8-5
Status Structure
8
8.2 Status Byte (STB) Register
The STB 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 defines
device status summary messages and the master summary status (MSS) trans-
ferred to bit 6 along with an STB in response to the ∗STB? common query.
8.2.1 ESB and MAV summary message
Let’s take a look at 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 STB 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 summary bit be-
comes 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 STB 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 example, 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 operations are sus-
pended until a response is received from the device.
8.2 Status Byte (STB) Register
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