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IEEE 1394 Fundamentals and Interface Overview 59Chapter 4
Optimizing the
Configuration
I/O performance is impacted slightly by the hardware configuration.The
VXI mainframe closest to the PC (root node) has the highest priority. For
example, if instruments in VXI mainframes 1 and 3 (Figure 4-1) contend for
the bus at the same time, the root node will grant mainframe 1 access to the
bus first. However, the bus’s fair arbitration protocol (covered in the next
section) ensures that each device has equal access to the bus and that devices
closer to the root are not continually granted the bus.
IEEE 1394
Data Transfer
Protocol
Data transfer over the IEEE 1394 bus can be either asynchronous or
isochronous
1
. Hewlett-Packard’s IEEE 1394 based VXI systems use
asynchronous data transfers and a “fair arbitration” protocol to ensure each
VXI mainframe has equal access to the bus. Figure 4-2 illustrates the
concepts of asynchronous data transfers and fair arbitration.
1. Isochronous data transfers broadcast variable amounts of data at regular intervals with no acknowledgement.
Isochronous and Asynchronous data transfers can occur on the same bus.
Figure 4-2. IEEE 1394 Data Transfer Protocol.
VXI mainframe 1 VXI mainframe 3
VXI mainframe 2
block transfer
data packet
data packet
data packet
data packet
data packet
data packet
block transfer
arb
seq
data
packet
ack
arb
seq packet
data
ack
arb
seq
data
packet
ack
mainframe 1 or 2 mainframe 1 or 2 mainframe 3
subaction 1 subaction 2 subaction 3
Fairness Interval
n
subaction gaps
arbitration rest gap arbitration gap
F I
n-1 n+1
F I