HP E8491A manual Ieee Data Transfer Protocol, Optimizing the Configuration

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

isochronous1. 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.

VXI mainframe 1 VXI mainframe 3

	block transfer	block transfer
VXI mainframe 2	data packet	data packet
	data packet	data packet
		data packet
data packet

F I n-1

Fairness Interval n

F I n+1

mainframe 1 or 2

mainframe 1 or 2

mainframe 3

arb

data

ack

arb

data

ack

arb

data

ack

seq

packet

seq

packet

seq

packet

subaction 1

subaction 2

subaction 3

arbitration rest gap

subaction gaps

arbitration gap

Figure 4-2. IEEE 1394 Data Transfer Protocol.

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.

Chapter 4 IEEE 1394 Fundamentals and Interface Overview 59