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RAM Expansion on the 17” iMac

This chapter tells how to gain access to the expansion slots in the 17” iMac computer and describes the RAM expansion modules.

In this section:

RAM Expansion Slots
The RAM Expansion Modules for Intrepid IC System

RAM Expansion Slots

The 17” iMac has two RAM expansion slots: one for an SO-DIMM and one for a DIMM, which is preloaded by the factory. The Intrepid configuration has standard 184-pin, 8-byte, DDR266B (PC2100) SO-DIMM (small outline, dual inline memory module) that uses DDR SDRAM devices.

The RAM Expansion Modules for Intrepid IC System

The RAM expansion slot accommodates a standard DDR266B (PC2100) SO-DIMM (small outline, dual inline memory module) that uses DDR SDRAM devices.

An SO-DIMM for the iMac computer can contain either 128, 256, or 512 MB of memory. Maximum total RAM capacity using devices currently available is 640 MB and is limited by the space available for the SO-DIMM.

Mechanical Design of RAM SO-DIMM

The mechanical characteristics of the RAM expansion module are given in the JEDEC specification JESD95 for the 184-pin 8-byte DRAM SO-DIMM. To find out how to obtain the specification, see “RAM Expansion Modules.”

The specification defines SO-DIMMs with nominal heights of 1.0, 1.25, 1.5, or 2.0 inches. The iMac can accommodate standard SO-DIMMs with a height of 1.0 or 1.25 inches.

The JEDEC specification defines the maximum depth or thickness of an SO-DIMM as 4 mm. That specification is also a maximum: Modules that exceed the specified thickness can cause reliability problems.

Electrical Design of RAM SO-DIMM

The electrical characteristics of the RAM SO-DIMM are given in section 4.5.6 of the JEDEC Standard 21-C, release 7 (JESD-21C). To find out how to obtain the specification, see “RAM Expansion Modules.”

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.

The JEDEC specification for the SO-DIMM defines a Serial Presence Detect (SPD) feature that contains the attributes of the module. SO-DIMMs for use in the iMac are required to have the SPD feature. Information about the required values to be stored in the presence detect EEPROM is in section 4.1.2.5 and Figure 4.5.6–C (200-pin DDR SDRAM SO–DIMM, PD INFORMATION) of the JEDEC standard 21-C specification, release 7.

Configuration of DDR SDRAM Devices

The DDR SDRAM devices used in the RAM expansion modules must be self-refresh type devices for operation from a 2.5 V power supply. The speed of the DDR SDRAM devices must be 133 MHz or greater. The preloaded DIMM is 186-pin DDR266B (PC2100).

The devices are programmed to operate with a CAS latency of 2.5. At these CAS latencies, the access time from the clock transition must be 7.5 ns or less. The burst length must be at least 4 and the minimum clock delay for back-to-back random column access cycles must be a latency of 1 clock cycle.

Table 7-1 shows information about the different sizes of DIMMs used in the iMac computer. The first three columns show the memory size, configuration, and number of banks in the DIMMs. The other three columns show the number, density, and configuration of the DDR SDRAM devices making up the memory modules.

Configuration of RAM SO-DIMMs

Table 7-2 shows information about the different sizes of SO-DIMMs used in the iMac computer. The first three columns show the memory size, configuration, and number of banks in the SO-DIMMs. The other three columns show the number, density, and configuration of the DDR SDRAM devices making up the memory modules.

Address Multiplexing

Signals A[0] – A[12] and BA[0] – BA[1] on each RAM SO-DIMM make up a 15-bit multiplexed address bus that can support several different types of DDR SDRAM devices. Table 7-3 lists the types of devices that can be used in the iMac by size, configuration, and sizes of row, column, and bank addresses.

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