User`s manual

Chapter 6 125
Advanced Operations
Status Reporting
Reading with a Serial Poll
The contents of the Status Byte Register can be read by a serial poll
from the Active Controller in response to some device on the bus
sending the Service Request (SRQ) message. When read with a serial
poll, bit 6 in the Status Byte Register represents the Request Service
(RQS) condition. Bit 6 is TRUE, logic 1, if the Test Set is sending the
Service Request (SRQ) message and FALSE, logic 0, if it is not. Bits 0-5
and bit 7 are defined as shown in Table 6-1 on page 124. When read by
a serial poll the RQS bit is cleared (set to 0) so that the RQS message
will be FALSE if the Test Set is polled again before a new reason for
requesting service has occurred. Bits 0-5 and bit 7 are unaffected by a
serial poll.
Reading with the *STB? Common Command
The contents of the Status Byte Register can be read by the application
program using the *STB? Common Command. When read with the
*STB? Common Command, bit 6 represents the Master Summary
Status (MSS) message. The MSS message is the inclusive OR of the
bitwise combination (excluding bit 6) of the Status Byte Register and
the Service Request Enable Register. For a discussion of Summary
Messages, see “Status Register Structure Overview” on page 127. Bit 6
is TRUE, logic 1, if the Test Set has at least one reason for requesting
service and FALSE, logic 0, if it does not. Bits 0-5 and bit 7 are defined
as shown in Table 6-1 on page 124. When read by the *STB? Common
Command, bits 0-5, bit 6, and bit 7 are unaffected.
The *STB? Status Byte Query allows the programmer to determine the
current contents (bit pattern) of the Status Byte Register and the
Master Summary Status (MSS) message as a single element. The Test
Set responds to the *STB? query by placing the binary-weighted
decimal value of the Status Byte Register and the MSS message into
the Output Queue. The response represents the sum of the
binary-weighted values of the Status Byte Register’s bits 0-5 and 7
(weights 1,2,4,8,16,32 and 128 respectively) and the MSS summary
message (weight 64). Thus, the response to *STB?, when considered as
a binary value, is identical to the response to a serial poll except that
the MSS message of 1 indicates that the Test Set has at least one
reason for requesting service (Refer to the IEEE 488.2-1987 Standard
for a complete description of the MSS message). The decimal value of
the bit pattern will be a positive integer in the range of 0 to 255. The
response data is obtained by reading the Output Queue into a numeric
variable, integer or real.