User Manual

2400 Series SourceMeter
®
User’s Manual IEEE-488 Bus Overview D-3
Bus description
The IEEE-488 bus, which is also frequently referred to as the GPIB (General Pur-
pose 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 com-
mands. 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 D-1. Generally, a sys-
tem will contain one controller and a number of other instruments to which the
commands are given. Device operation is categorized into three operators: con-
troller, 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. 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 talker, or communications 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 pri-
mary address. The primary address may have any value between 0 and 31, and is
generally set by rear panel DIP switches or programmed 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 #H20. For example, if the primary address is
#H16, the actual listen address is #H36 (#H36 = #H16 + #H20). In a similar man-
ner, the talk address is obtained by ORing the primary address with #H40. With
the present example, the talk address derived from a primary address of 16 deci-
mal would be #H56 (#H56 = #H16 + #H40).
The IEEE-488 standards also include another addressing mode called secondary
addressing. Secondary addresses lie in the range of #H60-#H7F. Note, however,
that many devices, including the SourceMeter, 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 appro-
priate software can be used to direct the information to the desired location.