Keithley 486 Instruction Manual page 223

APPENLHXE
IEEE-488 Bus Over&w
iiD &iTROL
E
--
/ Y-
DATA BYTE
TRANSFER
CONTROL
GENERAL
lNTERF*CE
MANAGEMENT
Figure E-l. IEEE-488 Bus Configuration
ple, if the primary address is 22 de&nal($16), the actual
listenaddressis$36($36=$16+$20).Inasimilarmanner,
the talk address is obtained by ORing the primary ad-
dress with $40. With the present example, the talk ad-
dress derived from a primary address of 22 decimal
would be $56 ($56 = $16 + 540).
The IEEE-488 standards also include another addressing
mode called secondary addressing. Secondary addresses
lie in the range of 564%57F. Note, however, that many de-
vices, including theMode 486/487, do not use secondary
addressing.
1
Once a device is addressed to talk or listen, the appropri-
ate bus transactions take place. For example: if theinstz'u-
ment is addressed to talk, it places its data string on the
bus one byte at a time. The controller reads the informa-
tion 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 approxjmately
zero volts representing a logic 1 (true). The following
pamgrephs describe the operation of these lines.
Data Lines
The IEEE-488 bus uses eight data lines that transfer data
one byte at a time. DIOl (Data Input/Output) through
DIO8 (Data Input/Output) are the eight data lines used
to bn.mit both data and mubiline commands and are
bidirectional. The data lines operate with low true logic.
Bus Management Lines
The five bus management lines help to ensure proper in-
terface control and management. These lines are used to
send the uniline commands.
ATN (Attention) -The ATN Line is one of the more im-
portent management lines in that the state of this line de
term&s how information on the data bus is to be inter-
preted.
IFC (Interface Clear) -As the name implies, the IFC line
controls clearing of instnunents from the bus.
REN(RemoteEnable)-TheRENlineisused toplacethe
instrument on the bus in the remote mode.
EOI (End or Identify) -The EOI is usually used to mark
the end of a multi-byte data transfer sequence.
SRQ (Service Request) - This line is used by device
when they require service from the controller.
E-2