Status Byte And Service Request (Srq) - Keithley 6517A User Manual

IEEE-488 Reference

3.8.6 Status byte and service request (SRQ)

Service request is controlled by two 8-bit registers; the Status
Byte Register and the Service Request Enable Register. The
structure of these registers is shown in Figure 3-13.
Status Byte Register — The summary messages from the
status registers and queues are used to set or clear the appro-
priate bits (B0, B2, B3, B4, B5 and B7) of the Status Byte
Register. These bits do not latch and their states (0 or 1) are
solely dependent on the summary messages (0 or 1). For ex-
ample, if the Standard Event Status Register is read, its reg-
ister will clear. As a result, its summary message will reset to
0, which in turn will clear the ESB bit in the Status Byte Reg-
ister.
Bit B6 in the Status Byte Register is either:
• The Master Summary Status (MSS) bit, sent in re-
sponse to the *STB? command, indicates the status of
any set bits with corresponding enable bits set.
• The Request for Service (RQS) bit, sent in response to
a serial poll, indicates which device was requesting ser-
vice by pulling on the SRQ line.
Request
Generation
Figure 3-13
Status byte and service request (SRQ)
3-16
Service
* STB?
OSB
Serial Poll (B7)
OR
* SRE
OSB
(B7)
* SRE?
OSB = Operation Summary Bit
MSS = Master Summary Status
RQS = Request for Service
ESB = Event Summary Bit
MAV = Message Available
QSB = Questionable Summary Bit
EAV = Error Available
MSB = Measurement Summary Bit
OR = Logical OR
For description of the other bits in the Status Byte Register,
refer to paragraph 3.11.12.
The IEEE-488.2 standard uses the following common query
command to read the Status Byte Register:
*STB?
When reading the Status Byte Register using the *STB?
command, bit B6 is called the MSS bit. None of the bits in
the Status Byte Register are cleared when using the *STB?
command to read it.
The IEEE-488.1 standard has a serial poll sequence that also
reads the Status Byte Register and is better suited to detect a
service request (SRQ). When using the serial poll, bit B6 is
called the RQS bit. Serial polling causes bit B6 (RQS) to re-
set. Serial polling is discussed in more detail later in this
paragraph (see Serial Poll and SRQ).
Any of the following operations clear all bits of the Status
Byte Register:
• Cycling power.
• Sending the *CLS common command.
Status Summary Messages
RQS
(B6) ESB MAV QSB EAV
(B5) (B4) (B3) (B2) (B1)
MSS
&
&
&
&
&
ESB MAV QSB EAV
(B6) (B5) (B4) (B3) (B2) (B1)
& = Logical AND
Read by Serial Poll
Status Byte
MSB
Register
(B0)
Read by *STB?
&
Service Request
MSB
Enable Register
(B0)