function code. This can include items such as coil or register addresses, the quantity of items to be handled, and the count of actual data bytes in the

field.

7.8.7. CRC Check Field

Messages include an error-checking field, operating on the basis of a Cyclical Redundancy Check (CRC) method. The CRC field checks the contents of

the entire message. It is applied regardless of any parity check method used for the individual characters of the message. The CRC value is calculated

by the transmitting device, which appends the CRC as the last field in the message. The receiving device recalculates a CRC during receipt of the message

and compares the calculated value to the actual value received in the CRC field. If the two values are unequal, a bus time-out results. The error-checking

field contains a 16-bit binary value implemented as two 8-bit bytes. When this is done, the low-order byte of the field is appended first, followed by the

high-order byte. The CRC high-order byte is the last byte sent in the message.

7.8.8. Coil Register Addressing

In Modbus, all data are organized in coils and holding registers. Coils hold a single bit, whereas holding registers hold a 2-byte word (i.e. 16 bits). All

data addresses in Modbus messages are referenced to zero. The first occurrence of a data item is addressed as item number zero. For example: The coil

known as 'coil 1' in a programmable controller is addressed as coil 0000 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 field already specifies a 'holding register'

operation. Therefore, the '4XXXX' reference is implicit. Holding register 40108 is addressed as register 006BHEX (107 decimal).

Coil Number

1-16

17-32

33-48

49-64

66-65536

158

Description

Frequency converter control word (see table below)

Frequency converter speed or set-point reference Range 0x0 – 0xFFFF (-200% ...

~200%)