Measurement Computing Serial488/4 - Manual (Page 55)

Section 3 Command Descriptions
3.16
Output Query O?
The Output Query command returns the number of characters w aiting to be
transmitted out on the selected serial port ( i. e. the port's serial output buffer ). The
Port Select command (Pn) is used to select the serial port regardless of addressing
mode in use.
O?
Return number of characters in the serial output buffer.
Example 1: Find number of output characters on port 1 using dual primary
addressing mode.
PRINT#1,"OUTPUT08;P1x"
select port 1
PRINT#1,"OUTPUT08;O?" request number of serial output characters
PRINT#1,"ENTER08"
INPUT#2,A$
read the response
PRINT A$
display shows O00010, 10 characters
Example 2: Find number of output characters on port 1 using secondary
addressing mode.
PRINT#1,"OUTPUT0800;P1xO?"
select port 1 and request number of output
PRINT#1,"ENTER0800" characters waiting to be sent
INPUT#2,A$
read the response
PRINT A$
display shows O00033, 33 characters