Agilent Technologies E1406A, User'S Manual And Scpi Programming Manual

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Status Byte Group
As Figure 4-6 indicates, the Status Byte is used to summarize information
from all the other status groups. The Status Byte differs from the other
groups in the way you read it and how its summary bit is processed.
The Status Byte can be read using either the
*STB?
common command or
by doing a SICL
ireadstb
function call. The
ireadstb
function reads the
Status Byte from the device specified.
The Status Byte Summary bit actually appears in bit 6 (RQS) of the Status
Byte. When bit 6 is set, it generates an SRQ interrupt. This interrupt is a
low-level GPIB message that signals the controller that at least one
instrument on the bus requires attention.
There are some subtle differences between *STB? and ireadstb . You can
use either method to read the state of bits 0-5 and bit 7. Bit 6 is treated
differently depending on whether you use
*STB?
or
ireadstb
. With
ireadstb
,
bit 6 returns RQS (request for service) which is cleared after the first
ireadstb . *STB? returns the MSS (master state summary). This is the
Summary bit of the Status Byte Register. It is like a condition bit and will
return to zero only when all enabled bits in the Status Byte are zero. In
general, use
ireadstb
inside interrupt service routines, not
*STB?
.
Note
In an SRQ interrupt service routine, you must clear the Event Register which
caused the SRQ (for example,
STAT:QUES:EVEN?
,
STAT:OPER:EVEN?
, or
*ESR?
). Failure to do so will prevent future SRQs from arriving.
Figure 4-6. Status Byte Register
Chapter 4 Triggering and System Status 107