PCIe M.2 — Key B+A Pinout
An M.2 Key B+A edge combines the overlapping B and A cut-outs into one notch across positions 8–19. It is not a standard PCI-SIG dual-key designation.
Electrical warning: mechanical fit in both Key A and Key B sockets does not make their pin assignments compatible. Several contacts map a signal in one socket to ground or to a different function in the other. Never insert a B+A board into either socket unless the complete host and board schematics explicitly define a safe shared mapping.
Connector overview
| Property | Value |
|---|---|
| Connector family | Custom PCI Express M.2 / NGFF module edge |
| Combined notch | Key A positions 8 to 15 plus Key B positions 12 to 19, forming a continuous opening from 8 to 19 |
| Contacts | 75 positions; 63 electrical contacts after the combined notch |
| Mechanical fit | May physically clear a Key A or Key B socket rib; this does not establish electrical compatibility |
| Standard status | No PCI-SIG Key B+A module profile identified; use only as a documented custom implementation |
| Intended use | Breakout, test fixture, or purpose-designed adapter |
| Supply | Depends on the explicitly selected host mapping; do not infer it from the notch |
Standard M.2 Key A and Key B modules use different socket families and different electrical assignments. Cutting both overlapping notches removes twelve contact positions, but it does not create a common pinout.
Contact diagrams
Top side — odd positions
Bottom side — even positions
Contact-position reference
The diagrams document contact numbering and notch geometry only. There is no universal B+A signal assignment to place in a conventional pinout table.
| Side | Positions present | Positions removed |
|---|---|---|
| Top / component side | Odd positions 1–7 and 21–75 | Odd positions 9, 11, 13, 15, 17, and 19 |
| Bottom side | Even positions 2–6 and 20–74 | Even positions 8, 10, 12, 14, 16, and 18 |
The combined opening spans positions 8–19 because the A and B keying zones overlap. Always verify the actual copper pads in the manufacturing data; the absence of a contact can be as important as its assigned signal.
Why the A and B pinouts cannot be merged
The examples below compare standard Key A and Key B socket assignments. They illustrate why a generic B+A electrical mapping would be unsafe.
| Contact(s) | Key A use | Key B use | Design consequence |
|---|---|---|---|
| 1 | GND | CONFIG_3 | Different functions |
| 3 / 5 | USB 2.0 D+ / D− | GND | Direct signal-to-ground conflict |
| 7 | GND | USB 2.0 D+ | Direct signal-to-ground conflict |
| 20–36 | DisplayPort, ground, or related signals | GPIO, UIM, PCIe/USB3 lane 1, and ground | No shared range-level mapping |
| 35 / 37 | PCIe PET pair for link 0 | PCIe PET pair for link 1 or USB 3 transmit | Differential pair serves a different link/function |
| 47 / 49 | PCIe reference clock for link 0 | PCIe/SATA transmit pair for link 0 | Clock-to-data conflict |
| 53 / 55 | PCIe clock request and wake | PCIe reference clock | Sideband-to-clock conflict |
| 59–75 | Optional PCIe link 1, management, control, power, and ground | Antenna control, UIM, configuration, power, and ground | Multiple unrelated assignments |
This table is a compatibility warning, not a substitute for either standardized pinout. Use the dedicated Key A reference and Key B reference when designing for one of those sockets.
Safe design approach
- Choose the one socket mapping that the product actually targets: Key A or Key B.
- Treat the second cut-out as a mechanical feature only unless a controlled host ecosystem defines otherwise.
- Remove or electrically isolate every pad that could meet an incompatible socket contact; do not rely on firmware to prevent an electrical conflict.
- Check power, ground, reset, clocks, differential pairs, sideband voltages, and reserved pins against the host and module schematics.
- Key the enclosure, label the assembly, or add another interlock so that users cannot insert it into an unsupported socket.
For a passive breakout intended to inspect either socket, keep the two mappings separate with selectors, isolation, or mutually exclusive assembly options. A straight-through connection is not a safe universal adapter.
Connector orientation
- The diagrams show the module contact edge, not the socket solder pads.
- The top/component side carries odd-numbered positions; the bottom side carries even-numbered positions.
- The continuous notch replaces positions 8–19.
- A socket footprint may appear mirrored compared with the module. Follow the connector manufacturer’s recommended PCB footprint.
Common mistakes
- Treating B+A as a standardized dual-key option comparable with A+E or B+M.
- Assuming that mechanical fit means compatible power, ground, or data signals.
- Leaving contacts 3, 5, or 7 exposed without resolving the USB-to-ground conflicts.
- Combining Key A and Key B signal names into one table without a product-specific schematic.
- Assuming that the wider notch protects the board from insertion into an unsupported socket.
- Mirroring the contact numbering between the module and socket footprint.
- Hot-plugging an M.2 board without explicit support from the host equipment.
Related connectors
| Connector | Typical use | Compatibility note |
|---|---|---|
| M.2 Key A | Wireless and display connectivity | One possible target mapping, but not electrically interchangeable with Key B |
| M.2 Key B | WWAN, SATA, USB, and limited-lane PCIe modules | One possible target mapping, but not electrically interchangeable with Key A |
| M.2 Key A+E | Wireless modules | Standardized dual-notch arrangement using non-overlapping A and E keying zones |
| M.2 Key B+M | SATA and PCIe storage modules | Standardized dual-notch arrangement using B and M keying zones |
All PCIe M.2 pinout pages
Compatibility guide · Key A · Key A+E · Key B · Key B+A · Key B+E · Key B+M · Key E · Key M
Official references
- PCI Express M.2 specification overview — PCI-SIG
- M.2 Socket 1 enhancements for Keys E and A+E — PCI-SIG
- SATA M.2 card-format technical proposal — SATA-IO
FAQ
Is B+A an official M.2 key combination?
No PCI-SIG B+A module profile was identified in the official material reviewed for this page. The label describes a custom edge with the overlapping B and A cut-outs.
Will a B+A board work in both Key A and Key B sockets?
Not by default. It may fit mechanically, but the two socket pinouts conflict electrically. Compatibility requires a board designed and documented for each specific host.
Can I make a passive universal A-to-B adapter?
Not as a straight-through board. Some contacts are signals in one mapping and ground in the other, while PCIe clocks and data pairs also occupy different positions.
Why are there 63 electrical contacts?
The A notch removes positions 8–15 and the B notch removes positions 12–19. Their overlap produces one twelve-position opening from 8 through 19, leaving 63 of the 75 positions.
Is the board hot-pluggable?
Do not assume it is. Power down the platform before insertion or removal unless the complete equipment documentation explicitly supports hot plugging.