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Table 2: Universal Access Registers
Modbus TCP/IP Register Function
9998 ASD register number
9999 Read / write data
To use this feature, first write the ASD register number that you wish to access
to Modbus register 9998. Then, any subsequent read/write accesses to
Modbus register 9999 will be redirected to the ASD register number defined in
register 9998. In this manner, any ASD register can be accessed simply by
changing the ASD register number held in Modbus register 9998, and then
performing a read or write command on the read/write data register (9999).
For example, let’s say we would like to read a group of ASD configuration
registers starting at register 100 from Modbus TCP/IP unit identifier 1
(UI1=ETH-100 channel A). Therefore, we would write a value of 100 to
Modbus UI1 register 9998. By then reading from Modbus UI1 register 9999,
we will actually be retrieving the data from register 100 on the ASD connected
to channel A. By alternating between incrementing the value contained in
register 9998 (100..101..102 etc.) and then reading from register 9999, we can
access all the desired configuration parameters from the ASD.
If the requested operation cannot be performed (typically due to such reasons
as a non-existent ASD register being targeted or a written data value being out
of range), a corresponding Modbus TCP/IP error code will be returned.