G-4 IEEE-488 Bus Overview Model 2700 Multimeter/Switch System User’s Manual
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 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 $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 would be $56 ($56 = $16 + $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 2700, 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.
Bus lines
The signal lines on the IEEE-488 bus are grouped into three different categories: data
lines, management lines, and handshake lines. The data lines handle bus data and
commands, while the management and handshake lines ensure that proper data transfer
and operation takes place. Each bus line is active low, with approximately zero volts
representing a logic 1 (true). The following paragraphs describe the operation of these
lines.
Data lines
The IEEE-488 bus uses eight data lines that transfer data one byte at a time. DIO1 (Data
Input/Output) through DIO8 (Data Input/Output) are the eight data lines used to transmit
both data and multiline commands and are bi-directional. The data lines operate with low
true logic.
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