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Lake Shore Model 325 Temperature Controller User’s Manual
6-16 Remote Operation
6.2.3 Character Format
A character is the smallest piece of information that can be transmitted by the interface. Each character is 10 bits long
and contains data bits, bits for character timing and an error detection bit. The instrument uses 7 bits for data in the
ASCII format. One start bit and one stop bit are necessary to synchronize consecutive characters. Parity is a method of
error detection. One parity bit configured for odd parity is included in each character.
ASCII letter and number characters are used most often as character data. Punctuation characters are used as delimiters
to separate different commands or pieces of data. Two special ASCII characters, carriage return (CR 0DH) and line feed
(LF 0AH), are used to indicate the end of a message string.
6.2.4 Message Strings
A message string is a group of characters assembled to perform an interface function. There are three types of message
strings commands, queries and responses. The computer issues command and query strings through user programs, the
instrument issues responses. Two or more command strings can be chained together in one communication but they must
be separated by a semi-colon (;). Only one query is permitted per communication but it can be chained to the end of a
command. The total communication string must not exceed 64 characters in length.
A command string is issued by the computer and instructs the instrument 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 6.3. 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 6.3.
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 only to the last
query 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 Section 6.3. 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.
6.2.5 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.