Keithley 6482, Справочное руководство, Страница: 314 / 350

6482-901-01 Rev. A / August 2012 Return to
Section Topics
C-5
Model 6482 Dual-Channel Picoammeter / Voltage Source Reference Manual Appendix C: IEEE-488 Bus Overview
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 line is one of the more important management lines. The state of this
line determines how information on the data bus is to be interpreted.
IFC (Interface Clear): As the name implies, 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.
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 devices when they require service from the
controller.
Handshake lines
The bus handshake lines operate in an interlocked sequence. This method ensures reliable data
transmission regardless of the transfer rate. Generally, data transfer will occur at a rate determined
by the slowest active device on the bus.
One of the three handshake lines is controlled by the source (the talker sending information), while
the remaining two lines are controlled by accepting devices (the listener or listeners receiving the
information). The three handshake lines are:
DAV (DATA VALID) : The source controls the state of the DAV line to indicate to any listening
devices whether or not data bus information is valid.
NRFD (Not Ready For Data) : The acceptor controls the state of NRFD. It is used to signal to the
transmitting device to hold off the byte transfer sequence until the accepting device is ready.
NDAC (Not Data Accepted) : NDAC is also controlled by the accepting device. The state of NDAC
tells the source whether or not the device has accepted the data byte.
The complete handshake sequence for one data byte is shown in Figure C-2 . Once data is placed
on the data lines, the source checks to see that NRFD is high, indicating that all active devices are
ready. At the same time, NDAC should be low from the previous byte transfer. If these conditions
are not met, the source must wait until NDAC and NRFD have the correct status. If the source is a
controller, NRFD and NDAC must be stable for at least 100 ns after ATN is set true. Because of the
possibility of a bus hang up, many controllers have time-out routines that display messages in
case the transfer sequence stops for any reason.
Once all NDAC and NRFD are properly set, the source sets DAV low, indicating to accepting
devices that the byte on the data lines is now valid. NRFD will then go low, and NDAC will go high
once all devices have accepted the data. Each device will release NDAC at its own rate, but NDAC
will not be released to go high until all devices have accepted the data byte.
The sequence just described is used to transfer both data, talk, and listen addresses, as well as
multiline commands. The state of the ATN line determines whether the data bus contains data,
addresses, or commands as described in
“Bus commands,” page C-6 .