Hardware Reference
In-Depth Information
would be damaged, but keying in the socket and on the DIMM prevents that.
Modern PC systems use only unbuffered 3.3V DIMMs. Apple and other non-PC systems can use the
buffered 5V versions. Fortunately, the key notches along the connector edge of a DIMM are spaced
differently for buffered/unbuffered and 3.3V/5V DIMMs, as shown in Figure 6.14 . This prevents
inserting a DIMM of the wrong type into a given socket.
Figure 6.14. The 168-pin DRAM DIMM notch key definitions.
DDR DIMM Details
The 184-pin DDR DIMMs use a single key notch to indicate voltage, as shown in Figure 6.15 .
Figure 6.15. The 184-pin DDR SDRAM DIMM keying.
DDR DIMMs also use two notches on each side to enable compatibility with both low- and high-
profile latched sockets. Note that the key position is offset with respect to the center of the DIMM to
prevent inserting it backward in the socket. The key notch is positioned to the left, centered, or to the
right of the area between pins 52 and 53. This indicates the I/O voltage for the DDR DIMM and
prevents installing the wrong type into a socket that might damage the DIMM.
DDR2 DIMM Details
The 240-pin DDR2 DIMMs use two notches on each side to enable compatibility with both low- and
high-profile latched sockets. The connector key is offset with respect to the center of the DIMM to
prevent inserting it backward in the socket. The key notch is positioned in the center of the area
between pins 64 and 65 on the front (184/185 on the back), and there is no voltage keying because all
DDR2 DIMMs run on 1.8V.
DDR3 DIMM Details
 
 
 
 
 
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