Status Structure 14-9
Service Request Enable Register
The generation of a service request is controlled by the Service Request Enable Register.
This register is programmed by you and is used to enable or disable the setting of bit B6 (RQS/
MSS) by the Status Summary Message bits (B0, B2, B3, B4, B5, and B7) of the Status Byte
Register. As shown in Figure 14-3, the summary bits are logically ANDed (&) with the corre-
sponding enable bits of the Service Request Enable Register. When a set (1) summary bit is
ANDed with an enabled (1) bit of the enable register, the logic “1” output is applied to the input
of the OR gate and, therefore, sets the MSS/RQS bit in the Status Byte Register.
The individual bits of the Service Request Enable Register can be set or cleared by using the
*SRE common command. To read the Service Request Enable Register, use the *SRE? query
command. The Service Request Enable Register clears when power is cycled or a parameter
value of 0 is sent with the *SRE command (i.e. *SRE 0). The commands to program and read
the SRQ Enable Register are listed in Table 14-3.
Serial polling and SRQ
Any enabled event summary bit that goes from 0 to 1 will set bit B6 and generate an SRQ
(service request). In your test program, you can periodically read the Status Byte to check if an
SRQ has occurred and what caused it. If an SRQ occurs, the program can, for example, branch
to an appropriate subroutine that will service the request.
Typically, SRQs are managed by the serial poll sequence of the SourceMeter. If an SRQ
does not occur, bit B6 (RQS) of the Status Byte Register will remain cleared, and the program
will simply proceed normally after the serial poll is performed. If an SRQ does occur, bit B6 of
the Status Byte Register will set, and the program can branch to a service subroutine when the
SRQ is detected by the serial poll.
The serial poll automatically resets RQS of the Status Byte Register. This allows subsequent
serial polls to monitor bit B6 for an SRQ occurrence generated by other event types. After a
serial poll, the same event can cause another SRQ, even if the event register that caused the first
SRQ has not been cleared.
The serial poll does not clear MSS. The MSS bit stays set until all Status Byte summary bits
are reset.
SPE, SPD (serial polling)
The SPE, SPD General Bus Command sequence is used to serial poll the SourceMeter.
Serial polling obtains the serial poll byte (status byte). Typically, serial polling is used by the
controller to determine which of several instruments has requested service with the SRQ line.
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