Agilent Technologies 6613C, 6634B Determining the Cause of a Service Interrupt, MSS Bit, RQS Bit

Programming the DC Source - 3

The MSS Bit

This is a real-time (unlatched) summary of all Status Byte register bits that are enabled by the Service Request Enable register. MSS is set whenever the dc source has one or more reasons for requesting service. *STB? reads the MSS in bit position 6 of the response but does not clear any of the bits in the Status Byte register.

The RQS Bit

The RQS bit is a latched version of the MSS bit. Whenever the dc source requests service, it sets the SRQ interrupt line true and latches RQS into bit 6 of the Status Byte register. When the controller does a serial poll, RQS is cleared inside the register and returned in bit position 6 of the response. The remaining bits of the Status Byte register are not disturbed.

The MAV Bit and Output Queue

The Output Queue is a first-in, first-out (FIFO) data register that stores dc source-to-controller messages until the controller reads them. Whenever the queue holds one or more bytes, it sets the MAV bit (4) of the Status Byte register.

Determining the Cause of a Service Interrupt

You can determine the reason for an SRQ by the following actions:

Step 1	Determine which summary bits are active. Use:
	*STB? or serial poll
Step 2	Read the corresponding Event register for each summary bit to determine which events
	caused the summary bit to be set. Use:
	STATus:QUEStionable:EVENt?
	STATus:OPERation:EVENt?
	ESR?
	When an Event register is read, it is cleared. This also clears the corresponding
	summary bit.
Step 3	Remove the specific condition that caused the event. If this is not possible, the event
	may be disabled by programming the corresponding bit of the status group Enable
	register or NTRPTR filter. A faster way to prevent the interrupt is to disable the service
	request by programming the appropriate bit of the Service Request Enable register

Servicing Operation Status and Questionable Status Events

This example assumes you want a service request generated whenever the dc source switches to the CC (constant current) operating mode, or whenever the dc source’s overvoltage, overcurrent, or overtemperature circuits have tripped. From figure 3-7, note the required path for a condition at bit 10

(CC)of the Operation Status register to set bit 6 (RQS) of the Status Byte register. Also note the required path for Questionable Status conditions at bits 0, 1, and 4 to generate a service request (RQS) at the Status Byte register. The required register programming is as follows:

Step 1	Program the Operation Status PTR register to allow a positive transition at bit 10 to be
	latched into the Operation Status Event register, and allow the latched event to be
	summed into the Operation summary bit. Use:
	STATus:OPERation:PTR 1024;ENABle 1024
Step 2	Program the Questionable Status PTR register to allow a positive transition at bits 0,
	1, or 4 to be latched into the Questionable Status Event register, and allow the latched

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