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Data lines
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Computer Control lines
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Digital instrument
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Figure 1567 Parallel data transmission
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Data lines 0 1 2 3 4 5 6 7 Handshake lines DAV NRFD NDAC Control lines IFC ATN REN SRQ EOI Interface clear Attention Remote enable Service request End or identify Data valid Not ready for data Not data accepted
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Figure 1568 IEEE 488 bus
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EXAMPLE 1516 ASCII to Binary Data Conversion Over IEEE 488 Bus
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Determine the actual binary data sent by a digital voltmeter over an IEEE 488 bus
Solution
Known Quantities: Digital voltmeter reading, V Find: Binary data sequence Schematics, Diagrams, Circuits, and Given Data: V = 3405 V ASCII conversion
table (Table 146)
Assumptions: Data is encoded in ASCII format Sequence is sent from most to least
signi cant digit
Analysis: Using Table 146, we can tabulate the conversion as follows:
Control character 3 4 0 5
ASCII (hex) 33 2E 34 30 35
Part II
Electronics
The actual binary data sent can therefore be determined by converting the hex ASCII sequence into binary data (see 13): 33 2E 34 30 35 110011 101110 110100 110000 110101
Comments: Note that the ASCII format is not very ef cient: if you directly performed a
base-10 to binary conversion (see 13), only eight bits (plus the decimal point) would be required
In an IEEE 488 bus system, devices may play different roles and are typically classi ed as controllers, which manage the data ow; talkers (eg, a digital voltmeter), which can only send data; listeners (eg, a printer), which can only receive data; and talkers/listeners (eg, a digital oscilloscope), which can receive as well as transmit data The simplest system con guration might consist of just a talker and a listener If more than two devices are present on the bus, a controller is necessary to determine when and how data transmission is to occur on the bus For example, one of the key rules implemented by the controller is that only one talker can transmit at any one time; it is possible, however, for several listeners to be active on the bus simultaneously If the data rates of the different listeners are different, the talker will have to transmit at the slowest rate, so that all of the listeners are assured of receiving the data correctly The set of rules by which the controller determines the order in which talking and listening are to take place is determined by a protocol One aspect of the protocol is the handshake procedure, which enables the transmission of data Since different devices (with different data rate capabilities) may be listening to the same talker, the handshake protocol must take into account these different capabilities Let us discuss a typical handshake sequence that leads to transmission of data on an IEEE 488 bus The three handshake lines used in the IEEE 488 have important characteristics that give the interface system wide exibility, allowing interconnection of multiple devices that may operate at different speeds The slowest active device controls the rate of data transfer, and more than one device can accept data simultaneously The timing diagram of Figure 1569 is used to illustrate the sequence in which the handshake and data transfer are performed:
Data transfer begins 1 4 Data transfer ends 1 NRFD Ready to receive data Data not valid yet 3 2 s NDAC Valid data on data lines 2 6 Time 5 6 0 Data byte accepted 7 0 1 0 1 0 1
Figure 1569 IEEE 488 data transmission protocol
15
Electronic Instrumentation and Measurements
1 All active listeners use the not ready for data (NRFD) line to indicate their state of readiness to accept a new piece of information Nonreadiness to accept data is indicated if the NRFD line is held at zero volts If even one active listener is not ready, the NRFD line of the entire bus is kept at zero volts and the active talker will not transmit the next byte When all active listeners are ready and they have released the NRFD line, it now goes high 2 The designated talker drives all eight data input/output lines, causing valid data to be placed on them 3 Two microseconds after putting valid data on the data lines, the active talker pulls the data valid (DAV) line to zero volts and thereby signals the active listeners to read the information on the data bus The 2- s interval is required to allow the data put on the data lines to reach (settle to) valid logic levels 4 After the DAV is asserted, the listeners respond by pulling the NRFD line back down to zero This prevents any additional data transfers from being initiated The listeners also begin accepting the data byte at their own rates 5 When each listener has accepted the data, it releases the not data accepted (NDAC) line Only when the last active listener has released its hold on the NDAC line will that line go to its high-voltage-level state 6 (a) When the active talker sees that NDAC has come up to its high state, it stops driving the data line (b) At the same time, the talker releases the DAV line, ending the data transfer The talker may now put the next byte on the data bus 7 The listeners pull down the NDAC line back to zero volts and put the byte away Each of the instruments present on the data bus is distinguished by its own address, which is known to the controller; thus, the controller determines who the active talkers and listeners are on the bus by addressing them To implement this and other functions, the controller uses the ve control lines Of these, ATN (attention) is used as a switch to indicate whether the controller is addressing or instructing the devices on the bus, or whether data transmission is taking place: when ATN is logic 1, the data lines contain either control information or addresses; with ATN = 1, only the controller is enabled to talk When ATN = 0, only the devices that have been addressed can use the data lines The IFC (interface clear) line is used to initialize the bus, or to clear it and reset it to a known condition in case of incorrect transmission The REN (remote enable) line enables a remote instrument to be controlled by the bus; thus, any function that might normally be performed manually on the instrument (eg, selecting a range or mode of operation) is now controlled by the bus via the data lines The SRQ (service request) line is used by instruments on the bus whenever the instrument is ready to send or receive data; however, it is the controller who decides when to service the request Finally, the EOI (end or identify) line can be used in two modes: when it is used by a talker, it signi es the end of a message; when it is used by the controller, it serves as a polling line, that is, a line used to interrogate the instrument about its data output Although it was mentioned earlier that the IEEE 488 bus can be used only over distances of up to 20 m, it is possible to extend its range of operation by connecting remote IEEE 488 bus systems over telephone communication lines This can be accomplished by means of bus extenders, or by converting the parallel data to serial form (typically, in RS-232 format) and by transmitting the serial
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