24
IEEE 488 BUS EXTENDER AND IEEE 488 FIBER BUS EXTENDER
5.7.11 G
ROUP
E
XECUTE
T
RIGGER
(GET)
This command usually signals a group of devices to begin executing a triggered
action. This allows actions of different devices to begin simultaneously. (IEEE 488
command code: $08.)
5.7.12 T
AKE
C
ONTROL
(TCT)
This command passes bus-control responsibilities from the current Controller to
another device which has the ability to control. (IEEE 488 command code: $09.)
5.7.13 S
ECONDARY
C
OMMAND
G
ROUP
(SCG)
These are any one of the 32 possible commands (0 to 31) in this group. They must
immediately follow a talk or listen address. (IEEE 488 command codes: $60 to
$7F.)
5.7.14 P
ARALLEL
P
OLL
C
ONFIGURE
(PPC)
This tells devices capable of performing a Parallel Poll which data bit they are
to assert in response to a Parallel Poll. (IEEE 488 command code: $05.)
5.7.15 P
ARALLEL
P
OLL
U
NCONFIGURE
(PPU)
This disables all devices from responding to a Parallel Poll. (IEEE 488 command
code: $15.)
5.8 More On Service Requests
Most of the commands covered, both uniline and multiline, are the responsibility
of the Active Controller to send and the bus devices to recognize. Most of these are
taken care of routinely by the interface and are totally transparent to the system
programmer. Other commands are used directly by the user to provide optimum
system control.
Of the uniline commands, SRQ (Service Request) is very important to the test
system, so the software designer has easy access to this line from most devices.
Service Request is the method by which a bus device can signal to the Controller
that an event has occurred. It is similar to an interrupt in a microprocessor-based
system.
Most intelligent bus peripherals have the ability to assert SRQ. A DMM might assert
it when its measurement is complete, if its input is overloaded, or for any of an
assortment of reasons. A power supply might assert SRQ if its output has current
limited. This is a powerful bus feature that removes the burden from the System
Содержание IC095A
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