Keithley 2182, Руководство пользователя, Страница: 296 / 339

F-4 IEEE-488 Bus Overview
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 $7, the actual listen address is $27 ($27 = $7 + $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 $7 would be $47 ($47 = $7 + $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 2182, 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.
Bus management lines
The five bus management lines help to ensure proper interface control and management.
These lines are used to send the uniline commands.
ATN (Attention) — The ATN state determines how information on the data bus is to be
interpreted.
IFC (Interface Clear) — The IFC line controls clearing of instruments from the bus.
REN (Remote Enable) —The REN line is used to place the instrument on the bus in the
remote mode.