MEMORY CARD RECEPTACLES WITH GUIDE RAILS AND GUIDE CHANNELS BACKGROUND [0001] Computing devices, and in particular, small-footprint computing devices, may employ portable electronic storage devices, such as memory cards, to store data. The memory cards may be inserted and removed from a receptacle of the computing devices. BRIEF DESCRIPTION OF THE DRAWINGS [0002] FIG.1A shows a perspective view of an example memory card receptacle with a guide rail and guide channel. [0003] FIG.1B shows the receptacle of FIG.1A with a memory card in a holder inserted to a reception position in the receptacle. [0004] FIG.1C shows the receptacle of FIG.1A with the memory card in the holder moved to an operational position in the receptacle. [0005] FIG.2 shows a perspective view of another example memory card receptacle with a guide rail and guide channel. [0006] FIG.3 shows a perspective view of a holder for inserting memory cards into the receptacle of FIG.2. [0007] FIG.4A shows a side view of the holder of FIG.3 in a closed configuration. [0008] FIG.4B shows a side view of the holder of FIG.3 in an open configuration. [0009] FIG.5A shows a side view of the holder of FIG.3 inserted to a reception position in the receptacle of FIG.2. [0010] FIG.5B shows a side view of the holder of FIG.3 moved to an operational position in the receptacle of FIG.2. [0011] FIG.5C shows a side view of the holder of FIG.3 moved to an operational position in the receptacle of FIG.2 with a closed latch. [0012] FIG.6 shows a partial perspective view of a computing device including the receptacle of FIG.2. DETAILED DESCRIPTION [0013] Computing devices may employ removable memory cards to store data. In some computing devices, such as small footprint computing devices, like laptops, notebooks, or the like, the receptacle to insert and remove memory cards may have access or line-of-sight partially obstructed by a cover or other components. Some computing devices may have guide rails to guide the movement of cards into slots by directing movement of the cards parallel to the direction of insertion, followed by a perpendicular movement. Such guide rails may still be cumbersome to effect the movement when access or line-of-sight is obstructed. Other computing devices have sockets which accept memory cards directly, and then are moved to a closed state. [0014] An example receptacle for a memory card includes a socket to couple to the memory card and a side wall having a guide rail to guide the memory card in a holder to contact the socket at a reception angle. As the holder is guided along the guide rail to the socket, a guide pin of the holder is received in a guide channel of the side wall. The guide pin is then redirected along the guide channel along a length of the guide channel, which rotates the holder to an operational angle in which the memory card is electrically coupled and in an operational position for use by a computing device connected to the receptacle. The guide rail and guide channel allow memory cards to be inserted and removed when line of sight and/or direct access to the socket of the receptacle is limited or obstructed, such as by a cover of the computing device. [0015] The holder may include two frames to carry two memory cards, substantially parallel to each other. The frames are connected on each side by two linkage bars to create two four-bar linkages. The frames are oriented such that when the holder is in the reception position, both memory cards contact their respective sockets in the receptacle, and movement of the guide pin along the guide channel moves both of the frames with their respective memory cards from the reception angle to the operational angle. In particular, the guide pin causes a rotation of the first memory card about its socket, and the four-bar linkages, in maintaining the parallel planes of the frames, causes a corresponding rotation of the second memory card about its socket to electrically couple the memory cards to their respective sockets. [0016] FIG.1A shows a perspective view of an example receptacle 100 to receive a memory card 102 in a holder 104. The receptacle 100 may be associated with a computing device (not shown), such as a laptop, notebook, other small-footprint personal computers, or the like. [0017] The memory card 102 is generally a portable electronic storage device to store electronic data. For example, the memory card 102 may be a small outline dual in-line memory module (SO-DIMM) card. The memory card 102 includes an electrical contact to electrically couple to the computing device to allow the computing device to access (e.g., read and write data to) the memory card 102. In particular, the electrical contact of the memory card 102 may interface with a corresponding electrical contact of a socket 108 of the receptacle 100. [0018] The holder 104 is to hold the memory card 102 and to cooperate with the receptacle 100 to electrically and mechanically couple the memory card 102 to the socket 108. The holder 104 includes a frame to hold the memory card 102 with the electrical contact of the memory card 102 oriented towards an open end of the frame to allow the memory card 102 to electrically couple with the socket 108 while supported in the holder 104. The frame may further include a retention mechanism, such as a spring, latch, or the like to retain the memory card 102 in the holder 104. The frame further includes a guide pin 106 extending from the frame. The guide pin 106 is to interface with the receptacle 100 to facilitate the electrical coupling of the memory card 102 with the socket 108, as described further below. [0019] The receptacle 100 includes the socket 108 to electrically couple to the memory card 102. As noted above, the socket 108 may include an electrical contact to interface with the electrical contact of the memory card 102. Additionally, the socket 108 may be electrically coupled to other components of a computing device (e.g., a system board, a processor, such as a central processing unit, a microprocessor, or the like) to allow the computing device to access the memory card 102. [0020] The receptacle 100 further includes a side wall 110 having a guide rail 112 and a guide channel 114. The guide rail 112 is to interface with the holder 104 to guide the holder 104 into a reception position in the receptacle 100. In particular, the guide rail 112 receives the frame of the holder 104 and supports a translation of the holder 104 along a length of the guide rail 112, in a direction A, towards the socket 108 to the reception position. For example, FIG.1B depicts the holder 104 in the receptacle 100 in the reception position. When the holder 104 is in the reception position within the receptacle 100, the memory card 102 contacts the socket 108 at a reception angle. [0021] The guide channel 114 is to interface with the guide pin 106 to guide the holder 104 from the reception position in the receptacle 100 to an operational position in the receptacle 100. In particular, the guide channel 114 receives the guide pin 106 and guides the guide pin 106 to an end 116 of the guide channel 114. Movement of the guide pin 106 to the end 116 of the guide channel rotates the holder 104 about the socket 108 from the reception position to the operational position. Further, movement of the guide pin 106 to the end 116 of the guide channel rotates the memory card 102 from the reception angle to an operational angle to electrically couple the memory card 102 to the socket 108. [0022] More particularly, in the present example, the guide channel 114 further includes a mouth 118 ending at a bend 120, and a length 122 extending from the bend 120 to the end 116. The mouth 118 is to receive the guide pin 106 into the guide channel 114. In particular, as the holder 104 is guided along the side wall 110 by the guide rail 112, the guide pin 106 is received in the mouth 118 of the guide channel 114. [0023] The mouth 118 ends at the bend 120. That is, when the holder 104 is in the reception position in the receptacle 100 and the memory card 102 contacts the socket 108 at the reception angle, the guide pin 106 is received at the bend 120. The bend 120 is further to redirect the guide pin 106 along the length 122 of the guide channel 114. That is, the bend 120 redirects the movement of the guide pin 106 from a translational movement towards the socket 108 to a rotational movement about the socket 108. [0024] In particular, the movement of the guide pin 106, in a direction B, along the length 122 of the guide channel 114 causes the rotational movement of the guide pin 106 about the socket 108. Accordingly, the movement of the guide pin 106 along the length 122 of the guide channel 114 to the end 116 rotates the holder 104 about the socket 108 from the reception position to the operational position as well as the memory card 102 about the socket 108 from the reception angle to the operational angle to electrically couple the memory card 102 to the socket 108. [0025] For example, FIG.1C depicts the holder 104 in the receptacle 100 in the operational position, after the guide pin 106 is moved along the length 122 of the guide channel 114. When the holder 104 is in the operational position within the receptacle 100, the memory card 102 contacts the socket 108 at an operational angle. [0026] To remove the holder 104 and the memory card 102 from the receptacle 100, the guide pin 106 is moved back along the length 122 of the guide channel 114 to the bend 120 to move the holder 104 to the reception position, and the holder 104 may be slid back along the guide rail 112 to remove the holder 104 and the memory card 102 from the receptacle 100. [0027] Thus, the receptacle 100 having the guide rail 112 and the guide channel 114 allows the memory card 102 in the holder 104 to be inserted and removed when line of sight and/or direct access to the socket 108 is limited or obstructed, such as by a cover of the computing device. Further, the configuration of the guide rail 112 and the guide channel 114 allows the memory card 102 to be inserted via a simple translation towards the socket 108 along the guide rail 112, and upon contacting the socket 108, a rotation or pivot about the socket 108 to an operational position in which the memory card 102 is electrically coupled to the socket 108. [0028] FIG.2 shows a perspective view of an example receptacle 200 to receive a first memory card 202-1 and a second memory card 202-2 in a holder 204. The receptacle 200 is similar to the receptacle 100 and may be associated with a computing device (not shown), such as a laptop, notebook, other small- footprint personal computer, or the like. [0029] The memory cards 202-1 and 202-2 are generally portable electronic storage devices to store electronic data. For example, the memory cards 202-1 and 202-2 may be small outline dual in-line memory module (SO-DIMM) cards. The memory cards 202-1 and 202-2 include electrical contacts to electrically couple to the computing device to allow the computing device to access (e.g., read and write data to) the memory cards 202-1 and 202-2. In particular, the electrical contacts of the memory cards 202-1 and 202-2 may interface with corresponding electrical contacts of a first socket 208-1 and a second socket 208-2, respectively, of the receptacle 200. [0030] The holder 204 is to hold the memory cards 202-1 and 202-2 and to cooperate with the receptacle 200 to electrically and mechanically couple the memory cards 202-1 and 202-2 to the sockets 208-1 and 208-2, respectively. In particular, the holder 204 includes a guide pin 206 to interface with the receptacle 200 to facilitate the electrical coupling of the memory cards 202-1 and 202-2 with the sockets 208-1 and 208-2, respectively, as described further below. [0031] Referring to FIG.3, the holder 204 is depicted in greater detail. The holder 204 includes a first frame 300-1 to hold the first memory card 202-1, and a second frame 300-2 to hold the second memory card 202-2. [0032] In particular, the first frame 300-1 includes first side arm 302-1 and second side arm 302-2. The first memory card 202-1 may be held in the first frame 300-1 by the side arms 302-1 and 302-2. For example, first frame 300-1 may further include a first retention feature 304-1, such as a spring, latch or the like to hold the first memory card 202-1 in the first frame 300-1. In some examples, the first frame 300-1 may include separate retention features 304-1 to maintain x-, y-, and z-axis positions of the first memory card 202-1 within the first frame 300-1. The first frame 300-1 further includes the guide pin 206 extending from the first frame 300-1. In particular, the holder 204 may include guide pins 206 extending from each of the side arms 302-1 and 302-2 of the first frame 300-1 for symmetry and stability. [0033] The second frame 300-2 includes third side arm 302-3 and fourth side arm 302-4. The second memory card 202-1 may be held in the second frame 300-2 by the side arms 302-3 and 302-4. For example, the second frame 300-2 may similarly include a second retention feature 304-2, such as a spring, latch, or the like to hold the second memory card 202-2 in the second frame 300-2. The second frame 300-2 may similarly include separate retention features 304-2 to maintain x-, y-, and z-axis positions of the second memory card 202-2 within the second frame 300-2 [0034] In particular, the frames 300-1 and 300-2 are to hold the memory cards 202-1 and 202-2 with the electrical contacts of the memory cards 202-1 and 202-2 at respective open ends of the frames 300-1 and 300-2, distal from the guide pin 206. [0035] The holder 204 further includes two sets of linkage bars 306-1 and 306-2, and 308-1 and 308-2. The two sets of linkage bars 306-1 and 306-2, and 308-1 and 308-2 are to connect the first frame 300-1 and the second frame 300- 2 in corresponding four-bar linkages at the sides of the first frame 300-1 and the second frame 300-2. More particularly, the linkage bars 306-1 and 306-2 are to couple the side arm 302-1 of the first frame 300-1 and the side arm 302-3 of the second frame 300-2 to form a first four-bar linkage. Similarly, the linkage bars 308-1 and 308-2 are to couple the side arm 302-2 of the first frame 300-1 and the side arm 302-4 of the second frame 300-2 to form a second four-bar linkage. [0036] The two sets of linkage bars 306-1 and 306-2, and 308-1 and 308-2 are to maintain the first frame 300-1 and the second frame 300-2 in substantially parallel planes. The four-bar linkages formed by the linkage bars 306-1 and 306-2, and 308-1 and 308-2 therefore link the respective movements of the first frame 300-1 and the second frame 300-2. That is, when one of the first frame 300-1 or the second frame 300-2 is moved or rotated, the other of the first frame 300-1 and the second frame 300-2 is prompted to move or rotate in a similar manner as a result of the four-bar linkages formed by the linkage bars 306-1 and 306-2, and 308-1 and 308-2. In particular, the four bar linkages are to maintain the first frame 300-1 and the second frame 300-2 in substantially parallel planes and to move the first frame 300-1 and the second frame 300-2 between a closed configuration and an open configuration. [0037] For example, referring to FIGS.4A and 4B, the holder 204 is depicted in a closed configuration 400 and an open configuration 402, respectively. As can be seen, the planes of the frames 300-1 and 300-2 are maintained in a substantially parallel spatial relationship. Additionally, as can be seen in the side views of FIGS.4A and 4B, the holder 204 may additionally include a closed configuration stopper 310 and an open configuration stopper 312. [0038] The closed configuration stopper 310 is to stop the linkage bar 306-1 at a predetermined angle to maintain a specific predetermined distance between the first frame 300-1 and the second frame 300-2 when the holder 204 is in the closed configuration. In particular, the distance may be selected to maintain adequate space for the memory cards 202-1 and 202-2 to be held in the holder 204, and for the memory cards 202-1 and 202-2 to be electrically coupled to the sockets 208-1 and 208-2 when the holder 204 is in the receptacle 200 in an operational position, as will be described further below. [0039] The open configuration stopper 312 is similarly to stop the linkage bar 306-1 at another predetermined angle to maintain another specific predetermined distance between the first frame 300-1 and the second frame 300-2 when the holder 204 is in the open configuration. In particular, the distance may be selected to maintain an appropriate distance between the memory cards 202-1 and 202-2 to contact the sockets 208-1 and 208-2 when the holder 204 is in the receptacle 200 in a reception position, as will be described further below. [0040] In the present example, the closed configuration stopper 310 and the open configuration stopper 312 are both disposed on the side arm 302-1 to stop the linkage bar 306-1 in the closed and open configurations of the holder 204. In other examples, the closed configuration stopper 310 and 312 may be disposed on any of the other side arms 302-2, 302-3, or 302-4, and positioned to stop any of the other linkage bars 306-2, 308-1, or 308-2. In other examples, the closed configuration stopper 310 and 312 may be disposed on different side arms 302- 1, 302-2, 302-3, and 302-4 from one another and may be positioned to stop different linkage bars 306-1, 306-2, 308-1 and 308-2 from one another. In still further examples, the holder 204 may include more than one closed configuration stopper 310 and/or more than one open configuration stopper 312. [0041] Returning to FIG.2, the receptacle 200 includes the sockets 208-1 and 208-2 to electrically couple to the memory cards 202-1 and 202-2. The sockets 208-1 and 208-2 may include electrical contacts to interface with the respective electrical contacts of the memory card 202-1 and 202-2. Additionally, the sockets 208-1 and 208-2 may be electrically coupled to other components of the computing device (e.g., a processor, such as a central processing unit, a microprocessor, or the like) to allow the computing device to access the memory cards 202-1 and 202-2. [0042] The receptacle 200 further includes a side wall 210 having a guide rail 212 and a guide channel 214. The side wall 210, the guide rail 212 and the guide channel 214 are similar to the side wall 110, the guide rail 112 and the guide channel 114. [0043] In particular, the guide rail 212 is to interface with the holder 204 to guide the holder 204 into a reception position in the receptacle 200. The guide rail 212 receives the holder 204, and in particular, the first frame 300-1 and supports a translation of the holder 204 along a length of the guide rail 212 in a direction towards the first socket 208-1 to the reception position. In particular, the holder 204 may be in its open configuration in the reception position. For example, FIG.5A depicts the holder 204 in the receptacle 200 in the reception position. When the holder 204 is in the reception position within the receptacle 200, the first memory card 202-1 contacts the second socket 208-1 at a reception angle, and the second memory card 202-2 contacts the second socket 208-2 at the reception angle. That is, the sockets 208-1 and 208-2 of the receptacle 200 are positioned so that the first memory card 202-1 and the second memory card 202-2 contact the first socket 208-1 and the second socket 208-2, respectively, when the holder 204, in its open configuration, is received in the receptacle 200 in the reception position. Further, the holder 204 maintains the first memory card 202-1 and the second memory card 202-2 at in substantially parallel planes and hence the memory cards 202-1 and 202-2 contact the respective sockets 208-1 and 208-2 at the same reception angle. [0044] The guide channel 214 is to interface with the guide pin 206 to guide the holder 204 from the reception position in the receptacle 200 to an operational position in the receptacle 200. In particular, the guide channel 214 receives the guide pin 206 and guides the guide pin 206 to an end 216 of the guide channel 214. More particularly, as the holder 204, and in particular, the first frame 300-1 is guided along the side wall 210 by the guide rail 212, the guide pin 206 is received in a mouth 218 of the guide channel 214. When the holder 204 is in the reception position in the receptacle 200 and the memory cards 202-1 and 202-2 contact the sockets 208-1 and 208-2 at the reception angle, the guide pin 206 is received at the bend 220. The bend 220 redirects the movement of the guide pin 206 from a translational movement towards the first socket 208-1 to a rotational movement about the socket 208-1 along a length 222 of the guide channel 214. [0045] In particular, the movement of the guide pin 206, in a direction along the length 222 of the guide channel 214 causes the rotational movement of the guide pin 206 about the first socket 208. Accordingly, the movement of the guide pin 206 along the length 222 of the guide channel 214 to the end 216 moves the holder 204 to the operational position in its closed configuration. In particular, the first memory card 202-1 is rotated about the first socket 208-1 from the reception angle to the operational angle to electrically couple the memory card 202-1 to the socket 208-1. Further, due to the construction of the holder 204 including the four-bar linkages, the second frame 300-2, and therefore the second memory card 202-2 is prompted to rotate in a similar manner about the second socket 208-2. The movement of the guide pin 206 along the length 222 of the guide channel 214 to the end 216 therefore causes the rotation of both the first memory card 202-1 about the first socket 208-1 and the second memory card 202-2 about the second socket 208-2 to the operational angle. [0046] For example, FIG.5B depicts the holder 204 in the receptacle 200 in the operational position, after the guide pin 206 is moved along the length 222 of the guide channel 214. In particular, when the holder 204 is in the operational position within the receptacle 200, the holder 204 is in its closed configuration, allowing the first memory card 202-1 to contact the first socket 208-1 at the operational angle, and the second memory card 202-2 to contact the second socket 208-2 at the operational angle. That is, as a result of the movement of the guide pin 206 to the end 216 of the guide channel 214, both the memory cards 202-1 and 202-2 are electrically coupled to their respective sockets 208-1 and 208-2. [0047] The receptacle 200 may further include a latch 224 to secure the guide pin 206 at the end 216 of the guide channel 214. In some examples, the latch 224 may rotate between an unlocked position to allow the guide pin 206 to move freely along the length 222 of the guide channel 214 to and from the end 216 and a locked position in which the guide pin 206 is prevented, by the latch 224, from moving along the length 222 of the guide channel 214. In particular, the latch 224 may secure the guide pin 206 at the end 216 of the guide channel 214. For example, in FIGS.5A and 5B, the latch 224 is in an unlocked position to allow the guide pin 206 to be directed to the end 216 of the guide channel 214. In FIG.5C, the latch 224 is in a locked position to secure the guide pin 206 at the end 216 of the guide channel 214. In some examples, the latch 224 may include a spring or other biasing mechanism to bias the latch 224 in the locked position. That is, the guide pin 206 may push the latch 224 to its unlocked position as it moves along the length 222 and the latch 224 may be biased, by the spring, back to its locked position when the guide pin 206 is at the end 216 of the guide channel 214. [0048] To remove the holder 204 and the memory cards 202-1 and 202-2 from the receptacle 200, the latch 224 may be released, for example by an operator, and the guide pin 206 is moved back along the length 222 of the guide channel 214 to the bend 220 to move the holder 204 to the reception position. The holder 204 may then be slid back along the guide rail 212 to remove the holder 204 and the memory cards 202-1 and 202-2 from the receptacle 200. [0049] Thus, the receptacle 200 similarly allows the memory cards 202-1 and 202-2 in the holder 204 to be inserted and removed when light of sight and/or direct access to the sockets 208-1 and 208-2 is limited or obstructed, such as by a cover of a computing device. [0050] For example, referring to FIG.6, a partial view of an example computing device 600 is depicted. The computing device 600 includes a cover 602 and the receptacle 200. As can be seen, the sockets 208-1 and 208-2 of the receptacle 200 are at least partially obstructed by the cover 602. The configuration of the guide rail 212 and the guide channel 214 allows the memory cards 202-1 and 202-2 to be inserted via a simple translation towards the socket 208 along the guide rail 212. In particular, when the holder 204 is in the reception position as provided by the guide rail 212, the memory cards 202-1 and 202-2 contact the sockets 208-1 and 208-2 at the reception angle. Advantageously, by using the guide rail 212, this insertion to the reception position may be completed without requiring line of sight to the sockets 208-1 and 208-2. Upon contacting the sockets 208-1 and 208-2, the guide pin 206 is rotated about the first socket 208-1 along the guide channel 214. The movement of the guide pin 206 along the guide channel 214 to the end 216 causes rotation of the first memory card 202-1 about the first socket 208-1, and the four-bar linkages cause a corresponding rotation of the second memory card 202-2 about the second socket 208-2 so that both memory cards 202-1 and 202-2 are electrically coupled to their respective sockets 208-1 and 208-2. Advantageously, by using the guide channel 214 in cooperation with the guide pins 206 of the holder 204, this movement to the operational position may similarly be completed without requiring line of sight to the sockets 208-1 and 208-2. [0051] The scope of the claims should not be limited by the above examples, but should be given the broadest interpretation consistent with the description as a whole.