UPGRADING REPAIRING PCs

The following features are common to all SIMMs:
• Pins numbered from left to right
• Same pins on both sides of the module
Memory Types 39
Tip
Note that all dimensions for both SIMMs and DIMMs in the following
figures are in both inches and millimeters (in parentheses).
30-Pin SIMM
The 30-pin SIMM is the oldest type of memory module still in use
(see Figure 2.7). It was popular on 386-based and early 486-based
systems, but became obsolete with the rise of Pentium-class 64-bit
CPUs. Although its capacities are extremely small compared to
more modern memory designs, its unpopularity since the early
1990s makes the 30-pin SIMM the most expensive memory type
per megabyte. If you are still supporting systems that use this type
of module, look for sources of used memory or replace the motherboard
with one that uses newer, 72-pin SIMM or DIMM memory
instead of buying new 30-pin modules.
.250 (6.35)
TYP.
.080 (2.03)
TYP.
PIN 1
3.505 (89.03)
3.495 (88.77)
.100 (2.54)
TYP.
.400 (10.16)
TYP.
.070 (1.78)
TYP.
.653 (16.59)
.647 (16.43)
.133 (3.38)
TYP.
.055 (1.40)
.047 (1.19)
.200 (5.08)
MAX.
Figure 2.7 A typical 30-pin SIMM. The one shown here is 9-bit,
although the dimensions would be the same for 8-bit.
72-Pin SIMM
The 72-pin SIMM was the most popular for a number of years, but
has now been superseded on newer systems by DIMM modules. 72pin
SIMMs are commonly found on late-model 486-based systems,
most Pentiums, and most early Pentium-compatible systems.
Because these modules are also becoming very expensive per
megabyte, try to salvage or swap memory to populate older systems
rather than purchase new. 72-pin SIMMs can be either fast-page or
extended data out (EDO). 486-class systems can use only fast-page
SIMMs, but Pentium-class systems that use SIMMs can use either
type. Fast-page and EDO SIMMs should not be mixed. Some