C H A P T E R 4
RAM Expansion
Total RAM capacity using devices currently available is 128 MB and is limited by the space available for the SO-DIMMs. The bottom slot can accommodate a 1.5-inch SO-DIMM. The top slot can accommodate a 2.0-inch SO-DIMM.
Note
Future devices with higher density may allow expansion beyond the current 128-MB limit.
Mechanical Design of RAM SO-DIMMs
The mechanical characteristics of the RAM expansion SO-DIMM are given in the JEDEC specification for the 144-pin 8-byte DRAM SO-DIMM. The specification number is JEDEC MO-190; it is available from the Electronics Industry Association’s web site, at
http://www.jedec.org/download/freestd/pub95/
The specification defines SO-DIMMs with nominal heights of 1.0, 1.25, 1.5, or
2.0inches. The iMac computer can accommodate two SO-DIMMS, one with a height up to 1.5 inches and the other with a height up to 2.0 inches.
IMPORTANT
The JEDEC specifications for the heights of the SO-DIMMs gives a plus-or-minus 0.15 mm tolerance. In the iMac, the specified heights for the SO-DIMMs are maximum heights.
The JEDEC specification defines the maximum depth or thickness of an SO-DIMM as 3.8 mm. That specification is also a maximum: Modules that exceed the specified thickness can cause reliability problems.
The electrical characteristics of the RAM SO-DIMM are given in section 4.5.6 of the JEDEC Standard 21-C, release 7. The specification is available from the Electronics Industry Association’s web site, at
http://www.jedec.org/download/jep21-j.pdf
The specification defines several attributes of the DIMM, including storage capacity and configuration, connector pin assignments, and electrical loading. The specification supports SO-DIMMs with either one or two banks of memory.
