Lake Shore Model 340 Temperature Controller User’s Manual
9-14
Remote Operation
9.2.3 Hardware
Support
The Model 340 interface hardware supports the following features. Asynchronous timing is used for the
individual bit data within a character. This timing requires start and stop bits as part of each character so the
transmitter and receiver can resynchronized between each character. Half duplex transmission allows the
instrument to be either a transmitter or a receiver of data but not at the same time. Communication speeds of
300, 1200, 2400, 4800, 9600, 19200 Baud are supported. The Baud rate is the only interface parameter that
can be changed by the user.
Hardware handshaking is not supported by the instrument. Handshaking is often used to guarantee that data
message strings do not collide and that no data is transmitted before the receiver is ready. In this instrument
appropriate software timing substitutes for hardware handshaking. User programs must take full responsibility
for flow control and timing as described in Paragraph 9.2.6.
9.2.4 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 9-4. Serial Interface Specifications
Transmission: Three-Wire
Connector: RJ-11 Modular Socket
Timing Format: Asynchronous
Transmission Mode: Half Duplex
Data Interface Levels: Transmits and Receives Using EIA Voltage Levels
BPS Rate: 300, 1200, 2400, 4800, 9600, 19200
Parity:
7 Data, Odd Parity, and 1 Stop
7 Data, Even Parity, and 1 Stop
8 Data, No Parity, and 1 Stop
Terminators: CR/LF, LF/CR, CR, or LF
9.2.5 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 9.3.
Terminators must be sent with every message string.