Remote Operation
Checking 5520A Status
5
5-39
76543210
0
RQS
MSS
ESB MAV EAV ISCB 0 0
Requesting service. The RQS bit is set to 1 whenever bits ESB, MAV, EAV, or ISCB
change from 0 to 1 and are enabled (1) in the SRE. When RQS is 1, the 5520A
asserts the SRQ control line on the IEEE-488 interface. You can do a serial poll to read
this bit to see if the 5520A is the source of an SRQ.
Master summary status. Set to 1 whenever bits ESB, MAV, EAV, or ISCB are 1 and
enabled (1) in the SRE. This bit can be read using the *STB? command in serial
remote control in place of doing a serial poll.
Set to 1 when one or more enabled ESR bits are 1.
Message available. The MAV bit is set to 1 whenever data is available in the 5520A’s
IEEE-488 interface output buffer.
Error available. An error has occurred and an error is available to be read from the
error queue by using the ERR? query.
ISCB One or more enabled ISCR bits are 1.
RQS
MSS
ESB
MAV
EAV
OPER
nn318f.eps
Figure 5-9. Serial Poll Status Byte (STB) and Service Request Enable (SRE)
5-44. Service Request (SRQ) Line
IEEE-488 Service Request (SRQ) is an IEEE-488.1 bus control line that the Calibrator
asserts to notify the controller that it requires some type of service. Many instruments
can be on the bus, but they all share a single SRQ line. To determine which instrument
set SRQ, the Controller normally does a serial poll of each instrument. The calibrator
asserts SRQ whenever the RQS bit in its Serial Poll Status Byte is 1. This bit informs the
controller that the Calibrator was the source of the SRQ.
RS-232 Remote operations using the RS-232 interface emulate the IEEE-488 SRQ line
by sending the SRQSTR string over the serial interface when the SRQ line is set. (See
the SRQSTR command description in Chapter 6 for more information.)
The Calibrator clears SRQ and RQS whenever the controller/host performs a serial poll,
sends *CLS, or whenever the MSS bit is cleared. The MSS bit is cleared only when
ESB, MAV, EAV, and ISCB are 0, or they are disabled by their associated enable bits in
the SRE register being set to 0.
5-45. Service Request Enable Register (SRE)
The Service Request Enable Register (SRE) enables or masks the bits of the Serial Poll
Status Byte. The SRE is cleared at power up. Refer to Figure 5-9 for the bit functions.
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