2.8.3 Error control/flow control Fundamentals of SCADA communications 45 For a half-duplex exchange of information frames, error control is by means of sequence numbers. Each end maintains a transmit sequence number and a receive sequence number. When a node successfully receives a frame, it responds with a supervisory frame containing a receiver ready (RR) indication and a receive sequence number. The number is that of the next frame expected, thus acknowledging all previous frames. If the receiving node responds with a negative acknowledgment (REJ) frame, the transmitter must transmit all frames from the receive sequence number in the REJ frame. This happens when the receiver detects an out-of-sequence frame. It is also possible for selective retransmission to be used. In this case the receiver would return a selection rejection frame containing only the sequence number of the missing frame. A slightly more complex approach is required for a point-to-point link using asynchronous balanced mode with full-duplex operation, where information frames are transmitted in two directions at the same time. The same philosophy is followed as for half-duplex operation except that checks for correct sequences of frame numbers must be maintained at both ends of the link. Flow control operates on the principle that the maximum number of information frames awaiting acknowledgment at any time is seven. If seven acknowledgments are outstanding, the transmitting node will suspend transmission until an acknowledgment is received. This can be either in the form of a receiver ready supervisory frame, or piggybacked in an information frame being returned from the receiver. If the sequence numbers at both ends of the link become so out of sequence that the number of frames awaiting acknowledgment exceeds seven, the secondary node transmits a frame reject or a command reject frame to the primary node. The primary node then sets up the link again, and on an acknowledgment from the secondary node, both sides reset all the sequence numbers and commence the transfer of information frames. It is possible for the receiver to run out of buffer space to store messages. When this happens it will transmit a receiver not ready (RNR) supervisory frame to the primary node to instruct it to stop sending any more information frames. 2.8.4 Modbus protocol General overview The Modbus transmission protocol was developed by Gould Modicon (now Schneider) for process control systems. In contrast to the many other buses discussed, no interface is defined. The user can therefore choose between EIA-232, EIA-422, EIA-485 or 20 mA current loop, all of which are suitable for the transmission rates which the protocol defines. Although Modbus is relatively slow in comparison with other buses, it has the advantage of wide acceptance among instrument manufacturers and users. About 20 to 30 manufacturers produce equipment with the Modbus protocol and many systems are in industrial operation. It can therefore be regarded as a de facto industrial standard with proven capabilities. A recent survey in the well-known American Control Engineering magazine indicated that over 40% of industrial communication applications use the Modbus protocol for interfacing. Besides the standard Modbus protocol, there is another Modbus protocol, called Modbus Plus.
46 Practical Modern SCADA Protocols: DNP3, 60870.5 andRelatedSystems The Modbus is accessed on the master/slave principle, the protocol providing for one master and up to 247 slaves. Only the master initiates a transaction. Transactions are either a query/response type where only a single slave is addressed, or a broadcast/no response type where all slaves are addressed. A transaction comprises a single query and single response frame or a single broadcast frame. Certain characteristics of the Modbus protocol are fixed, such as frame format, frame sequences, handling of communications errors and exception conditions and the functions performed. Other characteristics are selectable. These are transmission medium, transmission characteristics and transmission mode, RTU or ASCII. The user characteristics are set at each device and cannot be changed when the system is running. The Modbus protocol provides frames for the transmission of messages between master and slaves. The information in the message is the address of the intended receiver, what the receiver must do, the data needed to perform the action and a means of checking errors. The slave reads the messages, and if there is no error it performs the task and sends a response back to the master. The information in the response message is the slave address, the action performed, the result of the action and a means of checking errors. If the initial message was of a broadcast type, there is no response from the slaves. Normally, the master can send another query as soon as it has received the response message. A timeout function ensures that the system still functions when the query is not received correctly. Data can be exchanged in two transmission modes: • ASCII – readable, used e.g. for testing • RTU – compact and faster; used for normal operation (hex) The RTU mode (sometime also referred to as Modbus-B for Modbus Binary) is the preferred Modbus mode and will be discussed in this section. The ASCII transmission mode has a typical message that is about twice the length of the equivalent RTU message. The Modbus also provides an error check for transmission and communication errors. Communication errors are detected by character framing, a parity check, a redundancy check or CRC. The latter varies depending on whether the RTU or ASCII transmission mode is being used. Modbus functions All functions supported by the Modbus protocol are identified by an index number. They are designed as control commands for field instrumentation and actuators and are as follows: • Coil control commands for reading and setting a single coil or a group of coils • Input control commands for reading input status of a group of inputs • Register control commands for reading and setting one or more holding registers • Diagnostics test and report functions • Program functions • Polling control functions • Reset
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