SIMM

From Wikipedia, the free encyclopedia

30- (top) and 72-pin (bottom) SIMMs. Early 30-pin modules commonly had either 256 KB or 1 MB of memory.

A SIMM, or single in-line memory module, is a type of memory module used for random access memory in personal computers. It differs from a DIMM (the most predominant form of memory module today) in that the contacts on a SIMM are redundant on both sides of the module.

Most early PC motherboards (8088 based PCs and XTs) used socketed DIP chips. With the introduction of 80286 based PC/ATs, which could use larger amounts of memory, memory modules evolved from the need of saving motherboard space and to ease memory expansion. Instead of plugging in 8 or 9 single DRAM DIP chips, only one additional memory module was needed to increase the memory of the computer. A few 80286-based computers used (often non-standard) memory modules like SIPP memory (single in-line pin package). 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.

The first variant of SIMMs has 30 pins and provides 8 bits of data (9 bits in parity versions). Therefore, systems usually required that 30 pin SIMMs would have to be installed in sets of 4 (because processors such as the 80486 had 32 bit data paths).

The second variant of SIMMs—sometimes called PS/2 after the IBM line of personal computers in which it was first used—has 72 pins and provides 32 bits of data (36 bits in parity versions). Around 1997, 72-pin SIMMs replaced 30-pin SIMMs.

The Macintosh IIfx requires non-standard 64-pin SIMMs.

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. For instance, on 80386 or 80486 systems (data bus width of 32 bits), 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.

The earliest SIMM sockets were conventional push-type sockets. These were soon replaced by ZIF (Zero Insertion Force) sockets in which the SIMM was inserted and rotated until it locked into place. To install a SIMM, the module must be placed in the socket at an angle, then rotated (angled) into 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. The earlier sockets uses plastic retainer clips which were found to break, so steel clips replaced them.

RAM technologies used on SIMMs include EDO and FPM.

SIMM is standardised under JEDEC JESD-21C standard.

1 Standard sizes available
2 Pinouts
3 See also
4 External links

[edit] Standard sizes available

30-pin SIMM: 256KB, 1MB, 4MB, 16MB

72-pin SIMM: 1MB, 2MB, 4MB, 8MB, 16MB, 32MB, 64MB, 128MB

[edit] Pinouts

72-pin ECC SIMM Memory Module
Pin #	Non-Parity	Parity	Signal Description
1	V_SS	V_SS	Ground
2	DQ0	DQ0	Data 0
3	DQ1	DQ1	Data 1
4	DQ2	DQ2	Data 2
5	DQ3	DQ3	Data 3
6	DQ4	DQ4	Data 4
7	DQ5	DQ5	Data 5
8	DQ6	DQ6	Data 6
9	DQ7	DQ7	Data 7
10	V_CC	V_CC	+5 VDC
11	PD1	PD1	Presence Detect 1
12	A0	A0	Address 0
13	A1	A1	Address 1
14	A2	A2	Address 2
15	A3	A3	Address 3
16	A4	A4	Address 4
17	A5	A5	Address 5
18	A6	A6	Address 6
19	A10	A10	Address 10
20	n/c	PQ8	Data 8 (Parity 1)
21	DQ9	DQ9	Data 9
22	DQ10	DQ10	Data 10
23	DQ11	DQ11	Data 11
24	DQ12	DQ12	Data 12
25	DQ13	DQ13	Data 13
26	DQ14	DQ14	Data 14
27	DQ15	DQ15	Data 15
28	A7	A7	Address 7
29	A11	A11	Address 11
30	V_CC	V_CC	+5 VDC
31	A8	A8	Address 8
32	A9	A9	Address 9
33	/RAS3	RAS3	Row Address Strobe 3
34	/RAS2	RAS2	Row Address Strobe 2
35	DQ16	DQ16	Data 16
36	n/c	PQ17	Data 17 (Parity 2)
37	DQ18	DQ18	Data 18
38	DQ19	DQ19	Data 19
39	V_SS	V_SS	Ground
40	/CAS0	CAS0	Column Address Strobe 0
41	/CAS2	CAS2	Column Address Strobe 2
42	/CAS3	CAS3	Column Address Strobe 3
43	/CAS1	CAS1	Column Address Strobe 1
44	/RAS0	RAS0	Row Address Strobe 0
45	/RAS1	RAS1	Row Address Strobe 1
46	A12	A12	Address 12
47	/WE	WE	Read/Write
8	A13	A13	Address 13
49	DQ20	DQ20	Data 20
50	DQ21	DQ21	Data 21
51	DQ22	DQ22	Data 22
52	DQ23	DQ23	Data 23
53	DQ24	DQ24	Data 24
54	DQ25	DQ25	Data 25
55	n/c	PQ26	Data 26 (Parity 3)
56	DQ27	DQ27	Data 27
57	DQ28	DQ28	Data 28
58	DQ29	DQ29	Data 29
59	DQ31	DQ31	Data 31
60	DQ30	DQ30	Data 30
61	V_CC	V_CC	+5 VDC
62	DQ32	DQ32	Data 32
63	DQ33	DQ33	Data 33
64	DQ34	DQ34	Data 34
65	n/c	PQ35	Data 35 (Parity 4)
66	PD2	PD2	Presence Detect 2
67	PD3	PD3	Presence Detect 3
68	PD4	PD4	Presence Detect 4
69	PD5	PD5	Presence Detect 1
70	PD6	PD6	Presence Detect 6
71	PD7	PD7	Presence Detect 7
72	V_SS	V_SS	Ground