Communication systems including a case and an accessory are disclosed. In one example, the case supplies electric power. The accessory is connectable to the case with a charging line and a GND line. The charging line transmits and receives a charging signal. The GND line is set to a reference voltage. The accessory includes a requested-operation determination circuit that, based on a voltage level of the charging signal and on a duration time of the voltage level, determines one operation requested of the accessory. In another example, a power management IC is included in the accessory and connected to the case with a charging line and a GND line. The power management IC includes a requested-operation determination circuit that, based on a voltage level of the charging signal and on a duration time of the voltage level, determines one operation requested of the accessory.