node number) is the first to drive a 1 onto the bus during arbitration. The

node that sends the first 1 (asserting the bus) and reads it back wins the bus. All other nodes read back a 1,

which does not match the 0 (releasing the bus) they are sending, and drop out of contention. This arbitration

process requires the transceiver selected to be able to read from the bus at the same time it is driving the

bus.

For example, if three nodes, each with priority 0 and a node identifiers of 8, 7, and 2, were to arbitrate for the

bus the following would occur:

00

0

00

0

Driven by Node #2

(TSB14C01)

0

0

0

0

00

0

00

0

Driven by Node #7

(TSB14C01)

0

0

0

0

00

0

01

0

Driven by Node #8

(TSB14C01)

0

0

0

0

Bus Data Line

(voltage level on the bus)

Bus Read

NOTE A: This bus is reverse logic, a 1 being driven by the TSB14C01 is asserted by driving a 0 onto the bus by the transceiver.

Figure 3. Three Nodes Arbitrating for the Bus

Since the highest node number is 8 (1000b), node 8 outputs the first 1 (assert the bus) and wins the

arbitration. The other nodes drop out and do not try to drive their node number onto the bus.

The 1394 bus has two data lines that use Data-Strobe Encoding on the bus. This requires that the

transceivers be able to operate at a maximum frequency of one half of the maximum data transfer rate.

When operating at 50 Mbits/s, the maximum frequency the drivers are required to operate at is 25 MHz.

When operating at 100 Mbits/s, the maximum frequency the drivers are required to operate at is 50 MHz.