Advantech PCLS-848-P, User Manual

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CHAPTER 7 THEORY OF OPERATION 83
CHAPTER 9. BUS TUTORIAL
9.1. General Bus Description
The IEEE-488 bus is a carefully defined instrumentation interface which simplifies the
integration of instruments, peripherals and computers into systems. It minimizes
compatibility problems between devices and has sufficient flexibility to accommodate
future products. The bus has been formally accepted by the International Electrotech-
nical Commission (I.E.C.), as an international standard, and by the Institute of
Electrical and Electronic Engineers (I.E.E.E.) as an American standard.
The lEEE-488 bus employs a 16 line to interconnect up to 15 instruments. This bus is
normally the sole communication link e between the interconnected units. Each
instrument in the bus is Econnected in parallel to the 16 lines of the bus. Eight of the
lines are used to transmit data and the remaining eight are used for communication
timing and control.
Data is transmitted on the eight data lines as a series of eight- bit characters referred to
as “bytes”. Normally, a seven-bit ASCII (American Standard Code for Information
Interchange) code is used E with the eighth bit available for a parity check, if desired.
Data is E transferred by means of an interlocked “handshake” technique. e This
sequence permits asynchronous communication over a wide range of data rates.
Communication between devices on the IEEE-488 bus employs the E’ three basic
functional elements listed below. Every device on the e bus must be able to perform at
least one of these functions:
1) LISTENER. A device capable of receiving data from other devices. Typical
listeners are printers, programmable power E supplies, programmable signal
generators and the like.
2) TALKER. A device capable of transmitting data to other 2, devices. Typical
talkers are voltmeters, counters, audio analy zers and many other measurement
instruments.