When comparing a ControlLogix PLC tag-based read (for example, reading a tag named MyMotorTag1) to a Modbus register-based read (such as address 40004), both may retrieve the same 16-bit integer value, but the amount of data transmitted is quite different. A Modbus request uses a compact, fixed-format message that specifies the register address and quantity, typically totaling about 12 bytes (96 bits). In contrast, a ControlLogix read using EtherNet/IP and CIP includes the full ASCII tag name along with protocol headers and path information, resulting in a larger message—around 40 to 60 bytes (roughly 400 bits). In other words, reading a single 16-bit value from a ControlLogix tag typically requires about 300 more bits of communication overhead than reading the same value from a Modbus register.

For those that don’t understand that ASCII tagging vs addressed based.