Serial Interface (Modbus RTU)M-Max Series Adjustable Frequency Drive MN04020003E—October 2013 www.eaton.com 131Serial Interface (Modbus RTU)General Information About ModbusModbus is a centrally polled bus system in which a so-called master (PLC) controls the entiredata transfer on the bus. Cross-traffic between the individual slaves is not possible.Each data exchange is initiated only on request of the master. Only one request can be issuedon the cable. A slave cannot initiate a transfer but only react to a request with a response.Two types of dialog are possible between master and slave:● The master sends a request to a slave and waits for a response● The master sends a request to all slaves and does not wait for a response (broadcast)More information on Modbus can be found under www.modbus.org.Communications in a Modbus NetworkModbus Network with M-MaxThe figures shows a typical arrangement with a host computer (master) and any number(maximum 31 slaves) of frequency inverter M-Max (slaves). Each frequency inverter has aunique address in the network. This addressing is executed individually for each M-Max viasystem parameter S2.3 and is independent of the physical connection (position) in the network.Serial Interface A-BThe electrical connection between master and the slavesconnected in parallel is implemented via the serial interfaceA-B (A = negative, B = positive) with a shielded RS485twisted pair cable.The position of the connection terminals in the M-Max forthe serial interface A-B (see figure on Page 45).The M-Max’s built-in RS-485 port supports the Modbus RTUprotocol and therefore allows a direct network connectionwithout an additional interface module.The network cable must be provided at each physical end(last station) with a bus termination resistor (120 ohms) inorder to prevent signal reflections and the resulting transfererrors. This necessary resistor is already integrated in theM-Max frequency inverter and is switched on viamicroswitch S4 (see figure on Page 45).Host ComputerA B A B A BS4120 ohms1 2 nS4120 ohmsS4120 ohmsRS485– TermRS485– TermRS485– Term
