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Appendix D: IEEE-488 bus overview
Bus description
The IEEE-488 bus, which is also frequently referred to a GPIB (General Purpose Interface 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 setup for controlled operation is shown in the following figure. Generally, a system contains
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 controller 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 device 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. Therefore, 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 talker, or communications are
scrambled.
A device is placed in the talk or listen state by sending an appropriate talk or listen command. 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 listen address value sent out over
the bus is obtained by ORing the primary address with $20. For example, if the primary address is
$27, the actual listen address is $47 ($47 = $27 + $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 27 decimal would be $67 ($67 = $27 + $40).
The IEEE-488 standards also include another addressing mode called secondary addressing.
Secondary addresses lie in the range of $60-$7F. Note, however, that many devices, including the
Keithley Instruments Model 6517B Electrometer, 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.
D-2
Figure 142: IEEE-488 bus configuration
Model 6517B Electrometer Reference Manual
6517B-901-01 Rev. C / August 2015
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