Lake Shore 321-01 User Manual
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Lake Shore Model 321 Autotuning Temperature Controller User’s Manual4-2 Remote OperationPhysical Connection (Continued)Equipment with Data Communications Equipment (DCE) wiring can be connected to the instrumentwith a straight through cable. However, if the interface is for Data Terminal Equipment (DTE), a NullModem 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-232Cstandard. These voltages are considered safe under normal operating conditions because of theirrelatively low voltage and current limits. The drivers are designed to work with cables up to 50 feetin length.4.1.2 Hardware SupportThe Model 321 interface hardware supports the following features. Asynchronous timing is used forthe individual bit data within a character. This timing requires start and stop bits as part of eachcharacter so the transmitter and receiver can resynchronized between each character. Half duplextransmission allows the instrument to be either a transmitter or a receiver of data but not at the sametime. Communication speeds of 300 or 1200 baud are supported. The Baud rate is the only interfaceparameter that can be changed by the user.Hardware handshaking is not supported by the instrument. Handshaking is often used to guaranteethat 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 programsmust take full responsibility for flow control and timing as described in Paragraph 4.1.5.4.1.3 Character FormatA character is the smallest piece of information that can be transmitted by the interface. Eachcharacter 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 necessaryto synchronize consecutive characters. Parity is a method of error detection. One parity bit configuredfor odd parity is included in each character.ASCII letter and number characters are used most often as character data. Punctuation charactersare used as delimiters to separate different commands or pieces of data. Two special ASCIIcharacters, carriage return (CR 0DH) and line feed (LF 0AH), are used to indicate the end of amessage string.Table 4-1. Serial Interface SpecificationsConnector Type:Connector Wiring:Voltage Levels:Transmission Distance:Timing Format:Transmission Mode:Baud Rate:Handshake:Character Bits:Parity:Terminators:Command Rate:RJ-11 ConnectorDTEEIA RS-232C Specified50 feet maximumAsynchronousHalf Duplex300, 1200Software timing1 Start, 7 Data, 1 Parity, 1 StopOddCR(0DH) LF(0AH)20 commands per second maximum4.1.4 Message StringsA message string is a group of characters assembled to perform an interface function. There arethree types of message strings commands, queries and responses. The computer issues commandand query strings through user programs, the instrument issues responses. Two or more commandstrings 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. |
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