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Table 4-1 shows each of the Status Byte register bits and describes the event that
will set each bit.
Table 4-1. Status Byte Register
Bit
Number
Decimal
Weight Description
0 1 Instrument Specific (not used by most instruments)
1 2 Instrument Specific (not used by most instruments)
2 4 Instrument Specific (not used by most instruments)
3 8 Questionable Data Status Group Summary Bit. One or more events
in the Questionable Data Status group have occurred and set
bit(s) in those registers.
4 16 Message Available. The instrument’s output queue contains
information. This bit can be used to synchronize data exchange
with an external computer. For example, you can send a query
command to the instrument and then wait for this bit to be set.
The GPIB is then available for other use while the program is
waiting for the instrument to respond.
5 32 Standard Event Status Group Summary Bit. One or more enabled
events in the Standard Event Status Register have occurred and
set bit(s) in that register.
6 64 Service Request--Service is requested by the instrument and the
GPIB SRQ line is set true. This bit will be set when any other bit
of the Status Byte Register is set and has been enable to assert
SRQ by the *SRE command.
7 128 Operation Status Group Summary Bit. One or more events in the
Operation Status Group have occurred and set bit(s) in those
registers.
Reading the Status
Byte Register
You can read the Status Byte register using either the *STB? command or an
GPIB serial poll. Both methods return the decimal weighted sum of all set bits in
the register. The difference between the two methods is that *STB? does not
clear bit 6 (Service Request); serial poll does clear bit 6. No other status register
bits are cleared by either method with the exception of the Message Available
bit (bit 4) which may be cleared as a result of reading the response to *STB?. In
addition, using an GPIB serial poll lets you read the status byte without
interrupting the instrument parser. The *STB? method requires the instrument
to process the command. This can generate interrupt query errors if the
instrument is executing another query.
The following program uses the *STB? command to read the contents of the
system instrument’s (Command Module’s) Status Byte register.
10 OUTPUT 70900;"*STB?" Read Status Byte Register
20 ENTER 70900; A Enter weighted sum
30 PRINT A Print weighted sum
40 END
For example, assume bit 3 (weight = 8) and bit 7 (weight = 128) are set. The
above program returns the sum of the two weights (136).
6-4 Controlling Instruments Using GPIB