Modbus Data Base; Modbus Rtu Message Frame; Modbus Address; Fig 25: Modbus Transaction Format - rotork Modbus IQ Pro Technical Manual

7 MODBUS DATA BASE

The data in the Modbus Module may be collected using a number of different Modbus function codes.
Similarly the commands to the MFU can be either register or discrete based. This inbuilt flexibility is
designed to allow for any host device to be able to access the actuator over a Modbus network in the
simplest way.
This section provides information on the function codes supported and the data that may be collected
or commands issued using those codes.
In all cases registers and discrete locations are numbered from 0 and do not include any offsets.

7.1 Modbus RTU Message Frame

Message synchronisation is maintained by simulation, the receiving MFU monitors the elapsed time
between receipt of characters. If 3 and one-half character times elapse without a new character or
frame completion, then the next byte received will be the start of a new message and begin with the
address.
Modbus
Address
8 bits

Fig 25: Modbus transaction format

7.2 Modbus Address

The Modbus Module allows slave addresses in the range 1 to 247 to be used for devices; the
addresses above 247 are reserved for special functions. Address 0 is recognised by all the
slaves and is used for global Broadcast messages only. This leaves 247 addresses for use by
the connected Modbus Modules on the data highway.
Note that the dual highway unit uses a single address for both highway connections.

Single RS-485 highway plus one analogue input channel
Address range supported = 0 to 247 (0 for broadcast)

Dual Independent Isolated –RS-485 highways
Address range supported = 0 to 247 (0 for broadcast)
Both channels must share the same address

Single RS-485 highway with inbuilt isolating repeater
Address range supported = 0 to 247 (0 for broadcast)
Function Register or
Code Discrete
Address
8 bits 16 bits
Publication PUB091-003-00_0119
Number of Data
Registers or Field
Bits
16 bits N bits
Modbus Data Base
CRC
Check
16 bits
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