SIMM -- The Single In line Memory Module is a PC memory module introduced around 1990. The SIMM consists of a small circuit board with memory chips mounted on it. There are two standard formats -- 30 and 72 pin as well as a number of proprietary formats used by individual vendors. The number of pins refers to the number on one side of the SIMM. Double sided SIMMs exist and have 72 active pins on each side. The terms single and double sided refer to whether the pins on both sides are active, not to whether components are mounted on one or two sides. 1, 4, and 16mb SIMMs are generally single sided. 2, 8 and 32mb SIMMs are usually double sided. Single vs double sided seems to be largely a function of the pinout of the memory chips used and whether it is easy mechanically to wire the SIMM circuit board such that the two sets of input and output pins do not conflict. Some motherboards do not wire the second side of some or all SIMM sockets and will only use half of a double sided SIMM. These will generally work satisfactorily, but only half the memory on the SIMM will be available.
- A similar, but more advanced device called a DIMM format has a 168 pin (84 pins on each side) or 200 pin format. As of 1997, many motherboards will not operate properly with more than one DIMM module even though sockets may be present for several. 30 pin SIMMs have 8 bit wide interfaces. 72 pin SIMMs have 32 bit wide, per side, interfaces. DIMMs have 64 bit wide interfaces.
- SIMMs come in both parity and non-parity versions and in various speeds. SIMMs introduce a significant amount of capacitance and significant data transmission problems relative to socketed/soldered-in components. In general, highest speed and greatest reliability will be experienced when using the minimum number of SIMMs with the least number of mounted chips. 72 pin SIMMs are generally fully interchangeable, but older 486 chipsets sometimes have problems with interchangeability of 30 pin SIMMs. 30 pin SIMMs with 9 discrete ICs on them tend to work better with those chipsets than do the more modern 3 IC versions.
For pinouts refer to web site http://www.interfacebus.com/Memory_Modules_30Pin_SIMM_PinOut.html, http://www.interfacebus.com/Memory_Module_72Pin_EEC_SIMM_PinOut.html
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Copyright 1994-2002 by Donald Kenney.