The second device 120 may include the ODT circuit 124b connected to the signal line 130, and the ODT circuit 124b may be configured as a pull-up circuit connected to the power voltage line VDDQ. The ODT circuit 124b may include the pull-up resistor RU connected between the power voltage VDDQ line and the signal line 130 and the pull-up switch SU disposed in correspondence to the pull-up resistor RU. The pull-up switch SU may be turned on or off according to a second ODT control signal C2b. The second ODT control signal C2b may be generated by the control circuit 122. When the pull-up switch SU is turned on by the second ODT control signal C2b, the ODT circuit 124b may be ODT enabled. When the pull-up switch SU is turned off, the ODT circuit 124b may be ODT disabled.

The ODT circuit 114b of the first device 110 is configured as the pull-down circuit connected to the ground voltage VSSQ, whereas the ODT circuit 124b of the second device 120 is configured as the pull-up circuit, and thus the first device 110 may confirm that the signal line 130 is in an asymmetric ODT state through an ODT state check operation.

FIG. 6 shows a state in which when a signal is not transmitted through the signal line 130, the pull-down switch SD and the pull-up switch SU are turned off so that the ODT circuits 114b and 124b of the first and second devices 110 and 120 are disabled. Accordingly, during a signal non-transmission, the signal line 130 may maintain a high-impedance state.