Asynchronous Data During an asynchronous data transfer, a variable amount of data is
Transfers transferred to an explicit address in real time, and an acknowledgement is returned. Data is transferred across the IEEE 1394 bus in packets called
“subactions.” An asynchronous subaction is made up of three parts:
*arbitration sequence - the period when a device requests control of the bus in order to transmit a data packet.
*data packet - the data packet consists of a data prefix that contains information about the transaction, the data itself (e.g. VXI instrument commands), and a data end signal. The maximum packet size is 1 kByte for 200 Mbit host adapters and 2 kBytes for 400 Mbit adapters.
*acknowledgement - a code returned by the (addressed) data destination indicating the action taken by the receiver.
The periods between subactions are called subaction gaps. The subaction gap allows devices that have not had control of the bus during the current “fairness interval” to arbitrate for control.
Fair Arbitration Protocol The fair arbitration protocol is based on the fairness interval shown in Figure
2.A fairness interval consists of one or more subactions in which data packets are transferred over the bus. A fairness interval is as follows:
1.The interval begins when devices (HP E8491A’s) arbitrate for control of the bus.
2.When a device is granted control, it transfers its data packet and is then disabled from arbitrating until the next fairness interval.
3.A subaction gap occurs after the previous data packet is transferred. During this period, remaining devices arbitrate for the bus. The next device granted the bus transfers its data packet and is then disabled from arbitrating until the next fairness interval.
4.The fairness interval ends after each device has had an opportunity to access to the bus and the arbitration reset gap, which is longer than the subaction gap, occurs. The arbitration reset gap
VXI Data Transfers To take advantage of the IEEE 1394 data transfer protocol, large amounts of data should be transferred between VXI instruments and the PC using block transfers. During a block transfer, data is divided into the packets described previously; the number of packets depends on the amount of data and whether a 200 Mbit or 400 Mbit host adapter is used. Compared to protocols that transfer data one byte or one word at a time, transfer speed between the instrument and the PC is increased because the IEEE 1394 protocol overhead is associated with the fairness interval and with each packet, rather than with each byte or word transferred. Thus, transfer speeds (bits/second) over the IEEE 1394 bus increase as the amount of data transferred (block size) increases.
60 IEEE 1394 Fundamentals and Interface Overview | Chapter 4 |