Knowledge Base

Access to values in a PLC

In software mode (without using an Applicom card) the OPC server does not permit direct access to a Real format value in a PLC. Instead it converts Real variable in the PLC’s memory to two words.

Solution

You must create a ‘cyclic function’ to read the words in the PLC and copy them into the OPC Server’s internal database. There the words can be accessed in Real format by use of the ‘Fn‘ symbol (where n is the address of the memory area of the words, generally in the range 0 to 65535).

Reversed words

The second problem is to do with the format of the words for conversion to Real. There are two formats:

• ‘Intel’: the first word represents the least significant part of the value, while the second represents the most significant.
• ‘Motorola’: the first word represented the most significant part while the second represents the least significant.

In the Applicom console you can select the method to use. However this configuration does not affect the words in the Server’s database so it does not solve the problem.

Solution

You must add a specific symbol ‘_X2’ (as in ‘Fn_X2′) to cause the OPC Server to reverse the order of the words.