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| Fieldbus Guide |
Version 1.00
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4
Modbus R
TU
4.3. Modbus RTU Configuration Parameter
3. Parameter Configuration for Fieldbus Operation on page 6
4.4. Modbus RTU Status & Control Holding Registers
Modbus RTU status holding registers are shown in section
11. Control & Status Registers on page 35
Holding Register 1 – 4 are used for drive control.
Additional Holding registers provide status information which may be read from the drive.
Modbus Master devices differ between different manufacturers and so it may be necessary to review the documentation for the
specific Master device being used to understand the correct addressing method to access Holding Registers, e.g.
Some devices use zero based addressing
o In this case the first Holding Register address is Holding Register 0
o Subtract 1 from the Holding Register number shown in the documentation.
Some devices recognise Holding Registers with a 40000 prefix
o In this case the first Holding Register may be 40000 or 40001
o Add the appropriate prefix and if necessary, subtract the offset.
4.5. Parameter Access
All User Adjustable parameters are accessible by Modbus RTU using the corresponding Holding Register. The register number
associated to each parameter is shown in the Programming Guide document along with any specific information about how the value
is scaled or stored.
Parameter values may be read from the drive or written to, depending on the operating mode of the drive – some parameters cannot
be changed whilst the drive is enabled.
4.6. Modbus RTU Indirect Parameter Access
In addition to Direct Access, Indirect Read / Write access to all user adjustable parameters is supported using a simple method as
detailed below. This is achieved using the following two Modbus registers.
4.6.1. Register 44: Drive Parameter Index
This index value will be used by register 45 to carry out parameter read and write function.
If the value is set to a parameter number that does not exist in the drive, an exception response will be received.
4.6.2. Register 45: Drive Parameter Value
When reading this register, the value represents the value of the parameter specified by register 44.
When writing to this register, the value will be written to the drive parameter number specified by register 44.
If the value written to a parameter is outside of the parameter range, an exception response will be received.
4.6.3. Parameter Read Method
To read a parameter, firstly write the parameter number to register 44, then read the value from register 45, e.g. to Read the Value of
P-01.
Write 1 to Register 44.
Read the Value of Register 45.
4.6.4. Parameter Write Method
Writing parameter values can be achieved by the same method, however, register 45 is used to write the parameter value after the
parameter number has been selected using Register 44, e.g. to Write a Value of 60.0Hz to parameter P-01.
Write 1 to Register 44.
Register 45 will return the present value of P-01, which can be Read if required.
Referring to the Programming Guide, apply any scaling necessary
o In this case, 60.0Hz = 3600.
Write the scaled value to Register 45. P-01 now changes to 60.0Hz, or an exception code may be returned.
