Figure F; Bus Description - Keithley 6514 Instruction Manual

Bus description

The IEEE-488 bus, which is also referred to as the 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 Figure F-1. Generally, a system will con-
tain 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 con-
trols 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 sys-
tem. 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 talk-
ers 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 commu-
nications would be 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 $14, the actual listen address is $34 ($34 = $14 + $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 $14 would be $54 ($54 = $14 + $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 Model 6514, 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.
IEEE-488 Bus Overview F-3