Lake Shore Model 321 Autotuning Temperature Controller User’s Manual
4-2
Remote Operation
Physical Connection (Continued)
Equipment with Data Communications Equipment (DCE) wiring can be connected to the instrument
with a straight through cable. However, if the interface is for Data Terminal Equipment (DTE), a Null
Modem Adapter is required to exchange the transmit (TxD) and receive (RxD) lines.
The instrument uses drivers to generate the transmission voltage levels required by the RS-232C
standard. These voltages are considered safe under normal operating conditions because of their
relatively low voltage and current limits. The drivers are designed to work with cables up to 50 feet
in length.
4.1.2 Hardware
Support
The Model 321 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 or 1200 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 4.1.5.
4.1.3 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 4-1. 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
Software timing
1 Start, 7 Data, 1 Parity, 1 Stop
Odd
CR(0DH) LF(0AH)
20 commands per second maximum
4.1.4 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.
