Lakeshore Learning Materials 642 User Manual
Page 75
Lake Shore Model 642 Electromagnet Power Supply User’s Manual
Computer Interface Operation
5-23
Message Strings (Continued)
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 Paragraph 5.3.
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 instruments response or answer to a query string. The instrument will respond to the last query
or queries it receives. 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 Paragraph 5.3. The response is sent as soon as possible after
the instrument receives the query. Typically it takes 10 ms for the instrument to begin the response. Some responses
take longer.
5.2.6
Message Flow Control
It is important to remember that the user program is in charge of the serial communication at all times. 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.
When issuing commands only the user program should:
•
Properly format and transmit the command including terminators as one string.
•
Guarantee that no other communication is started for 50 ms after the last character is transmitted.
•
Not initiate communication more than 20 times per second.
When issuing queries or queries and commands together the user program should:
•
Properly format and transmit the query including terminators as one string.
•
Prepare to receive a response immediately.
•
Receive the entire response from the instrument including the terminators.
•
Guarantee that no other communication is started during the response or for 50 ms after it completes.
•
Not initiate communication more than 20 times per second.
Failure to follow these simple rules will result in inability to establish communication with the instrument or intermittent
failures in communication.
5.2.7
Serial Interface Example Programs
A Visual BASIC program is included to illustrate the serial communication functions of the instrument. Refer to
Paragraph 5.2.7.1 for instructions on how to setup the program. The Visual Basic code is provided in Table 5-8. Refer to
Paragraph 5.2.7.2 for instructions on how to setup the program. A description of operation is provided in Paragraph
5.2.7.2. While the hardware and software required to produce and implement these programs not included with the
instrument, the concepts illustrated apply to almost any application where these tools are available.