Lake Shore 460, User Manual

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Lake Shore Model 460 Gaussmeter User’s Manual
Remote Operation
4-15
4.2.2 Hardware Support
The Model 460 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 or 9600 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 4.2.5.
4.2.3 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 4-4. Serial Interface Specifications
Connector Type:
Connector Wiring:
Voltage Levels:
Transmission Distance:
Timing Format:
Transmission Mode:
Baud Rate:
Handshake:
Character Bits:
Parity:
Terminators:
Command Rate: RJ-11 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
4.2.4 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.