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IEEE Bus
Controller (Listener)
Multimeter (Talker)
Signal Generator
IEEE Bus
Controller (Talker)
Multimeter
\x
Signal Generator (Listener)
Figure 6. Messages are transmitted over an IEEE-488 bus between "talker" devices and "listener" devices under direction of the computer, which functions as the bus controller (and, as such, can also be a talker or listener). In this example, a voltage reading from a multimeter is converted to an output signal from the signal generator linked to the same bus
tween instrument and computer in a digital data acquisition system. It defines the 16-line bus (physical cable) linking the instrument and computer (called the controller); the shape, size, and number of pins in the connectors; all voltages; the handshaking procedure (including addressing); and the method of data transfer. Hundreds of instruments having widely diverse applications have been designed to meet the IEEE-488 standard. If the instrument and interface strictly adhere to IEEE-488, researchers can expect full compatibility in method of data transfer at both devices. However, IEEE-488 intentionally doesn't in any way specify the organization of data that are transmitted. The 16 lines in the IEEE bus are shared by the computer and the one or more instruments that are linked to the bus in the data acquisition system. The standard defines each line of the bus and specifies exactly when and how an instrument may use that line. Eight of the lines, called data lines, are for parallel transmission of the ASCII characters sent out on the bus. Three are handshaking lines, and the remaining five lines are for general bus management. IEEE-defined communications software in the instruments and the computer's interface modules provides for exchange of messages on the bus between what are called talkers and listeners. As bus controller, the computer oversees activity on the bus and issues commands designating which instruments talk and which listen at any given time. In Figure 6, for instance, the controller has instructed the multimeter as talker to send the controller as listener a message that reports a voltage reading. The controller, now as talker, sends a message to the signal generator as listener that creates an output signal based on the voltage reading. The talker and listen.
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er exchange handshake signals for every transmitted character in a message. Each instrument on the bus is assigned an address between zero and 30 that the controller uses to identify that instrument when instructing it to talk or listen. Each instrument's address can be set with switches located on the instrument itself. The IEEE-488 standard describes a number of message routines through which instruments communicate over the bus. The most frequently used routines, as might be expected, are SEND and RECEIVE, in which the controller transmits and receives, respectively, messages from a designated instrument. The contents of each message string and the vocabulary of characters that are meaningful are unique to the particular instrument (and are provided by its user's guide).
Joseph Liscouski is product manager, Small Systems and Graphics, for Digital Equipment Corporation. He earned his undergraduate degree at Montclair State College in New Jersey and his MS in computer science at Fairleigh Dickinson University. His research interests include use of small machines in laboratory automation and applications of graphics to understanding laboratory data.
