Ieee-488 Bus Lines; 4-3-L Bus Management Lines - Keithley 776 Instruction Manual

IEEE-488 Operation
Figure
4-1. IEEE Bus Conjiguration
like an individual is trying to select a single conversa-
tion out of a large crowd.
Before a device can talk or listen, it must be appro-
priately addressed. Devices are selected on the basis of
their primary address. The addressed device is sent a
talk or listen command derived from its primary ad-
dress. Normally, each device on the bus has a unique
primary address so that each may be addressed individu-
ally. The bus also has another addressing mode called
secondary addressing, but not all devices use this ad-
dressing mode.
Once the device is addressed to talk or listen,
appropriate bus transactions may be initiated. For
example, if an instrument is addressed to talk, it will
usually place its data on the bus one byte at a time.
The listening device will then read this information,
and the appropriate software is then be used to chan-
nel the information to the desired location.
4-3. IEEE-488 Bus Lines
The signal lines on the IEEE-488 bus are grouped into
three general categories. The data lines handle bus
information. while the handshake and bus manage-
ment lines assurc that proper data lransfcr and bus
4-2
operation takes place. Each of the bus lines is "active
low" so that approximately zero volts is a logic "one".
The followingparagraphsdescribe the purposeofthese
lines, whichare showninFigure4-1.
4-3-l. Bus Management Lines
The bus management group is made up of five signal
lines that provide orderly transfer of data. These lines
are used to send the uniline commands described in
paragraph 4-S- 1.
1.
ATN
(Attention) - the ATN line is one of the more
important management lines. The state of the ATN line
determines whether controller information on the data
bus is to be considered data or a multiline command as
described in paragraph 4-S-2.
2.
IFC
(Interface Clear) - Setting the IFC line true (low)
causes the bus to go to a known state.
3.
REN
(Remote Enable) - Setting the REM line low
sends the REM command. This sets up instruments on
the bus for remote operation.
4.
EOI
(End Or Identify) - The EOI line is used to send
the EOI command that usually terminates a multi-byte
transfer sequence.
5.
SRQ
(Service Request) - the SRQ line
is
set low by
a device when it requires service from the controller.