HP 8924C - Page 203

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S:\HP8924C\USRGUIDE\BOOK\chapters\hpibstat.fb
Chapter 4, Status Reporting
Status Reporting
Reading the Status Byte Register
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 20 on
page 202
. 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.
When read with the Serial Poll (SPOLL) command, bit 6 represents the RQS
condition.
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 108. 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 20 on page 202. 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.
When read with the *STB? Common Command, bit 6 represents the Master
Summary Status (MSS) message.