TSB14C01A, TSB14C01AI, TSB14C01AM

5-V IEEE 1394-1995 BACKPLANE TRANSCEIVER/ARBITER

SGLS107A – FEBRUARY 1999 – REVISED NOVEMBER 1999

24

POST OFFICE BOX 655303

PRINCIPLES OF OPERATION

fairness intervals (continued)

Fairness Interval (N)

arb

arb

data

arb

arb

data

data

arb

Owner A

Owner B

Owner M

Fairness

Interval (N –1)

Fairness

Interval (N +1)

Subaction

Arbitration

Reset Gap

Subaction Gaps

Arbitration

Reset Gap

Figure 11. Fairness Interval

The implementation of the fair arbitration protocol is defined in terms of these fairness intervals as discussed

in the following paragraphs.

fair arbitration

When using this arbitration class, an active node can send an asynchronous packet only once each fairness

interval. Once a subaction gap is detected, a node can begin arbitration when its arbitration_enable signal is

set. The arbitration_enable signal is set at the beginning of the fairness interval and is cleared when the node

successfully accesses the bus through fair arbitration. This disables further fair arbitration attempts by that node

for the remainder of the fairness interval. In the absence of urgent nodes, a fairness interval ends once all of

the nodes attempting fair arbitration have successfully accessed the bus. At this time, all of the fair nodes have

their arbitration_enable signals reset and cannot arbitrate for the bus. The bus remains idle until an arbitration

reset gap occurs. Once this happens, the next fairness interval begins. All of the nodes set their

arbitration_enable signal and can begin to arbitrate for the bus. This process is illustrated in Figure 12.

Fairness Interval (N)

arb

arb

node A

arb

Fairness

Interval (N –1)

Fairness

Interval (N +1)

Cleared When Node Wins Arbitration

node B

arb

node C

Arbitration

Reset Gap

Arbitration

Reset Gap

Set at Arbitration Reset Gap

Node A

Node B

Node C

Arbitration

Enable

NOTE A: The arbitration number of A > B > C.

Figure 12. Fair Arbitration Timing

Note that a node sending a concatenated subaction does not reset its arb_enable bit.