In a memory system having a data bus transferring data in either direction, highly reliable data transfer is provided regardless of the direction in which data is transferred. The signal lines of a data bus ( 12 ) bidirectionally transfer data. That is to say, during data write operations to a DIMM, the signal lines transfer data from a memory controller ( 10 ) to the DIMM, and during data read operations, they transfer data from the DIMM to the memory controller ( 10 ). The signal lines have, as terminating resistors, terminating variable resistors (VRt) whose impedance is controlled by the memory controller ( 10 ). During data write operations to the DIMM, the memory controller ( 10 ) sets the impedance of each terminating variable resistor (VRt) at a value suitable for writing, and during data read operations, it sets the impedance of each terminating variable resistor (VRt) at a value suitable for reading.
1. A data bus comprising:
a signal line capable of bidirectionally transferring data, said signal line including a first end and a second end; and
at least one terminating element connected between at least one of said first end and said second end and a voltage supply line,
said at least one terminating element having impedance that can be varied in accordance with the direction in which said data is transferred.
2. The data bus according to claim 1 , further comprising a variable impedance element connected in series to said signal line separated from said at least one terminating element,
wherein said variable impedance element is configured to provide a second variable impedance in accordance with the direction in which said data is transferred.
3. A data bus comprising:
a signal line including a first end, a second end, and at least one third end branching off between said first end and said second end and capable of bidirectionally transferring data;
a memory capable of performing read and write operations of said data being connected to each of said at least one third end; and
a variable impedance element connected in series to said signal line,
said variable impedance element having impedance that can be varied in accordance with said read and write operations.
4. A data bus comprising:
a signal line capable of bidirectionally transferring data; and
at least one terminating element for terminating said signal line, wherein said at least one terminating element having impedance that can be varied in accordance with the direction in which said data is transferred;
said signal line comprises a first end, a second end, and at least one third end branching off between said first end and said second end,
a memory capable of performing read and write operations of said data is connected to each said at least one third end, and
said at least one terminating element further comprises a first terminating variable resistor connected to said second end and being configured to provide different resistance values in said read and write operations.
5. The data bus according to claim 4 , further comprising:
a memory controller configured to send write data to said memory and configured to receive read data from said memory is connected to said first end, and
the resistance value of said first terminating variable resistor is smaller in said write operation than in said read operation.
6. The data bus according to claim 4 , wherein a second terminating variable resistor is connected to said first end and configured to provide different resistance values in said read and write operations.
7. The data bus according to claim 6 , further comprising:
a memory controller configured to send write data to said memory and configured to receive read data from said memory, and
the resistance value of said second terminating variable resistor is infinity in said write operation and the resistance value of said first terminating variable resistor is infinity in said read operation.