HP 8112A, Operating, Programming And Servicing Manual

Page: 89 / 406

Hints for solving
Problems that might
occur
Reading the Status Byte
When programming the HP 81 12A it must be considered that the
instrument needs some time to:
•
receive the commands
•
interpret the commands
•
update the Status Byte.
The hardware settling time must also be considered. (See Chapter 2
Specifications
for more details).
When a command is sent to the HP 81 12A, the device places all
received characters into a buffer. Interpretation of the command( s)
starts as soon as a valid terminator is received.
To determine if the buffer is empty or not, read the Status Byte by
conducting a SPOLL. If bit 7 is set (decimal value of the STB is
bigger than 127) then the buffer is not yet empty.
Whenever the status byte is read directly after sending a command,
the HP 81 12A should be given sufficient time to update the status
byte. The time needed, varies with the type and number of the
commands sent. For single commands 100 to 200 ms should be
adequate.
Example:
10 CLEAR 712
20 A = SPOLL (712)
30
40 OUTPUT "WID 10 MS "
!
causes error
50 WAIT . 2
! give HP 8 1 12A t ime to update the STB
60 PRINT " Status Byte : " ; A
70 END
If the wait period in line 50 is too low, the value of the Status Byte
printed in line 60 will be 128. This indicates tlhat the buffer is not
yet empty. An adequate wait period (here 200 ms) will give the HP
81 12A enough time to process the command and update the status
byte. As expected, the value of the status byte printed in line 60 will
therefore be 7
4.
Programming
6-19