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GPIB
IEEE488
MANUAL
1.1
IEEE488
SYSTEM
BUS
OPERATIONS
The sequence
of actions
for
all
data
transfer
commands on the bus
is as follows:
OUTPUT
INPUT
1. IBM-PC Talk
Address
1. Device
Talk
Address
2. UNLISTEN
2. UNLISTEN
3. Device
Listen
Address
3. IBM-PC Listen
Address
After
the
transfer
of data
is
complete
the
bus remains
in the
last
programmed
condition.
SECONDARY
COHMANDS
When
communicating
to/from
devices
which
use
secondary
addressing,
the
devices
extended
address
is
specified
by
separating
the
primary
address
with
a ".11 (period).
The use of
extended
addressing
or secondary
commands (maximum level
of five)
complies
with
the
1978
IEEE488
standard.
The bus sequence
for
secondary
or extended
addressing
is as follows:
1. IBM-PC Talk
Address
2. Unlisten
3. Device
Primary
Address
4. Secondary
Commands/Address
5. Data
NOT
IN
CONTROL
ADDRESSING
When the IBM-PC is not the active
controller
on the bus,
no other
devices
may be addressed
by the
IBM-PC (all
controller
commands).
The IBM-PC may still
transfer
data
as a talker/listener
if
setup
by the controller
in charge.
During
an ENTER command the
IBM-PC
waits
until
the active
talker
transmits
the data
if
the
IBM-PC
was addressed
by the talker/listener.
If
the
address
codes
are
not satisfied,
ah9000 will
be returned
in the FLG% variable.
This
allows
the
user
to
perform
a program
wait
loop
for
maximum
efficiency
in error
handling.
The OUTPUT statement
waits
until
the controller
addresses
the IBM-PC to talk
and the ENTER command
waits
until
the
controller
addresses
the
IBM-PC to listen.
The
IBM-PC may assume control
if
the
controller
in charge
sends the
Take Control
Message
after
it
programs
the
IBM-PC
as a Talker.
During
a DMA transfer
the
IBM will
wait
until
addressed
before
data
is transferred.
A time
out code is returned
if
the data
is
not
accepted
by the
talker/listener.
The programmer
may use this
return
code for
a time
out
loop
also.
Page 5
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