Model 6220/6221 Reference Manual IEEE-488 Bus Overview C-5
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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 man-
agement. 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.
EOI (End or Identify) — The EOI line is used to mark the end of a multi-byte data
transfer sequence.
SRQ (Service Request) — The SRQ 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.
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