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.

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