Lake Shore Cryotronics Measure Ready M91 FastHall, User Manual

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4.2.7 Terminators All data in a given SCPI message is encoded in the American Standard Code for Infor-
mation Interchange (ASCII) format. A special ASCII character, the line feed (hex 0A,
decimal 10), is required by the instrument to know where the SCPI message ends. The
instrument also allows an optional carriage return (hex 0D, decimal 13) to precede
the line feed.
4.2.8 Message Flow
Control It is important to remember that the user program is in charge of SCPI communica-
tion at all times, independent of the communications bus used to control the instru-
ment. The instrument cannot initiate communication, determine which device
should be transmitting at a given time, or guarantee timing between messages. All of
this is the responsibility of the user program.
Most commands sent to the M91 FastHall™ measurement controller will execute
within 100 ms of the last character being transmitted. However, the SCPI command
protocol does not specify that an instrument return an acknowledge message when a
given command is finished executing. Because of this, a user program could poten-
tially flood and overload the instruments SCPI buffer.
Therefore, it is highly recommended to append the Operation Complete Query
(*OPC?) to the end of any given command string. The operation complete query is a
SCPI mandated IEEE 488.2 query that will cause the instrument to send a “1” back to
the user program when all commands have been processed. This scheme closes the
timing loop and provides the proper balance for a user program to maximize com-
mand throughput yet not risk overrunning the instrument SCPI buffer. *OPC? can be
appended onto a single command or a chained SCPI message.
The following example sets digital output 1 on, sets the analog output to 10 V, and
waits for “1” to be returned when the commands have been processed:
DIGital:OSETting 1;: ANALOG:OUTPUT:SETTING 10;*OPC? 1
4.2.9 Output Types The results can be outputted in JSON format or standard SCPI format. JSON
(Javascript Object Notation) is a common way of serializing data with a human read-
able output. For more information, see
http://www.json.org . Standard SCPI results
follow the format defined in the SCPI-99 standard:
http://www.ivifoundation.org/docs/scpi-99.pdf .