Lake Shore 625, User Manual

Page: 68 / 132

Lake Shore Model 625 Superconducting MPS User’s Manual
5-14 Computer Interface Operation
5.1.4.4.4 Using Status Byte Query (*STB?)
The Status Byte Query (*STB?) command is similar to a Serial Poll except it is processed like any other instrument
command. The *STB? command returns the same result as a Serial Poll except that the Status Byte bit 6 (RQS/MSS) is
not cleared. In this case bit 6 is considered the MSS bit. Using the *STB? command does not clear any bits in the Status
Byte Register.
5.1.4.4.5 Using the Message Available (MAV) bit
Status Byte summary bit 4 (MAV) indicates that data is available to read into your bus controller. This message may be
used to synchronize information exchange with the bus controller. The bus controller can, for example, send a query
command to the Model 625 and then wait for MAV to set. If the MAV bit has been enabled to initiate an SRQ, the user’s
program can direct the bus controller to look for the SRQ leaving the bus available for other use. The MAV bit will be
clear whenever the output buffer is empty.
5.1.4.4.6 Using Operation Complete (*OPC) and Operation Complete Query (*OPC?)
The Operation Complete (*OPC) and Operation Complete Query (*OPC?) are both used to indicate when pending
device operations complete. However, the commands operate with two distinct methods.
The *OPC command is used in conjunction with bit 0 (OPC) of the Standard Event Status Register. If *OPC is sent as
the last command in a command sequence, bit 0 will be set when the instrument completes the operation that was
initiated by the command sequence. Additional commands may be sent between the instrument and the bus controller
while waiting for the initial pending operation to complete. A typical use of this function would be to enable the OPC bit
to generate an SRQ and include the *OPC command when programming the instrument. The bus controller could then
be instructed to look for an SRQ allowing additional communication with the instrument while the initial process
executes.
The *OPC? query has no interaction with bit 0 (OPC) of the Standard Event Status Register. If the *OPC? query is sent
at the end of a command sequence, the bus will be held until the instrument completes the operation that was initiated by
the command sequence. Additional commands (except *RST) should not be sent until the operation is complete as erratic
operation will occur. Once the sequence is complete a 1 will be placed in the output buffer. This function is typically
used to signal a completed operation without monitoring the SRQ. It is also used when it is important to prevent any
additional communication on the bus during a pending operation.