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IEEE SIGNALS AND DEFINITIONS
The 16 transmission lines of the IEEE·488 bus are each assigned a specific signal. Table 7-23 gives the
bus group, name, abbreviation and functional description for each of these signals.
LOGIC LEVEL CONVENTION
The "true" or logical "1"is low with common collector type outputs. This allows any device to hold the
bus in the "true" or logical "1" state.
Bu,
Signal
Name
Functional
Group
Abbrev.
Description
Manager
ATN
Attention
The PET (controller) sets this
signal low while it is sending
commands on the data bus.
When ATN is low, only periph-
eral addresses and control
messages are on the data bus.
When ATN is high, only pre·
veiously assigned devices can
transfer data.
Transfer
DAV
Data Valid
When DAV is low, this signi-
fies that data is valid on
data bus.
Manager
EO'
End or
When the last byte of data is
Identify
being transferred, the talker
has the option of setting EOI
low. The PET always sets EOI
low while the last data byte is
being transferred from the
PET.
Manager
'Fe
Interface
The PET sends its internal reo
Clear
set signal aslFC low (true) to
initialize all devices to the idle
state. When PET is switched
on or reset, IFC goes low for
about 100 milliseconds.
Transfer
NDAC
Data Not
This signal is held low (true)
Accepted
by the listener while reading.
When the data byte has been
read, the listener sets NDAC
high. This signals the talker
that data has been accepted.
Transfer
NRFD
Not Ready
When NRFD is low (true),
for Data
one or more listeners are not
ready for the
next
byte of
data. When all devices are
ready, NRFD goes high.
Manager
SRa
Service
Not implemented in BASIC,
Request
but available to the PET user.
Manager
REN
Remote
R EN is held low by the bus
Enable
controller. The PET has a pin
grounded that keeps REN
permanently low.
Table 7.23. IEEE·488 bus signal.
Table continued on next page.
87
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