Detection and Blocking of Replay, False Command, and False Access Injection Commands in SCADA Systems with Modbus Protocol

Table 1

Various Modbus function codes and their actions on data.

Function code	Action to be performed
1	Reading of coils
2	Reading of status registers
3	Reading holding registers
4	Reading of input registers
5	Writing to single coil
6	Writing to single holding register
15	Multiple coils: write
16	Multiple holding registers: write