National Instruments GPIB-SCSI-A, Руководство пользователя

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© National Instruments Corp. C-1 GPIB-SCSI-A User Manual
Appendix C
Operation of the GPIB
This appendix describes the operation of the GPIB.
Communication among interconnected GPIB devices is achieved by passing
messages through the interface system.
Types of Messages
The GPIB carries device-dependent messages and interface messages.
• Device-dependent messages, often called data or data messages,
contain device-specific information such as programming instructions,
measurement results, machine status, and data files.
• Interface messages manage the bus itself. They are usually called
commands or command messages. Interface messages perform such
tasks as initializing the bus, addressing and unaddressing devices, and
setting device modes for remote or local programming.
The term command as used here should not be confused with some device
instructions which can also be called commands. Such device-specific
instructions are actually data messages.
Talkers, Listeners, and Controllers
A Talker sends data messages to one or more Listeners. The Controller
manages the flow of information on the GPIB by sending commands to all
devices.
Devices can be Listeners, Talkers, and/or Controllers. A digital voltmeter,
for example, is a Talker when sending measurements and is a Listener when
receiving programming messages.
The GPIB is a bus like an ordinary computer bus, except that the computer
has its circuit cards interconnected via a backplane bus, whereas the GPIB
has stand-alone devices interconnected via a cable bus.