Lake Shore Model 218 Temperature Monitor User’s Manual
Remote Operation
6-9
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.
Table 6-2. Serial Interface Specifications
Connector Type:
Connector Wiring:
Voltage Levels:
Transmission Distance:
Timing Format:
Transmission Mode:
Baud Rate:
Handshake:
Character Bits:
Parity:
Terminators:
Command Rate:
DE-9 D-Style Connector
DTE
EIA RS-232C Specified
50 feet maximum
Asynchronous
Half Duplex
300, 1200, 9600
Software timing
1 Start, 7 Data, 1 Parity, 1 Stop
Odd
CR(0DH) LF(0AH)
20 commands per second maximum
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 Paragraph 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 Paragraph 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
Paragraph 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.
