72 pin SO DIMM
The 72-pin SO-DIMM (Small Outline Dual Inline Memory Module) is the compact, laptop-oriented counterpart of the full-size 72-pin SIMM, providing a 32-bit Fast Page Mode (FPM) or Extended Data-Out (EDO) DRAM interface on a reduced-footprint PCB. It is standardised under JEDEC JESD21-C (referenced as module-444) and operates at 5 V or 3.3 V. The format was prevalent in notebook computers and compact desktop machines from approximately 1992 to 1998, before being superseded by the 144-pin SDRAM SO-DIMM.
- SO = Small Outline
- DIMM = Dual Inline Memory Module
- DRAM = Dynamic Random Access Memory
| PIN | FRONT | PIN | BACK | PIN | FRONT | PIN | BACK |
|---|---|---|---|---|---|---|---|
| 1 | VSS | 2 | DQ0 | 37 | DQ16 | 38 | DQ17 |
| 3 | DQ1 | 4 | DQ2 | 39 | VSS | 40 | CAS0# |
| 5 | DQ3 | 6 | DQ4 | 41 | CAS2# | 42 | CAS3# |
| 7 | DQ5 | 8 | DQ6 | 43 | CAS1# | 44 | RAS0# |
| 9 | DQ7 | 10 | VDD | 45 | NC/RAS1#** | 46 | NC (A12) |
| 11 | PRD1 | 12 | A0 | 47 | WE# | 48 | NC (A13) |
| 13 | A1 | 14 | A2 | 49 | DQ18 | 50 | DQ19 |
| 15 | A3 | 16 | A4 | 51 | DQ20 | 52 | DQ21 |
| 17 | A5 | 18 | A6 | 53 | DQ22 | 54 | DQ23 |
| 19 | A10 | 20 | NC | 55 | NC | 56 | DQ24 |
| 21 | DQ8 | 22 | DQ9 | 57 | DQ25 | 58 | DQ26 |
| 23 | DQ10 | 24 | DQ11 | 59 | DQ28 | 60 | DQ27 |
| 25 | DQ12 | 26 | DQ13 | 61 | VDD | 62 | DQ29 |
| 27 | DQ14 | 28 | A7 | 63 | DQ30 | 64 | DQ31 |
| 29 | NC (A11) | 30 | VDD | 65 | NC | 66 | PRD2 |
| 31 | A8 | 32 | A9 | 67 | PRD3 | 68 | PRD4 |
| 33 | NC/RAS3#** | 34 | RAS2# | 69 | PRD5 | 70 | PRD6 |
| 35 | DQ15 | 36 | NC | 71 | PRD7 | 72 | VSS |
(**) 8MB version only
Notes
- The module carries contacts on both the front and back edges, hence “Dual Inline”.
- Pins labelled NC/RAS1# (pin 45) and NC/RAS3# (pin 33) are No Connect on standard modules and are used as RAS1# and RAS3# only on 8 MB double-sided modules.
- Pins NC (A11) (pin 29), NC (A12) (pin 46), and NC (A13) (pin 48) are No Connect on smaller-capacity modules; A11, A12, and A13 become active on higher-density modules that require additional address lines.
- PRD1–PRD7 (pins 11, 66–71) are presence-detect pins used to encode module capacity and speed grade to the host system.
- The 72-pin SO-DIMM is keyed differently from the full-size 72-pin SIMM and is not mechanically interchangeable with it.
- Both 5.0 V and 3.3 V variants exist; the socket voltage key differentiates them, matching the corresponding JEDEC sub-family definition.
Signals
- VSS — Ground reference (pins 1, 39, 72).
- VDD — Supply voltage; 5.0 V or 3.3 V depending on module variant (pins 10, 30, 61).
- DQ0–DQ31 — Bidirectional 32-bit data bus.
- A0–A10 — Multiplexed row/column address bus (A0–A10 active on all modules).
- A11 — Additional address line; NC on smaller-capacity modules, active on higher-density devices (pin 29).
- A12 — Additional address line; NC on lower-density modules (pin 46).
- A13 — Additional address line; NC on lower-density modules (pin 48).
- RAS0# — Row Address Strobe, bank 0 (pin 44); latches row address on falling edge.
- RAS1# — Row Address Strobe, bank 1 (pin 45); 8 MB modules only; NC otherwise.
- RAS2# — Row Address Strobe, bank 2 (pin 34).
- RAS3# — Row Address Strobe, bank 3 (pin 33); 8 MB modules only; NC otherwise.
- CAS0#–CAS3# — Column Address Strobe signals for byte-lane or bank selection (pins 40–43).
- WE# — Write Enable; low selects a write cycle (pin 47).
- PRD1–PRD7 — Presence-detect pins; resistor-coded to identify module capacity and speed to the host controller (pins 11, 66–71).
- NC — No Connect; unused on the specified module density.