Serial Interface (Modbus RTU)134 M-Max Series Adjustable Frequency Drive MN04020003E—October 2013 www.eaton.comStructure of the Master RequestAddress:● The address (1 to 255) of the frequency inverter to be sentthe request is entered in parameter S2.3. Only thefrequency inverter with this address can respond to therequest● Address 0 is used as a so-called Broadcast (message to allslaves) from the master. In this mode, individual slavescannot be addressed and data cannot be output from theslavesFunction Code:The function code defines the type of message. Thefollowing operations can be executed on the M-Max:DataThe length of the data block (Data: N x 1 Byte) depends onthe function code. This consists of two hexadecimalcharacter sets between 00 and FF. The data block containsadditional information for the slave in order to carry out theoperation defined by the master in the function code.Example: number of parameters to be processed.CRCThe telegrams in the Modbus RTU have a CRC (CyclicalRedundancy Check). This CRC field consists of two bytesthat contain a binary 16 bit value. The CRC check is alwaysrun independently of the parity check for the individualcharacters of the telegram. The CRC result is attached to theend of the telegram by the master. The slave recalculateswhile receiving the telegram and compares the calculatedvalue with the actual value in the CRC field. If both values arenot identical, an error is set.Structure of the Slave ResponseRequired Transfer Time● The time between receiving a request from the masterand the response of the frequency inverter is at least3.5 characters (rest time)● Once the master has received a response from thefrequency inverter, it must wait for at least the rest timebefore it can send a new requestNormal Slave Response● If the master request contains a write register function(function code 06 or 16), the frequency inverterimmediately returns the request as a response● If the master request contains a read register function(function code 03 or 04), the frequency inverter returns theread data with the slave address and the function code asa responseSlave Response in the Event of an ErrorIf a request contains an error (for example, incorrect dataaddress or incorrect data value) other than a transfer error,the frequency inverter returns an exception message withoutexecuting anything. You can evaluate the exception messageFunction Code(hex) Designation Description03 Read Holding Registers Reading of the holding registers in the slave (process data, parameters, configuration)A master request enables up to 11 registers to be read04 Read Input Registers Reading of the input registers in the slave (process data, parameters, configuration)A master request enables up to 11 registers to be read06 Write Single Register Writing of a holding register in the slaveWith a general telegram (Broadcast) the appropriate holding registers are written in all slavesThe register is read back for comparison10 Write Multiple Register Writing of several holding registers in the slaveWith a general telegram (Broadcast) the appropriate holding registers are written in all slavesA master request enables up to 11 registers to be read
