Hardware Reference
In-Depth Information
Pin number
Abbreviation
Function
8
NDAC
Not data accepted. This signal is asserted by a listener whilst data is being
accepted. When several devices are simultaneously listening, each device releases
this line at its own rate (the slowest device will be the last to release the line).
9
IFC
Interface clear. Asserted by the controller in order to initialize the system in a
known state.
10
SRQ
Service request. This signal is asserted by a device wishing to gain the attention
of the controller. This line is wire - OR'd.
11
ATN
Attention. Asserted by the controller when placing a command on to the bus.
When the line is asserted this indicates that the information placed by the
controller on the data lines is to be interpreted as a command. When it is not
asserted, information placed on the data lines by the controller must be
interpreted as data. ATN is always driven by the active controller.
12
SHIELD
Shield
13
DIO5
Data line 5
14
DIO6
Data line 6
15
DIO7
Data line 7
16
DIO8
Data line 8
17
REN
Remote enable. This line is used to enable or disable bus control (thus
permitting an instrument to be controlled from its own front panel rather than
from the bus).
18-24
GND
Ground/common signal return
Notes:
1
Handshake signals (DAV, NRFD, and NDAC) employ active low open-collector outputs which may be used in a
wired-CR configuration.
2
All remaining signals are fully TTL compatible and are active low (asserted low).
3
Pins 18 to 23 are intended for use with twisted pair grounds for the control signals (DAV, NRFD, etc.) that appear on pins
6 to 11 on the other side of the connector).
Commands
Bus commands are signalled by taking the ATN line low. Commands are
then placed on the bus by the controller and directed to individual devices
by placing a unique address on the lower five data bus lines. Alternatively,
universal commands may be issued to all of the participating devices (see
page 255).
Handshaking
The IEEE-488 bus uses three handshake lines (DAV, NRFD, and NDAC). The
handshake protocol adopted ensures that reliable data transfer occurs at a rate
determined by the slowest listener.
A talker wishing to place data on the bus first ensures that NDAC is in a
released state. This indicates that all of the listeners have accepted the previous
data byte. The talker than places the byte on the bus and waits until NRFD is
released. This indicates that all of the addressed listeners are ready to accept
Figure 8.3
IEEE-488 bus
connector
