Model 6487 Reference Manual
IEEE-488 Bus Overview
D-5
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.
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 informa-
tion), while the remaining two lines are controlled by accepting devices (the listener or lis-
teners 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
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
