5-5
5.2 Status Byte (STB) Register
5.2 Status Byte (STB) Register
The STB register consists of device STB and RQS (or MSS) messages. The
IEEE488.1 standard defines the method of reporting STB and RQS messages, but not
the setting and clearing of protocols or the meaning of STB. The IEEE488.2 standard
defines the device status summary message and the Master Summary Status (MSS)
which is sent to bit 6 together with STB in response to an *STB? common query.
5.2.1 ESB and MAV summary messages
The following describes the ESB and MAV summary messages.
(1) ESB summary messages
The ESB (Event Summary Bit) summary message is a message defined by
IEEE488.2, and is represented by bit 5 of the STB register. This bit indicates
whether at least one of the events defined in IEEE488.2 has occurred when the
service request enable register is set to enable events after the final reading or
clearing of the standard event register.
The ESB summary message bit becomes 1 when the setting permits events to
occur if any of the events recorded in the standard event status register becomes
1. The ESB summary bit becomes true when the setting permits events to occur
if any of the events registered in the standard event status register is true. Con-
versely, it is false if none of the recorded events occurs even if events are set to
occur.
This bit becomes FALSE (0) when the ESR register is read by the *ESR? query
and the ESR register is cleared by the *CLS command.
(2) MAV summary messages
The MAV summary message is a message defined in IEEE488.2 and repre-
sented by bit 4 in the STB register. This bit indicates whether the output queue
is empty. The MAV summary message bit is set to 1 (true) when a device is
ready to receive a request for a response message from the controller and to 0
(false) when the output queue is empty. This message is used to synchronize the
exchange of information with the controller. For example, this message can be
used to make the controller wait until MAV is true after it sends a query com-
mand to the device. While the controller is waiting for a response from the
device, it can process other jobs. Reading the output queue without first check-
ing MAV delay all system bus operations until the device responds.
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