Setpoint provides the value communication channel is to be accepted by the RTU. Demodulators detect change used,so that other RTUs A remote-to-master check- back message is derived in the signal,and output do not decode a batch data transfer message. from the RTU point selec- transmitted information The purpose of the first two tion hardware in order to at the receiving end of messages is to prepare the verify that the RTU has RTU to receive the longer- acted correctly in interpret- the communication channel than-normal message. ing the control selection. The third message trans- A master-to-remote execute mits the data and the message is transmitted only upon receipt of a fourth indicates that the data was correctly proper checkback message. received at the RTU. A remote-to-master execute acknowledge message is a positive indication that the desired control Remote-to-Master Data Transfer action was initiated. All remote-to-master data transfer is accomplished with one basic message sequence by using MASTER-TO-REMOTE CONTROL SELECT MESSAGE variations in the field definitions to FUNCTION CODE CONTROL ADDRESS accommodate different types of data. SETPOINT The basic sequence is shown in Figure 7.The following explanatory notes REMOTE-TO-MASTER CHECKBACK MESSAGE apply to Figure 7: FUNCTION CODE CONTROL ADDRESS SETPOINT Message establishment and mes- sage termination fields are not shown. MASTER-TO-REMOTE EXECUTE MESSAGE Function code specifies the type of FUNCTION CODE CONTROL ADDRESS data to be transferred by the RTU. Data identification specifies the REMOTE-TO-MASTER EXECUTE ACKNOWLEDGE MESSAGE amount and type of data request- FUNCTION CODE ed by the master. CONTROL ADDRESS On each message transmitted by Figure 5.Sequence of messages for control the RTU (except for messages containing only current data),it is necessary to retain the transmit- The sequence of messages shown in Figure 5 pro- ted message in a RTU buffer,so that if the master vides additional security by the checkback and execute does not receive the message correctly,it can messages,since undetected errors must occur in the request a retransmission.Otherwise this informa- control,select,checkback,and execute messages in tion would be lost. order to operate the wrong control device.Prior to Three basic types of data are transferred using the transmission of the sequence,a control operator or dispatcher per- forms a similar select-verify-exe- MASTER-TO-REMOTE CONTROL MESSAGE cute-acknowledge sequence via the FUNCTION CODE DATA ADDRESS control console. Batch data transfers from master to remote include such data as REMOTE-TO-MASTER ACKNOWLEDGE MESSAGE parameters for report by exception FUNCTION CODE DATA ADDRESS and parameters for locally con- trolled devices.This type of transfer is accomplished by the sequence MASTER-TO-REMOTE BATCH DATA TRANSFER MESSAGE shown in Figure 6.The following FUNCTION CODE DATA ADDRESS DATA explanatory notes apply to Figure 6: Message establishment and message termination fields REMOTE-TO-MASTER ACKNOWLEDGE MESSAGE are not shown. FUNCTION CODE DATA ADDRESS Special security precautions are required if a party line Figure 6.Sequence of messages for batch data transfer 48 IEEE Computer Applications in Power