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802BT/802R Video Test Generator User Guide 101
For example, the following number can be represented in three ways:
• Integer (42)
• Floating point (42.00)
• Scaled floating point (4.200E+01)
Queries
Queries are comprised of a header followed immediately by a question mark (?). If there
are any characters between the query header and the question mark character (including
whitespace), a command error will result. Queries, return a response message upon
completion of execution. A response message is comprised of the requested data
terminated with an NL (ASCII 10) character. For example the following text would be
returned in response to the HRAT? (horizontal rate query) message:
3.1500E+04<NL>
Note that the HRAT? query returns its parameter in exponential form. Possible returned
parameter forms are:
• Integer (3965)
• Exponential (+3.965E+03)
• String ("text string")
Output queue
When a query is executed, the resulting response message is placed in an output queue
where it can be read by the controller. The 802 has an output queue that is 255 bytes long.
When a message is present in the output buffer, the MAV (message available) bit in the
Status Byte register is set. This varies slightly from the 488.2 standard in that the MAV bit
will be set only when at least one complete response message is present in the output
queue. A complete response message consists of response message text and a message
terminator (NL).
Buffer deadlock
Buffer deadlock occurs when the 802 tries to put a response message in the output queue
when the output queue is full and the controller is held off while sending a new message
because the input buffer is full. If deadlock occurs, the 802 will clear its output queue, set
the query error (QYE) bit in the Event Status register and proceed to parse incoming
messages. If any additional queries are requested while in deadlock, those response
messages will be discarded.
The 802 will clear the buffer deadlock when it finishes parsing the current command/query.
The QYE bit will remain set until read with the *ESR? query or cleared with the *CLS
command.