Bedienungsanleitung Keithley 2000 (Seite 239 von 256) (Englisch)

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The IEEE-488 bus, which is also frequently referred to a the GPIB (General Purpose In-

terface Bus), was designed as a parallel transfer medium to optimize data transfer without

using an excessive number of bus lines. In keeping with this goal, the bus has only eight

data lines that are used for both data and with most commands. Five bus management lines

and three handshake lines round out the complement of bus signal lines

A typical set up for controlled operation is shown in Figure E-1. Generally, a system will

contain one controller and a number of other instruments to which the commands are given.

Device operation is categorized into three operators: controller, talker and listener. The con

troller does what its name implies; it controls the instruments on the bus. The talker sends

data while a listener receives data. Depending on the type of instrument, any particular de

vice can be a talker only, a listener only or both a talker and listener.

There are two categories of controllers: system controller, and basic controller. Both are

able to control other instruments, but only the system controller has the absolute authority

in the system. In a system with more than one controller, only one controller may be active

at any given time. Certain protocol is used to pass control from one controller to another.

The IEEE-488 bus is limited to 15 devices, including the controller. Thus, any number of

talkers and listeners up to that limit may be present on the bus at one time. Although several

devices may be commanded to listen simultaneously, the bus can have only one active talk

er, or communications would be scrambled.

A device is placed in the talk or listen state by sending an appropriate talk or listen com-

mand. These talk and listen commands are derived from an instrument’s primary address.

The primary address may have any value between 0 and 31, and is generally set by rear

panel DIP switches or programmed in from the front panel of the instrument. The actual lis

ten address value sent out over the bus is obtained by ORing the primary address with $20.

For example, if the primary address is $16, the actual listen address is $36 ($36 = $16 +

$20). In a similar manner, the talk address is obtained by ORing the primary address with

$40. With the present example, the talk address derived from a primary address of 16 dec

imal would be $56 ($56 = $16 + $40).

The IEEE-488 standards also include another addressing mode called secondary ad-

dressing. Secondary addresses lie in the range of $60-$7F. Note, however, that many de-

vices, including the Model 2000, do not use secondary addressing.

Once a device is addressed to talk or listen, the appropriate bus transactions take place.

For example: if the instrument is addressed to talk, it places its data string on the bus one

byte at a time. The controller reads the information and the appropriate software can be used

to direct the information to the desired location.

E-4 IEEE-488 Bus Overview

2000-900 (J - Aug 2010) BOOK.fm Page 4 Wednesday, October 12, 2011 12:29 PM

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