7.8.8 Coil Register AddressingIn Modbus, all data are organised in coils and holding registers. Coils hold a single bit, whereas holding registers hold a 2byte word (16 bits). All data addresses in Modbus messages are referenced to zero. The first occurrence of a data item isaddressed as item number zero. For example: The coil known as ‘coil 1’ in a programmable controller is addressed as coil0000 in the data address field of a Modbus message. Coil 127 decimal is addressed as coil 007EHEX (126 decimal).Holding register 40001 is addressed as register 0000 in the data address field of the message. The function code fieldalready specifies a ‘holding register’ operation. Therefore, the ‘4XXXX’ reference is implicit. Holding register 40108 isaddressed as register 006BHEX (107 decimal).Coil number Description Signal direction1–16 Frequency converter control word (see Table 7.13) Master to follower17–32 Frequency converter speed or set-point reference Range 0x0–0xFFFF (-200% ...~200%) Master to follower33–48 Frequency converter status word (see Table 7.13) Follower to master49–64 Open loop mode: Frequency converter output frequency Closed loop mode:Frequency converter feedback signal Follower to master65 Parameter write control (master to follower)Master to follower0 = Parameter changes are written to the RAM of the frequency converter1 =Parameter changes are written to the RAM and EEPROM of the frequencyconverter.66-65536 ReservedTable 7.12 Coils and Holding RegistersRS-485 Installation and Set... Design GuideMG20Z202 Danfoss A/S © Rev. 06/2014 All rights reserved. 1577 7
