Using function code 1, Modbus can reads the coils in a Modbus slave, which is commonly used by SCADA and DCS field devices. Coils refer to the binary output settings and are typically mapped to actuators. A sample of coil settings read from the device are provided by the plugin output. The ability to read coils may help an attacker profile a system and identify ranges of registers to alter via a write coil message.

Using function code 2, Modbus can read the discrete inputs from a Modbus slave, which is commonly used by SCADA and DCS field devices. Discrete inputs represent binary (i.e boolean) values that often map to switches, relays, or other sensors. A sample of discrete inputs read from the device are provided by the plugin output. The ability to read discrete inputs may help an attacker profile a system.