Lakeshore Learning Materials 642 - Manual (Page 74)

Lake Shore Model 642 Electromagnet Power Supply User’s Manual
5-22 Computer Interface Operation
5.2.3 Hardware Support
The Model 642 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 be 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 9600, 19200, 38400, or 57600 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 5.2.6.
5.2.4 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 5-6. Serial Interface Specifications
Connector Type:
Connector Wiring:
Voltage Levels:
Transmission Distance:
Timing Format:
Transmission Mode:
Baud Rate:
Handshake:
Character Bits:
Parity:
Terminators:
Command Rate: 9-pin D-style connector plug
DTE
EIA RS-232C Specified
50 feet maximum
Asynchronous
Half Duplex
9600, 19200, 38400, 57600
Software timing
1 Start, 7 Data, 1 Parity, 1 Stop
Odd
CR(0DH) LF(0AH)
20 commands per second maximum
5.2.5 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 or query strings can be chained together in one communication but
they must be separated by a semi-colon (;). The total communication string must not exceed 255 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 5.3. Terminators must be
sent with every message string.