Table 1.Comparison of communication with performed on the data and compared with the asynchronous and synchronous modems received check bits.The message is accepted if they are identical;otherwise,a retransmission of Asynchronous Synchronous the original message is requested. Modem synchro- Separate timing Timing signal A typical example of commonly used asynchronous nizing technique sources at transmitted message format is shown in Figure 4.The efficiency of transmitter and continuousty receiver the example format is 12/32 or 37.5 percent,which is typical for the asynchronous format. Message lengths Usually 32 blts Several hundred The security of this format is provided by the five-bit allowed maximum bits or more BCH code,which detects all single-bit and double-bit Efficlency Best for short Best for long random errors and all bursts (consecutive bit stream. messages messages where first and last bits,as a minimum,are in error)of Data rates 150-9600bp5 five or less.Security codes must also provide protection 2400 bps-1 Mbps from undetected errors caused by false message syn- Modem cost Very low Moderate,but chronization.Since the typical asynchronous format decreasing requires only a mark-to-space transition to signal the Channel require- Unconditioned Unconditioned start of a message,a false start could occur several bits ment voice grade or conditioned prior to a message due to a noise spike.One way to voice grade reduce this problem,commonly called sync slip,is to invert all security code bits in the message.This pro- vides protection equal to one undetected error per 2n Since verifying channel quality is a time-consuming task,it false synchronizations,where n is the number of securi- is wise to buy modems that have the minimum channel ty code bits.With this approach,longer security codes requirements and the most diagnostic features.These fea- provide better protection from sync slip. tures may include loopback to allow end-to-end tests:indi- cator lights for transmit,receive,carrier,request-to-send Master-to-Remote Data Transfer and clear-to-send;a switch to force the modem to a Information transmitted from master to remote is for the known state;and a self-test mode.The self test mode may purpose of device control,setpoint control,or batch data include a remotely controlled digital or analog loopback transfer.Due to the possible severe consequences of oper- with a test signal transmis- sion.Also,plug-in jacks are recommended to quickly connect test equipment 12 BITS BITS BITS BITS BIT Message Formats 8 MULSECOND The transmission of infor- RTU PRETRANSMISSION FUNCTON 8CH ADDRESS SECURITY AODITONAL CODE mation (both directions) MARK CODE MESSAGE移 between the master and RTUs using TDM techniques SYNCHRONIZATION requires the use of serial MESSAGE INFORMA TION MESSAGE digital messages.All mes- ESTA BLISHMENT TERMINA TION sages are divided into three basic parts: Message establish- Figure 4.Typical asynchronous message format ment,which provides the signals to synchronize the receiver and trans- ating the wrong device or receiving a bad control mes- mitter and the unique RTU address sage,additional security is required for control.This is Information,which provides the data in a coded provided in the form of a sequence of messages,common- form to allow the receiver to decode the informa ly called a select-before-operate sequence,as shown in Fig- tion and properly utilize it ure 5.The following explanatory notes apply to Figure 5: Message termination,which provides the message Message establishment and message termination security checks and a means of denoting the end of fields are not shown. the message.Message security checks consist of Function code specifies the operation to be per- logical operations on the data which result in a pre- formed by the RTU. defined number of check bits transmitted with the Control address specifies the device or setpoint to message.At the receiver,the same operations are be controlled. uy199347