Lake Shore Cryotronics 121, User Manual

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A command string is issued by the computer and instructs the instru-
ment to perform a function or change a parameter setting. The format is:
<command mnemonic><space><parameter data><terminators>.
Command mnemonics and parameter data necessary for each one is
described in section 4.3. Terminators must be sent with every message
string.
A query string is issued by the computer and instructs the instrument to
send a response. The query format is:
<query mnemonic><?><space><parameter data><terminators>.
Query mnemonics are often the same as commands with the addition of
a question mark. Parameter data is often unnecessary when sending
queries. Query mnemonics and parameter data if necessary is described
in section 4.3. Terminators must be sent with every message string. The
computer should expect a response very soon after a query is sent.
A response string is the instrument's response or answer to a query
string. The response can be a reading value, status report or the present
value of a parameter. Response data formats are listed along with the
associated queries in section 4.3. The response is sent as soon as possible
after the instrument receives the query.
4.2.5 Message Flow Control
It is important to remember that the user program is in charge of the
USB communication at all times. The instrument cannot initiate com-
munication, determine which device should be transmitting at a given
time, or guarantee timing between messages. All of this is the responsi-
bility of the user program.
When issuing commands the user program alone should:
D
Properly format and transmit the command including the termina-
tor as one string
D
Guarantee that no other communication is started for 50 ms after
the last character is transmitted
D
Not initiate communication more than 20 times per second
When issuing queries or queries and commands together, the user pro-
gram should: