Keysight Technologies VXI bus 75000 C Series Configuration And User's Manual
Also see for VXI bus 75000 C Series: Service guideUser manual
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70 Keysight E8491B IEEE 1394 PC Link to VXI User GuideIEEE 1394 Fundamentals and Interface Overview Using this ChapterAsynchronous Data TransfersDuring an asynchronous data transfer, a variable amount of data is transferred toan explicit address in real time, and an acknowledgement is returned. Data istransferred across the IEEE 1394 bus in packets called “subactions.” Anasynchronous subaction is made up of three parts:– arbitration sequence - the period when a device requests control of thebus in order to transmit a data packet.– data packet - the data packet consists of a data prefix that containsinformation about the transaction, the data itself (e.g. VXI instrumentcommands), and a data end signal. The maximum packet size is 2 kByte for400 Mbit adapters such as the adapter supplied with the E8491B.– acknowledgement - a code returned by the (addressed) data destinationindicating the action taken by the receiver.The periods between subactions are called subaction gaps. The subaction gapallows devices that have not had control of the bus during the current “fairnessinterval” to arbitrate for control.Fair Arbitration ProtocolThe fair arbitration protocol is based on the fairness interval shown in Figure 2. Afairness interval consists of one or more subactions in which data packets aretransferred over the bus. A fairness interval is as follows:1 The interval begins when devices (E8491B’s) arbitrate for control of the bus.2 When a device is granted control, it transfers its data packet and is thendisabled from arbitrating until the next fairness interval.3 A subaction gap occurs after the previous data packet is transferred. Duringthis period, remaining devices arbitrate for the bus. The next device grantedthe bus transfers its data packet and is then disabled from arbitrating until thenext fairness interval.4 The fairness interval ends after each device has had an opportunity to accessto the bus and the arbitration reset gap, which is longer than the subactiongap, occurs. The arbitration reset gap re-enables each device for arbitrationduring the next fairness interval.VXI Data TransfersTo take advantage of the IEEE 1394 data transfer protocol, large amounts of datashould be transferred between VXI instruments and the PC using block transfers.During a block transfer, data is divided into the packets described previously; thenumber of packets depends on the amount of data and whether a 200 Mbit or400 Mbit host adapter is used. Compared to protocols that transfer data onebyte or one word at a time, transfer speed between the instrument and the PC isincreased because the IEEE 1394 protocol overhead is associated with the |
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