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Section 5 Remote Control
5-26
5.5 Status Byte Register
The status byte register consists of device STB and RQS (or MSS) messages.
IEEE 488.1 defines the method of reporting STB and RQS messages, but it does
not define the setting and clearing protocols and STB meaning. IEEE 488.2 de-
fines device status summary messages and MSS transferred to bit 6 along with an
STB in response to the
∗
STB? common query.
5.5.1 ESB and MAV Summary Message
The followings are the explanations of an ESB summary message and an MAV
summary message.
(1) ESB summary message
The ESB (event summary bit) summary message is defined by IEEE 488.2.
It appears in status byte register bit 5. This bit indicates whether one or
more IEEE 488.2 defined events have occurred, with the service request
enable register set to allow events to occur, after the standard event status
register was read or cleared last. The ESB summary message bit becomes
True when at least one event registered in the standard event status register
becomes True with event occurrence enabled. Conversely, the ESB sum-
mary bit becomes False when none of the registered events has occurred
even if event occurrence is enabled.
(2) MAV summary message
The MAV (message available) summary message is defined by IEEE 488.2.
It appears in status byte register bit 4. This bit indicates whether the output
queue is empty. When a device is ready for accepting response messages
from the controller, the MAV summary message bit becomes 1 (True).
When the output queue is empty, this bit becomes 0 (False). This message is
used to synchronize information exchange with the controller. For ex-
ample, the controller can send a query message to the device and wait for the
MAV to become True. The controller can perform another processing
while waiting for a response from the device. If the controller has started
reading the output queue without checking the MAV, all system bus opera-
tions are suspended until a response is received from the device.