Abstract:
A memory device is adapted to be connected in a daisy chain with a memory controller and one or more other memory devices. The memory device includes at least one data input port and at least one data output port for communicating data along the daisy-chain between the memory devices and the memory controller. The memory device is adapted to selectively enable/disable at least one of the data input or data output ports in response to whether a command received from the memory controller is intended for the memory device, or for one of the other memory devices.

Description:
BACKGROUND AND SUMMARY 
   1. Field 
   This invention relates, in general, to a memory device, and more particularly a memory device adapted to be connected in a daisy chain and having a data input port and output port that can be selectively enabled, and a memory module and memory system including the same. 
   2. Description 
   In general, a memory system includes a memory controller and a plurality of memory module connected to the memory controller. As memory systems having a higher density are demanded, an increasing number of memory modules are employed. Although a memory system with higher density can be obtained by using more memory modules, the capacitive loading of each of the signal lines between the memory controller and memory modules increases. This, in turn, limits the operating speed of the memory system. For this reason, the number of memory devices connected to one data signal line is limited, for example, to eight devices in a memory system employing synchronous dynamic random access memory (SDRAM) to four devices in a memory system employing double data rate (DDR) SDRAM, and to two devices in a memory system employing DDR2/3 SDRAM. 
   To solve the foregoing problem, a memory system employing a point-to-point (PTP) connection between a memory controller and a memory module has been adopted in memory system architectures. This arrangement is also sometimes referred to as a “daisy chain.” Also, in this PTP arrangement, to increase the density of memory system the memory devices on one memory module employ a stacking package technology including lower memory device  132 - 1  and upper memory device  134 - 1  and each memory device is connected by the PTP arrangement. 
     FIG. 1A  is a block diagram of an exemplary memory system  100  having a daisy chain structure. Memory system  100  includes a memory controller  110  and a memory module  120 . Memory module  120  includes a plurality of memory groups  130 - 1 ˜ 130 - n . In turn, each memory group  130 - i  includes a primary memory device  132 - i  and a secondary memory device  134 -I which are connected together in a daisy chain or PTP arrangement. 
   Memory controller  110  includes first output ports (Tx 1 ˜Txn) to output commands, addresses, and write data (C/A/WD) to memory module  120 , and first input ports (Rx 1 ˜Rxn) to input read data from memory module  120 . 
   In the memory system  100 : C/A/WD indicates merged signal lines for command and addresses and write data for write operations; RD indicates read data lines for read operations; Rx_p indicates an input port of primary memory device  132 - i ; Rx_s indicates an input port of secondary memory device  134 - i ; Tx_p indicates an output port of primary memory device  132 -I for sending command and addresses and write data; Tx_rdp indicates an output port of primary memory device  132 - i  for outputting read data; Rx_rdp indicates an input port of primary memory device  132 - i ; Rx_rds indicates an input port of secondary memory device  134 - i  for receiving read data; and Tx_rds indicates an output port of secondary memory device  134 - i  for outputting read data. The input ports Rx_rdp of primary memory devices  132 - 1 ˜N are all disabled based upon the memory devices&#39; connection as primary memory devices, rather than secondary memory devices, in the configuration of memory system  100 . 
   Operationally, a read operation of memory system  100  will be explained with reference to  FIG. 1A . Consider a case where data is being read out of a primary memory device  132 - i  to memory controller  110 . In that case, read data of the memory device  132 - i  is transferred to memory controller  110  through the Tx_rdp port of primary memory device  132 - i , the Rx_rds port of secondary memory device  134 - i , and the Tx_rds port of secondary memory device  134 - i , sequentially in that order. 
   Now, for a read data operation, the Rx_rds port and Tx_rds port of secondary memory device  134 - i  are always enabled or activated. That is, because secondary memory device  134 - i  doesn&#39;t know when a read operation for primary memory device  132 - i  occurs and when it will receive the read data from primary memory device  132 - i  and repeat the read data to memory controller  110 , the circuits comprising the Rx_rds port and Tx_rds port of secondary memory device  134 - i  should always be in an operating condition. 
   Accordingly, power consumption in memory system  100  is larger than necessary and therefore wasted 
     FIG. 1B  is a block diagram of another exemplary memory system  150  having a daisy chain structure. 
   Memory system  150  is configured the same as memory system  100  of  FIG. 1A , except for the following differences. 
   While the signal line for commands, addresses, and write data (C/A/WD) is merged in memory system,  100  of  FIG. 1A , the signal lines for commands and addresses (C/A) and the signal lines for write data (WD) are separated from each other in memory system  150  of  FIG. 1B . 
   Therefore, in memory system  150  of  FIG. 1B , the input ports Rx_rdp of the primary memory devices  132 - 1 ˜N are all enabled to receive write data from memory controller  110 . Also in a write operation for writing data to secondary memory device  134 - i , primary memory device  132 - i  repeats the write data from memory controller  110  to secondary memory device  134 - i  through the output port Tx_rdp of primary memory device  132 - i.    
   That is, the input port Rx_rdp and the output port Tx_rdp of the primary memory device  132 - i  are always enabled or activated to repeat write data to secondary memory device  134 - i  because primary memory device  132 - i  doesn&#39;t know when it will have to repeat the write data and output the write data to secondary memory device  134 - i.    
   Accordingly, power consumption in memory system  150  is larger than necessary and therefore wasted. 
   Accordingly, it would be advantageous to provide a memory device capable of selectively enabling/disabling an input port and/or an output port depending upon whether the port is needed for a current operation being performed in a memory system in which the memory device operates. It would also be advantageous to provide a memory module including a plurality of such memory devices. It would further be advantageous to provide a memory system including a plurality such a memory module including such a plurality of memory devices. 
   In one aspect of the invention, a memory device is adapted to be connected in a daisy chain with a memory controller and one or more other memory devices. The memory device comprises: a plurality of memory cells; a data input port adapted to receive read data; a data output port adapted to output the read data; a command/address input port adapted to receive a command and address packet; a decoder adapted to receive and decode the command and address packet and to output one or more detection signals, wherein when the command and address packet includes a read command, the one or more detection signals indicate whether the read command is intended for memory cells of the memory device, or for another memory device in the daisy chain; and a port controller adapted to selectively enable and disable at least one of the data input port and the data output port in response to at least one of the one or more detection signals from the decoder. 
   In another aspect of the invention, a memory module comprises a plurality of memory devices connected in a daisy chain. Each memory device comprises: a plurality of memory cells; a data input port adapted to receive read data; a data output port adapted to output the read data; a command/address input port adapted to receive a command and address packet; a decoder adapted to receive and decode the command and address packet and to output one or more detection signals, wherein when the command and address packet includes a read command the one or more detection signals indicate whether the read command is intended for memory cells of the memory device, or for one of the other memory device(s) in the daisy chain; and a port controller adapted to selectively enable and disable at least one of the data input port and the data output port in response to at least one of the one or more detection signals from the decoder. 
   In a further aspect of the invention, a memory system includes: a memory controller; and at least one memory module. Each memory module includes a plurality of memory devices connected in a daisy chain with the memory controller. Each memory device comprises: a plurality of memory cells; a data input port adapted to receive read data; a data output port adapted to output read data; a command/address input port adapted to receive a command and address packet; a decoder adapted to receive and decode the command and address packet and to output one or more detection signals, wherein when the command and address packet includes a read data command the one or more detection signals indicate whether the read data command is intended for memory cells of the memory device, or for one of the other memory device(s) in the daisy chain; and a port controller adapted to selectively enable and disable at least one of the data input port and the data output port in response to at least one detection signal from the decoder. 
   In yet another aspect of the invention, a memory device is adapted to be connected in a daisy chain with a memory controller and one or more other memory devices. The memory device includes at least one data input port and at least one data output port for communicating data along the daisy-chain between the memory devices and the memory controller. The memory device is adapted to selectively enable/disable at least one of the data input or data output ports in response to whether a command received from the memory controller is intended for the memory device, or for one of the other memory devices. 
   In still another aspect of the invention, a memory system includes: a memory controller; and at least one memory module. Each memory module includes a plurality of memory devices connected in a daisy chain with the memory controller. Each memory device includes at least one data input port and at least one data output port for communicating data along the daisy-chain between the memory devices and the memory controller, the memory device being adapted to selectively enable/disable at least one of the data input or data output ports in response to whether a command received from the memory controller is intended for the memory device, or for one of the other memory devices. 
   In a still further aspect of the invention, a memory device is adapted to be connected in a daisy chain with a memory controller and one or more other memory devices. The memory device comprises: a plurality of memory cells; a data input port adapted to receive read data; a data output port adapted to output read data; a command/address input port adapted to receive a command and address packet; a decoder adapted to receive and decode the command and address packet and to output a self read detection signal and a repeat read detection signal, wherein when the command and address packet includes a read command intended for memory cells of the memory device, then the self read detection signal is activated and the repeat read detection signal is inactivated, and when the command and address packet includes a read command intended for another memory device in the daisy chain which is connected to pass the read data to the memory controller through the memory device, then the self read detection signal is inactivated and the repeat read detection signal is activated; and a port controller adapted to selectively enable and disable at least one of the data input port and the data output port in response to at least one of the self read detection signal and the repeat read detection signal. 
   In an even further aspect of the invention, a memory device is adapted to be connected in a daisy chain with a memory controller and one or more other memory devices. The memory device comprises: a plurality of memory cells; a data input port adapted to receive write data; a data output port adapted to output the write data; a command/address input port adapted to receive a command and address packet; a decoder adapted to receive and decode the command and address packet and to output one or more detection signals, wherein when the command and address packet includes a write command the one or more detection signals indicate whether the write command is intended for memory cells of the memory device, or for one of the other memory device(s) in the daisy chain; and a port controller adapted to selectively enable and disable at least one of the data input port and the data output port in response to at least one detection signal from the decoder. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIGS. 1A-B  are block diagrams of two memory systems each having a daisy chain arrangement. 
       FIG. 2  is a functional block diagram of one embodiment of a memory device. 
       FIG. 3  shows one embodiment of a port controller that may be employed in the memory device of  FIG. 2 . 
       FIG. 4  shows one embodiment of a command, address and write data (C/A/WD) packet format. 
       FIG. 5A  is a high level block diagram of a memory system that may include the memory device of  FIG. 2 . 
       FIG. 5B  is a timing diagram illustrating a read operation of the memory system of  FIG. 5A . 
       FIG. 6  is a functional block diagram of another embodiment of a memory device. 
       FIGS. 7A-B  show two different embodiments of a port controller that might be employed in the memory device of  FIG. 6 . 
       FIG. 8A  shows a block diagram of one embodiment of a memory system, which might include the memory device of  FIG. 2 . 
       FIG. 8B  shows a block diagram of a second embodiment of a memory system, which might include the memory device of  FIG. 6 . 
   

   DETAILED DESCRIPTION 
     FIG. 2  is a functional block diagram of one embodiment of a memory device  200 . Memory device  200  includes: first input port  202 ; first output port  204 ; ID register  206 ; packet decoder  208 ; port controller  210 ; data input port  212 ; memory core  214 ; selector  216 ; and data output port  218 . First input port  202  includes buffer B 1 , and first output port  204  includes buffer B 2 . Data input port  212  includes buffer B 3  and serial-to-parallel-converter (SPC)  213 . Data output port  214  includes buffer B 4  and parallel-to-serial-converter  219 . 
   Referring to  FIG. 2 , when memory device  200  is used as a primary memory device in a daisy chain structure, then first input port  202  receives a command/address/write data (C/A/WD) packet from a memory controller, and generates an internal C/A/WD packet. On the other hand, when memory device  200  is not used as a primary memory device (e.g., is used as a secondary memory device), then first input port  202  receives a C/A/WD packet from a preceding memory device in the daisy chain (e.g., a primary memory device) and generates an internal C/A/WD packet. 
   First output port  204  receives the internal C/A/WD packet from first input port  202  and outputs the internal C/A/WD packet to a next memory device in the daisy chain. When memory device  200  is a last memory device in the daisy chain, then first output port  204  remains in a disabled state, perhaps by one or more pins on memory device  200  detecting a voltage level indicating the last device “slot” in the daisy chain. Other arrangements are of course possible. 
   ID Register  206  stores device identification information for the daisy chain structure in which memory device  200  is currently provided. For example, if a daisy chain includes four memory devices  200 , then each memory device  200  stores one of “00”, “01”, “10” and “11” in ID Register  206 . Again, memory device  200  may determine its position in the daisy chain by detecting a voltage level(s) on one or more pins of memory device  200 , indicating the corresponding device “slot” in the daisy chain. Other arrangements are of course possible. 
   Packet decoder  208  receives the internal C/A/WD packet. In addition to command, address, and write data (in a data writing operation), a C/A/WD packet also includes device identification (ID) information. Packet decoder  208  compares the ID information included in the C/A/WD packet and the ID information stored in ID Register  206 , and in response to the comparison generates command, address, and control signals (SRD, RP_RD) for operation of memory device  200 . 
   The SRD signal is activated when the ID information included in the C/A/WD packet and the ID information stored in ID Register  206  are same, and the decoded command is for a read operation. That is, the SRD signal is activated to have a logic “high” state when a self read command is detected by packet decoder  208 . On the other hand, the RP_RD signal is activated when the ID information included in the C/A/WD packet and the ID information stored in ID Register  206  are not same, and the decoded command is for read operation. That is, the RP_RD signal is activated to have a logic “high” state when a read command for another memory device is detected by packet decoder  208 . 
   The point of time when the SRD signal is activated may be determined by a CL (CAS Latency) of memory device  200 , and the duration for which the SRD signal is activated may be decided by a BL (Burst Length). CL is the time, measured as a number of clock cycles, from receiving a read command to outputting read data. BL is the number of data which is successively outputted or inputted to/from memory device. 
   The point of time when the RP_RD signal is activated may be determined by the CL (CAS Latency) of a preceding memory device in a daisy chain, and the time for repeating data between memory devices. The duration for which the RP_RD signal is activated may be determined by a BL (Burst Length) of a preceding memory device in the daisy chain. 
   Port controller  210  receives the SRD and RP_RD signals, and outputs data input port and data output port enable signals (Rx_en, Tx_en) for determining when data input port  212  and data output port  218  are enabled. In particular, the data input port enable signal (Rx_en) is activated in response to RP_RD, and the data output port enable signal (Tx_en) is activated in response to RP_RD or SRD. 
   As noted above, data input port  212  includes buffer B 3  and a Serial-to-Parallel-Converter (SPC)  213  and is enabled by the Rx_en signal. SPC  213  parallelizes a serial read data packet from a preceding memory device in a daisy chain, and outputs 1st read data to a selector  216 . If memory device  200  is used as a primary memory device, the circuits comprising data input port  212 , i.e., the buffer B 3  and SPC  213 , are always disabled. 
   Memory core  214  outputs 2nd read data in response to the read command and address from packet decoder  208 . 
   Selector  216  selects and outputs one of 1st read data and 2nd read data to the data output port is response to the SRD signal from packet decoder  208 . That is, selector  216  outputs 2nd read data to data output port  218  when packet decoder  208  detects a self read command, and outputs 1st read data when packet decoder  208  detects a read command for another memory device. 
   As noted above, data output port  218  includes Parallel-to-Serial-Converter (PSC)  219  and buffer B 4  and is enabled in response to the Tx_en signal. PSC  219  serializes the parallel read data from selector  216 , and outputs the serial read data from memory device  200 . 
     FIG. 3  shows one embodiment of port controller  210  that may be employed in the memory device of  FIG. 2 . Port controller  210  includes a delay element “R” and an OR logic gate  211 . OR Logic gate activates the Tx_en signal to enable data output port  218  when either the SRD signal or the RP_RD is activated. Meanwhile the Rx_en signal is activated to enable data input port  212  when the RP_RD signal is activated. It is desirable for delay element “R” to have a delay that is less than the sum of the delay through data input port  212  and selector  216 . 
     FIG. 4  shows one embodiment of a command, address, and write data (C/A/WD) packet format. As shown in  FIG. 4 , the C/A/WD packet can be transferred by 6 pins, and each pin may provide up to 8 bits of information synchronized with the clock signal. The C/A/WD packet may be for an active operation, a read operation, a write operation etc. If the C/A/WD packet is for write operation, the packet may be extended to include write data in the same manner. 
   The first bit of the C/A/WD packet includes a command type indicated C 0 ˜C 2  and device identification information CS 0 ˜CS 1 . The second and third bits of the C/A/WD packet include BA 0 ˜BA 3  address bits for bank addresses, and A 0 ˜A 13  address bits to select a specific memory cell. 
     FIG. 5A  is a high level block diagram of a memory system  500  that may include a memory device  200  as shown in  FIG. 2 . 
   Memory system  500  includes a memory controller  510  and a memory group having a primary memory device  200   p  and a secondary memory device  200   s.    
   In  FIG. 5A , CRD 0  denotes a C/A/WD packet from memory controller  510  to primary memory device  200   p , and CRD 1  denotes a C/A/WD packet from primary memory device  200   p  to secondary memory device  200   s . RD 0  denotes read data from primary memory device  200   p  and RD 1  denotes read data from secondary memory device  200   s . RD 1  can be RD 0  when read operation is for primary memory device  200   p.    
   Although  FIG. 5A  shows only one memory group, the memory system may include more than one memory group. Also, although each memory group of the memory system in  FIG. 5A  has two memory devices, this is used for illustrative purposes only and that the teaching of this invention can be extended to other memory group having more than two memory devices in a daisy chain. 
     FIG. 5B  is a timing diagram illustrating a read operation of the memory system  500  of  FIG. 5A . 
   In  FIG. 5B , the CL and BL of primary memory device  200   p  and secondary memory device  200   s  are 6 clocks and 2 clocks respectively. 
   Referring to  FIGS. 2-5 , a successive read operation of primary and secondary memory devices  200   p  and  200   s  in memory system  500  will be explained. 
   Primary memory device  200   p  receives CRD_P and CRD_S successively and repeats and outputs the CRD_P and CRD_S. Packet decoder  208  decodes the CRD_P packet and activates the SRD_P signal because the device identification information included in the CRD_P and the identification information stored in IDR  206  of memory device  200   p  are the same. 
   The Tx_en signal of primary memory device  200   p  (Tx_en_p) is activated responsive to the SRD_P signal after a pre-determined time of the CL lapses. The duration of activation of Tx_en_p is long enough to output all of the read data as determined by the BL. 
   First read data (RD_ 0 ) from primary memory device  200   p  is transferred in response to the Tx_en_p signal to data input port  212  of secondary memory device  200   s.    
   Secondary memory device  200   s  receives the CRD_P and CRD_S packets successively through primary memory device  200   p  after a repeating time delay tRP. 
   Secondary memory device  200   s  decodes the CRD_P packet, detects that the read command is for another memory device (i.e., primary memory device  200   s ) and activates the RP_RD_s signal. Port controller  210  of secondary memory device  200   s  activates the Rx_en and the Tx_en signals in response to the RP_RD_s signal. Data input port  212  of secondary memory device  200   s  receives the first read data (RD_ 0 ) and transfers RD_ 0  to data output port  218  through the selector  216 . Data output port  218  of secondary memory device  200   s  outputs the RD_ 0  data to memory controller  510  in response to the Tx_en signal. 
   Also, secondary memory device  200   s  decodes the CRD_S packet, detects self read command and activates the SRD_s signal. Port controller  210  maintains the activation of the Tx_en signal in response to the SRD_s signal until the second read data RD_ 1  from primary memory device  200   p  is output to memory controller  510 . 
   By the process outlined above, data output port  218  of secondary memory device  200   s  can output RD_ 0  and RD_ 1  packets successively to memory controller  510 . 
   Accordingly, as the data input port and data output port of memory device  200  comprising a daisy chain structure can be selectively operated by detecting a command for other memory devices as well as commands for itself, power consumption of the data input and output ports can be reduced because the data input and output port operate only when they are needed. 
     FIG. 6  is a functional block diagram of another embodiment of a memory device  600 . The memory device  600  of  FIG. 6  may be used in a memory system similar to memory system  150  of  FIG. 1B , where the signal lines for commands and addresses (C/A) and the signal lines for write data (WD) are separated from each other. While the signal lines for commands and addresses, and the signal lines for write data are separated each other, the write data lines are merged with the read data lines. Accordingly, the interface for memory device  600  is different from for memory device  200 . 
   Referring to  FIG. 6 , only the differences from memory device  200  will be explained. 
   Packet decoder  608  decodes a C/A packet and detects whether a write command is for its own memory device  600 , or for another memory device. If the decoded command is for a write operation and the ID information in the C/A packet matches the ID information in IDR  206 , then the SWR (self write) signal is activated. If the decoded command is for a write operation and the ID information in the C/A packet does not match the ID information in IDR  206 , then the RP_WR (repeating write data) signal is activated. 
   Port controller  610  receives the SRD signal, the RP_RD signal, the SWR signal, and the RP_WR signal from packet decoder  608 , and outputs Rx_en and Tx_en signals to data input port  212  and data output port  218 , respectively. In addition, port controller  610  activates the Rx_en and Tx_en signals in response to the SRD signal and the RP_RD signal, as in the memory device  200  of  FIG. 2 , but it also activates the Rx_en signal when the SWR signal is activated, and activates the Rx_en and Tx_en signals when the RP_WR signal is activated. 
   The point of time when the Rx_en signal is activated in response to the SWR signal may be determined by the Write Latency (WL), and the duration of activation of the Rx en signal in response to SWR may be also decided by a Burst Length (BL). Also, the point of time when the Tx_en is activated in response to RP_WR signal may be decided by the WL and a repeating time, and the duration of activation of the Tx_en signal in response to the RP_WR signal may be also decided by a BL. 
   Data input port  212  of memory device  600  is the same as that of memory device  200  of  FIG. 2 . However, data input port  212  of memory device  600  receives write data (WD) from the memory controller when memory device  600  is used as a primary memory device, and receives write data from a preceding memory device in a daisy chain when it is not used as the primary memory device. 
   Switch  612  transfers the write data received from data input port  212  to memory core  214  or selector  216  in response to the SWR signal. That is, switch  612  transfers write data to memory core  213  only when a self write command is detected. 
   Selector  216  outputs self read data in response to the SRD signal only when a self read command is detected, otherwise it outputs the read data or write data from a preceding memory device in a daisy chain to either a subsequent memory device, or the memory controller in the case of a read command, when it is the last memory device in the daisy chain. 
     FIGS. 7A-B  show two different embodiments of a port controller  610  that might be employed in the memory device of  FIG. 6 . 
   While  FIG. 7A  illustrates a configuration for port controller  610  when memory device  600  is used as a primary memory device in a daisy chain,  FIG. 7B  illustrates a configuration for port controller  610  when memory device is not used as a primary memory device. 
   Referring  FIG. 7A , the Rx_en signal is activated in response to the SWR signal or the RP_WR signal, and the Tx_en signal is activated in response to the SRD signal or the RP_WR signal. The Tx_en signal can be activated through a delay element “R” after the RP_WR signal is activated. It is desirable for the delay element “R” to have a delay that is less than or equal to a delay equaling the sum of the delays of data input port  212 , switch  612 , and selector  216 . 
   Referring  FIG. 7B , the Rx_en signal is activated in response to the SWR signal, the RP_WR signal, and the RP_RD signal, and the Tx_en signal is activated in response to the SRD signal, the RP_RD signal, and the RP_WR signal. The Tx_en signal can be activated through a delay element “R” after the RP_WR signal or the RP_RD signal is activated. It is desirable for the delay element “R” to have a delay that is less than or the same as the sum of the delays of data input port  212 , switch  612 , and selector  216 . 
     FIG. 8A  shows a block diagram of one embodiment of a memory system  800 , which might include the memory device  200  of  FIG. 2 . 
   Although the memory system  800  shows just one memory group (S 0 ), in general the memory system may have a plurality of memory groups. 
   Memory group S 0  of memory system  800  includes primary (P), secondary (S), third (T) and fourth (F) memory devices  200 . IDR  206  of primary memory device  200  stores self ID information (ID 0 ). IDR  206  of secondary memory device  200  stores self ID information (ID 1 ) and ID information (ID 0 ) of primary memory device  200 . IDR  206  of third memory device  200  stores: self ID information (ID 2 ), ID information (ID 0 ) of primary memory device  200 , and ID information (ID 1 ) of secondary memory device  200 . IDR  206  of fourth memory device  200  stores: self ID information (ID 3 ), ID information (ID 0 ) of primary memory device  200 , ID information (ID 1 ) of secondary memory device  200 , and ID information (ID 2 ) of third memory device  200 . 
   Each memory device  200  compares device identification information included in a command packet and stored ID information in IDR  206  of the memory device  200  and determines form the comparison whether a read command is for itself or for another memory device in a daisy chain. Whether or not data input port  212  and/or data output port  218  should be enabled can be determined selectively by the result of the comparison. 
   For example, data input port  212  and data output port  218  of secondary memory device  200  can be enabled when secondary memory device  200  detects a read command for primary memory device  200 . Also, data input port  212  and data output port  218  of third memory device  200  can be enabled when third memory device  200  detects a read command for primary memory device  200  or secondary memory device  200 . Furthermore, data input port  212  and data output port  218  of fourth memory device  200  can be enabled when fourth memory device  200  detects a read command for primary memory device  200 , secondary memory device  200 , or third memory device  200 . 
     FIG. 8B  shows a block diagram of a second embodiment of a memory system, which might include the memory device  600  of  FIG. 6 . 
   Each of memory devices  600  comprising a daisy chain has an IDR  206  which stores all ID information of all of the memory devices  600  in the daisy chain. 
   Each memory device  600  compares device identification information included in a command packet, with the stored ID information, and determines as a result of that comparison whether a write command is for itself or another memory device, and also whether a read command is for itself or another memory device. Whether or not data input port  212  and/or data output port  218  should be enabled can be determined selectively by the result of the comparison when a read operation or a write operation is performed for a memory device  600  in the daisy chain. 
   For example, data input port  212  and data output port  218  of primary memory device  600  can be enabled when primary memory device  200  detects a write command for secondary memory device  600 , or third memory device  600 , or fourth memory device  600 . Also, data input port  212  and data output port  218  of secondary memory device  600  can be enabled when secondary memory device  600  detects a write command for third memory device  600  or fourth memory device  600 . Furthermore, data input port  212  and data output port  218  of third memory device  600  can be enabled when third memory device  600  detects a read command for fourth memory device  600 . 
   As the number of memory devices in a daisy chain increase, the benefits described above also increase. 
   While preferred embodiments are disclosed herein, many variations are possible which remain within the concept and scope of the invention. Such variations would become clear to one of ordinary skill in the art after inspection of the specification, drawings and claims herein. The invention therefore is not to be restricted except within the scope of the appended claims.