How to compare string value in micrologix plc?

Hopefully by Micrologix you mean a ML1100, 1200, or 1500 and not an ML1000. The ML1000 does not support strings. Take a look at these instrucitons in the Micrologix instruction set reference manual available at AB's online literature library. ACI Converts a string to an integer value. ACN Links two strings into one. AEX Extracts a portion of a string to create a new string. AIC Converts an integer value to a string. ASC Searches a string. ASR Compares two strings.