National Instruments GPIB-SCSI-A, Руководство пользователя

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Operation of the GPIB Appendix C
GPIB-SCSI-A User Manual C-2 © National Instruments Corp.
The role of the GPIB Controller can also be compared to the role of the
CPU of a computer, but a better analogy is to the switching center of a city
telephone system.
The switching center (Controller) monitors the communications network
(GPIB). When the center (Controller) notices that a party (device) wants to
make a call (send a data message), it connects the caller (Talker) to the
receiver (Listener).
The Controller addresses a Talker and a Listener before the Talker can send
its message to the Listener. After the message is transmitted, the Controller
may unaddress both devices.
Some bus configurations do not require a Controller. For example, one
device may always be a Talker (called a talk-only device) and there may be
one or more listen-only devices.
A Controller is necessary when the active or addressed Talker or Listener
must be changed. The Controller function is usually handled by a
computer.
If the GPIB-SCSI-A is in S mode, your SCSI host plays all three roles.
• Controller–to manage the GPIB
• Talker–to send data to an attached GPIB device
• Listener–to receive data from an attached GPIB device
If the GPIB-SCSI-A is in G mode, your SCSI host is never the Controller,
as it is considered a GPIB device. It only plays the following two roles:
• Talker–to send data to the GPIB host
• Listener–to receive data from the GPIB host
The Controller-In-Charge and System Controller
Although there can be multiple Controllers on the GPIB, only one
Controller at a time is active or Controller-In-Charge (CIC). Active control
can be passed from the current CIC to an idle Controller. Only one device