SIMM

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This article is about a type of memory module for computers. For other uses, see SIMM (disambiguation).

A SIMM, or single in-line memory module, is a type of memory module containing random access memory used in computers from the early 1980s to the late 1990s. It differs from a dual in-line memory module (DIMM), the most predominant form of memory module today, in that the contacts on a SIMM are redundant on both sides of the module. SIMMs were standardised under the JEDEC JESD-21C standard.

30-pin SIMM, 256 KB capacity

Most early PC motherboards (8088-based PCs, XTs, and early ATs) used socketed DIP chips. With the introduction of 286-based IBM XT/286, which could use larger amounts of memory, memory modules evolved to save motherboard space and to ease memory expansion. Instead of plugging in eight or nine single DIP DRAM chips, only one additional memory module was needed to increase the memory of the computer. A few 286-based computers used (often non-standard) memory modules like SIPP memory (single in-line pin package). The SIPP's 30 pins often bent or broke during installation, which is why they were quickly replaced by SIMMs which used contact plates rather than pins.

SIMMs were invented and patented by Wang Laboratories. Wang invented what was to become the basic memory module, now known as a SIMM (single in-line memory module) in 1983. The original memory modules were built upon ceramic and had pins. Later the pins were removed and the modules were built on standard PCB material.

The first variant of SIMMs has 30 pins and provides 8 bits of data (plus a 9th error-detection bit in parity SIMMs). They were used in AT (286), 386, 486, Macintosh Plus, Macintosh II, Quadra, Atari STE and Wang VS systems.

The second variant of SIMMs has 72 pins and provides 32 bits of data (36 bits in parity and ECC versions). These appeared first in the early 1990s in the IBM PS/2, and later in systems based on the 486, Pentium, Pentium Pro, early Pentium II, and contemporary/competing chips of other brands. By the mid 90s, 72-pin SIMMs had replaced 30-pin SIMMs in new-build computers, and were starting to themselves be replaced by DIMMs.

Non-IBM PC computers such as UNIX workstations may use proprietary non-standard SIMMs. The Macintosh IIfx uses proprietary non-standard SIMMs with 64 pins.

DRAM technologies used in SIMMs include FPM (Fast Page Mode memory, used in all 30-pin and early 72-pin modules), and the higher-performance EDO DRAM (used in later 72-pin modules).

Due to the differing data bus widths of the memory modules and some processors, sometimes several modules must be installed in identical pairs or in identical groups of four to fill a memory bank. The general rule of thumb is a 286, 386SX, 68000 or low-end 68020 / 68030 (e.g. Atari, Mac LC) system (using a 16 bit wide data bus) would require two 30-pin SIMMs for a memory bank. On 386DX, 486, and full-spec 68020 through 68060 (e.g. Amiga 4000, Mac II) systems (32 bit data bus), either four 30-pin SIMMs or one 72-pin SIMM are required for one memory bank. On Pentium systems (data bus width of 64 bits), two 72-pin SIMMs are required. However, some Pentium systems have support for a "half bank mode", in which the data bus would be shortened to only 32 bits to allow operation of a single SIMM. Conversely, some 386 and 486 systems use what is known as "memory interleaving", which requires twice as many SIMMs and effectively doubles the bandwidth.

The earliest SIMM sockets were conventional push-type sockets. These were soon replaced by ZIF sockets in which the SIMM was inserted at an angle, then tilted into an upright position. To remove one, the two metal or plastic clips at each end must be pulled to the side, then the SIMM must be tilted back and pulled out (low-profile sockets reversed this convention somewhat, like SODIMMs - the modules are inserted at a "high" angle, then pushed down to become more flush with the motherboard). The earlier sockets used plastic retainer clips which were found to break, so steel clips replaced them.

Some SIMMs support presence detect (PD). Connections are made to some of the pins that encode the capacity and speed of the SIMM, so that compatible equipment can detect the properties of the SIMM. PD SIMMs can be used in equipment which does not support PD; the information is ignored. Standard SIMMs can easily be converted to support PD by fitting jumpers, if the SIMMs have solder pads to do so, or by soldering wires on.[1]

30-pin SIMMs[edit]

30-pin SIMM, 256 KB capacity

Standard sizes: 256 KB, 1 MB, 4 MB, 16 MB

30-pin SIMMS have 12 address lines, which can provide a total of 24 address bits. With an 8 bit data width, this leads to an absolute maximum capacity of 16 MB for both parity and non-parity modules (the additional redundancy bit chip usually does not contribute to the usable capacity).

30-pin SIMM Memory Module
Pin # Name Signal Description Pin # Name Signal Description
1 VCC +5 VDC 16 DQ4 Data 4
2 /CAS Column Address Strobe 17 A8 Address 8
3 DQ0 Data 0 18 A9 Address 9
4 A0 Address 0 19 A10 Address 10
5 A1 Address 1 20 DQ5 Data 5
6 DQ1 Data 1 21 /WE Write Enable
7 A2 Address 2 22 VSS Ground
8 A3 Address 3 23 DQ6 Data 6
9 VSS Ground 24 A11 Address 11
10 DQ2 Data 2 25 DQ7 Data 7
11 A4 Address 4 26 QP* Data parity out
12 A5 Address 5 27 /RAS Row Address Strobe
13 DQ3 Data 3 28 /CASP* Parity Column Address Strobe
14 A6 Address 6 29 DP* Data parity in
15 A7 Address 7 30 VCC +5 VDC

* Pins 26, 28 and 29 are not connected on non-parity SIMMs.

72-pin SIMMs[edit]

72-pin EDO DRAM SIMM

Standard sizes: 1 MB, 2 MB, 4 MB, 8 MB, 16 MB, 32 MB, 64 MB, 128 MB (the standard also defines 3.3 V modules with additional address lines and up to 2 GB)

With 12 address lines, which can provide a total of 24 address bits, two ranks of chips, and 32 bit data output, the absolute maximum capacity is 227 = 128 MB.

5 V 72-pin SIMM Memory Module
Pin # Name Signal Description Pin # Name Signal Description
1 VSS Ground 37 MDP1* Data Parity 1 (MD8..15)
2 MD0 Data 0 38 MDP3* Data Parity 3 (MD24..31)
3 MD16 Data 16 39 VSS Ground
4 MD1 Data 1 40 /CAS0 Column Address Strobe 0
5 MD17 Data 17 41 /CAS2 Column Address Strobe 2
6 MD2 Data 2 42 /CAS3 Column Address Strobe 3
7 MD18 Data 18 43 /CAS1 Column Address Strobe 1
8 MD3 Data 3 44 /RAS0 Row Address Strobe 0
9 MD19 Data 19 45 /RAS1 Row Address Strobe 1
10 VCC +5 VDC 46 NC Not Connected
11 NU [PD5#] Not Used [Presence Detect 5 (3v3)] 47 /WE Read/Write Enable
12 MA0 Address 0 48 NC [/ECC#] Not Connected [ECC presence (if grounded) (3v3)]
13 MA1 Address 1 49 MD8 Data 8
14 MA2 Address 2 50 MD24 Data 24
15 MA3 Address 3 51 MD9 Data 9
16 MA4 Address 4 52 MD25 Data 25
17 MA5 Address 5 53 MD10 Data 10
18 MA6 Address 6 54 MD26 Data 26
19 MA10 Address 10 55 MD11 Data 11
20 MD4 Data 4 56 MD27 Data 27
21 MD20 Data 20 57 MD12 Data 12
22 MD5 Data 5 58 MD28 Data 28
23 MD21 Data 21 59 VCC +5 VDC
24 MD6 Data 6 60 D29 Data 29
25 MD22 Data 22 61 MD13 Data 13
26 MD7 Data 7 62 MD30 Data 30
27 MD23 Data 23 63 MD14 Data 14
28 MA7 Address 7 64 MD31 Data 31
29 MA11 Address 11 65 MD15 Data 15
30 VCC +5 VDC 66 NC [/EDO#] Not Connected [EDO presence (if grounded) (3v3)]
31 MA8 Address 8 67 PD1x Presence Detect 1
32 MA9 Address 9 68 PD2x Presence Detect 2
33 /RAS3 Row Address Strobe 3 69 PD3x Presence Detect 3
34 /RAS2 Row Address Strobe 2 70 PD4x Presence Detect 4
35 MDP2* Data Parity 2 (MD16..23) 71 NC [PD (ref)#] Not Connected [Presence Detect (ref) (3v3)]
36 MDP0* Data Parity 0 (MD0..7) 72 VSS Ground

* Pins 35, 36, 37 and 38 are not connected on non-parity SIMMs. [2]

/RAS1 and /RAS3 are only used on two-rank SIMMS: 2, 8, 32, and 128 MB.

# These lines are only defined on 3.3V modules.

x Presence Detect signals are detailed in JEDEC Standard.

Proprietary SIMMs[edit]

GVP 64-pin[edit]

Several CPU cards from Great Valley Products for the Commodore Amiga used special 64-pin SIMMs (32 bits wide, 1, 4 or 16 MB, 60 ns).

Apple 64-pin[edit]

Dual-ported 64-pin SIMMs were used in Apple Macintosh IIfx computers to allow overlapping read/write cycles (1, 4, 8, 16 MB, 80 ns).[3][4]

5V 64-pin Mac IIfx SIMM Memory Module[5]
Pin # Name Signal Description Pin # Name Signal Description
1 GND Ground 33 Q4 Data output bus, bit 4
2 NC Not connected 34 /W4 Write-enable input for RAM IC 4
3 +5V +5 volts 35 A8 Address bus, bit 8
4 +5V +5 volts 36 NC Not connected
5 /CAS Column address strobe 37 A9 Address bus, bit 9
6 D0 Data input bus, bit 0 38 A10 Address bus, bit 10
7 Q0 Data output bus, bit 0 39 A11 Address bus, bit 11
8 /W0 Write-enable input for RAM IC 0 40 D5 Data input bus, bit 5
9 A0 Address bus, bit 0 41 Q5 Data output bus, bit 5
10 NC Not connected 42 /W5 Write-enable input for RAM IC 5
11 A1 Address bus, bit 1 43 NC Not connected
12 D1 Data input bus, bit 1 44 NC Not connected
13 Q1 Data output bus, bit 1 45 GND Ground
14 /W1 Write-enable input for RAM IC 1 46 D6 Data input bus, bit 6
15 A2 Address bus, bit 2 47 Q6 Data output bus, bit 6
16 NC Not connected 48 /W6 Write-enable input for RAM IC 6
17 A3 Address bus, bit 3 49 NC Not connected
18 GND Ground 50 D7 Data input bus, bit 7
19 GND Ground 51 Q7 Data output bus, bit 7
20 D2 Data input bus, bit 2 52 /W7 Write-enable input for RAM IC 7
21 Q2 Data output bus, bit 2 53 /QB Reserved (parity)
22 /W2 Write-enable input for RAM IC 2 54 NC Not connected
23 A4 Address bus, bit 4 55 /RAS Row address strobe
24 NC Not connected 56 NC Not connected
25 A5 Address bus, bit 5 57 NC Not connected
26 D3 Data input bus, bit 3 58 Q Parity-check output
27 Q3 Data output bus, bit 3 59 /WWP Write wrong parity
28 /W3 Write-enable input for RAM IC 3 60 PDCI Parity daisy-chain input
29 A6 Address bus, bit 6 61 +5V +5 volts
30 NC Not connected 62 +5V +5 volts
31 A7 Address bus, bit 7 63 PDCO Parity daisy-chain output
32 D4 Data input bus, bit 4 64 GND Ground

See also[edit]

References[edit]

  1. ^ Making Standard SIMMs Work – Memory Upgrade on the HP LaserJet 6MP/5MP Article on fitting jumpers to add Presence Detect to standard SIMMs
  2. ^ JEDEC Standard No. 21-C, Section 4.4.2 "72 pin SIMM DRAM Module Family"
  3. ^ Macintosh IIfx
  4. ^ Apple Computer, Inc. (1990). Guide to the Macintosh Family Hardware (2nd ed.). Addison-Wesley, Inc. p. 230. 
  5. ^ Apple Computer, Inc. (1990). Guide to the Macintosh Family Hardware (2nd ed.). Addison-Wesley, Inc. pp. 214–222. 

External links[edit]