A+ Core 1 3.5: Motherboard Expansion Slots — Class Notes

Class notes · Professor Messer A+ Core 1 (220-1201) · Section 3, lesson 5

Quick reference: the short version of this lesson lives in the A+ Core 1 Section 3 resource sheets. This is lesson 3.5, sitting between Storage devices (3.4) and Computer power (3.6), and it deals with the expansion buses you use to add cards to a board.

Learning objectives

By the end of these notes you should be able to:

1. Explain what a computer bus is and why a motherboard is built from several of them.
2. Describe conventional PCI: its parallel signalling, its 32-bit and 64-bit forms, and the keying that stops a card seating the wrong way.
3. Describe PCI Express (PCIe): its serial signalling and the idea of lanes, and read an interface label such as “PCIe x1”.
4. Compare PCI and PCIe physically on a real board (keyway position, slot length, retention latch).
5. Install and remove an add-in card safely without damaging the board.

1. The motherboard as a system of buses

Look at a motherboard from above and it resembles a small city seen from the air: distinct districts of components, with roads running between them. Those roads are buses. A bus is simply a shared pathway that carries information from one part of the board to another, so that many very different components behave as a single working system.

There is not one bus but several. A memory bus connects the CPU to the memory slots. A separate expansion bus connects the expansion slots to the rest of the system. Keeping these on their own pathways lets each be designed for its own job while still tying everything together.

Buses are also how you grow a system. The expansion bus exists precisely so that you can drop additional cards onto the board and add capability that wasn’t there before: a faster graphics card, a network card, a capture card, extra storage controllers. If you trace the fine copper lines (the traces) on a board, you can follow one side of a bus connection all the way round to where it lands on the other side.

+---------+   +---------+   +-----------------------------+
|   CPU   |   |   RAM   |   |   Expansion slots (cards)   |
+----+----+   +----+----+   +--------------+--------------+
     |             |                       |
     +=====+=======+===========+===========+
           |                   |
      memory bus          expansion bus
   (CPU <-> RAM)     (CPU <-> add-in cards)

The two expansion technologies you meet in this lesson are PCI, the older one, and PCIe, the one on essentially every modern board.

2. PCI — Peripheral Component Interconnect

On older motherboards one of the available buses is the PCI bus, short for Peripheral Component Interconnect. Newer boards have usually dropped it in favour of PCI Express, but you will still meet it in the field and on the exam.

Note (beyond this lesson): the transcript dates PCI to 1994. The first PCI Local Bus specification actually appeared in 1992, with the 2.x revisions that made it ubiquitous following across the mid-1990s, so “early 1990s” is the safest way to place it. Nothing else about how it works changes.

2.1 Parallel signalling, 32-bit and 64-bit

PCI sends data over parallel communication, and it comes in two widths: a 32-bit bus and a 64-bit bus. Parallel means the bits of a single transfer travel side by side on their own separate wires and are meant to arrive together at the far end. A 32-bit PCI bus therefore has 32 separate connections; when you send 32 bits, one bit rides each of the 32 lines and, ideally, all 32 appear simultaneously on the other side. A 64-bit PCI bus works in exactly the same way but doubles the count to 64 lines carrying 64 bits at once.

PCI (parallel), 32-bit:            PCIe (serial), one x1 lane:

  bit  0 ----------------->          --- one bit at a time --->
  bit  1 ----------------->          <-- one bit at a time ----
  bit  2 ----------------->
   ...   (32 wires total)           one lane = two paths, one
  bit 31 ----------------->         per direction (full duplex)

  32 bits arrive together            x4 = four such lanes, and
  across 32 separate wires           roughly 4x the throughput

2.2 Keying: the tabs in the slot

Look closely at a PCI slot and you will see small tabs or keys part way along the connector. These keys designate the power arrangement for the cards that plug in, and they physically stop a card seating in a slot it does not belong in. The matching card has notches cut into its edge connector that slide past those tabs, so a card only drops in when it is the right type and the right way round.

A 64-bit card is longer than a 32-bit card: it carries the extra 32 bits on an additional run of contacts beyond the 32-bit section, with its own separate keyway marking it out as a 64-bit card.

Exam tip: the key/notch scheme is what enforces compatibility. If a card will not drop into place, do not force it — the keying is telling you the card and slot do not match.

3. Installing an add-in card

Fitting a card is mechanically simple, but the “carefully” matters. Line the card’s notches up with the keys in the slot so it is correctly oriented, rest it squarely on top of the slot, then press straight down with steady, even pressure. Too much force, or pressing at an angle, risks cracking the board or its components.

When it is seated correctly the card sits snugly and you should no longer see any of the gold copper contacts above the slot — if a strip of copper is still visible, it usually needs pushing down a little further. Once home, the card is screwed to the computer’s case at its metal bracket so it cannot work loose or be knocked out of the slot.

Caution: as with any work inside the case, power down and follow ESD precautions first. A card that is only half-seated can behave like a faulty card — no output, intermittent errors — so reseating is a sensible first check before condemning the hardware.

4. PCI Express (PCIe)

PCI Express, or PCIe, is the newer bus and the one you will find on the most recent computers. Its key difference from PCI is that it communicates over a serial connection rather than a parallel one. Instead of spreading a transfer across many wires at once, serial sends one bit at a time down the same pathway.

4.1 Lanes

Each PCIe pathway is called a lane. A lane is not a single wire but a pair of paths: one carrying data in each direction, so a lane is full duplex.

An interface is labelled by how many lanes it has. “PCIe x1” is spoken “PCIe by 1” and means a single-lane interface; “x2” is “by 2”, then “by 4”, “by 8”, and so on. Because the signalling is serial you do not need the wide 32- or 64-wire pathways of PCI to go faster — you simply add more lanes. A PCIe x4 interface is four lanes wide and moves data at roughly four times the throughput of a PCIe x1.

4.2 Worked example — reading a slot and picking a card

You are fitting a low-profile network card. The card’s edge connector is short and its notch is near the end closest to the bracket. On the board you see two empty slots: a very short one and a much longer one.

1. The short notch tells you this is a PCIe card (PCIe notches sit closer to the board edge than PCI’s), and its short length points to a PCIe x1 card.
2. The short slot is a PCIe x1 slot; the long slot is a PCIe x16 slot (the length you would use for a graphics card).
3. A x1 card physically fits and works in the x1 slot, so use the short slot and save the long x16 slot for something that needs the extra lanes.
4. Orient the notch to the slot’s key, press straight down until no copper shows, then screw the bracket to the case.

The general rule the example shows: a smaller PCIe interface (fewer lanes) is a smaller physical slot, which lets a board dedicate only as much room as a card actually needs.

5. Telling PCI and PCIe apart on a board

A board can carry both PCI and PCIe slots at once, and at a glance they look similar. Three cues separate them.

Keyway position. On a PCI slot the key/notch sits further back, away from the edge of the motherboard. On a PCIe slot the key sits closer to the edge. This is also how you read a card: a PCIe card has its notch nearer the bracket end.

Slot length. The slots are different sizes depending on the bus. The shortest PCI interface is the 32-bit slot; the shortest PCIe interface is the x1 slot. Because PCIe scales by lane count, its slots range from the tiny x1 up to the long x16.

Retention latch. Many PCIe cards — x16 graphics cards especially — have a small hook on the back of the card that clips into a retention latch on the motherboard. That fastens the card down to the board itself, on top of the screw holding its bracket to the case.

Board edge (nearest the case bracket)
|
v
+--------------------------------------------------+
| PCIe x1 :  [==]#[====]                            |  shortest
| PCIe x16:  [==]#[==========================]      |  longest PCIe
| PCI 32  :  [========]##[==============]           |  key set back
| PCI 64  :  [========]##[=====================]    |  longest PCI
+--------------------------------------------------+
   #  = notch near the edge  -> PCIe
   ## = notch set further back -> PCI
   (schematic: relative length and notch position, not to scale)

Exam tip: when removing a PCIe card, undo the case screw and release the retention latch at the far end. Pulling a latched card out by force is a common way to damage the slot.

6. Security perspective

Expansion slots are a physical-access story, and the risk is specific to what a card on the bus can reach:

• A card in a slot sits on the system bus. An add-in card is not a peripheral held at arm’s length by the OS; it is wired into the machine’s internal pathways. That is exactly why an unexpected card in a previously empty slot is worth treating as an incident rather than a curiosity.
• PCIe devices can use DMA. Direct Memory Access lets a device read and write system memory without going through the CPU. That is what makes PCIe fast, and also what makes a rogue PCIe card (the PCILeech class of attack tooling) able to lift keys, credentials and decrypted data straight out of RAM — even on a machine with an encrypted disk, because the disk is unlocked while running. The same DMA exposure is why external Thunderbolt, which tunnels PCIe, is treated with such caution.
• Open slots and case access are the control. The defence is boringly physical: lock the case, account for every slot, and know what is supposed to be installed. Full-disk encryption protects a powered-off machine; it does not protect memory on a running one, so a DMA-capable card fitted to a live system defeats it.
• Half-seated is not just a fault, it’s noise. A card that only sometimes registers muddies the picture during triage — rule out a poor seat before assuming tampering or failure.

Summary

• A bus is a shared pathway on the board; a motherboard is built from several (memory bus, expansion bus, and others).
• PCI is the older expansion bus: parallel, in 32-bit and 64-bit widths, sending all the bits of a transfer at once across many wires.
• PCI slots are keyed; 64-bit cards are longer with an extra keyed section.
• PCIe is the modern bus: serial, one bit at a time, organised into lanes. “PCIe x1/x4/x16” states the lane count, and more lanes means more throughput. Each lane is full duplex.
• Physically: PCIe notches sit closer to the edge, PCI notches further back; PCIe slots range from tiny x1 to long x16; many PCIe cards add a retention latch.
• Install by aligning the key, pressing straight down until no copper shows, and screwing the bracket to the case. Remove a PCIe card by undoing the screw and releasing the latch.

Glossary

Review questions

1. In one sentence, what is a computer bus?
2. Name two different buses found on a motherboard and what each connects.
3. Does PCI use parallel or serial communication, and what does that mean for how a 32-bit transfer travels?
4. What are the two width variants of the PCI bus?
5. How does a 64-bit PCI card differ physically from a 32-bit one?
6. What is the purpose of the tabs/keys in a PCI slot?
7. Does PCIe use parallel or serial communication?
8. What is a PCIe “lane”, and how is “PCIe x4” read aloud?
9. Roughly how does the throughput of a PCIe x4 interface compare to x1, and why can PCIe add speed without wider physical slots?
10. Give two physical ways to tell a PCIe slot from a PCI slot on the same board.
11. A graphics card will not drop into its slot no matter how you orient it. What is the most likely cause, and what should you not do?
12. Scenario: you must remove a seated PCIe x16 graphics card. List the two things you release before lifting it out, and why forcing it is risky.

Answer key

1. A shared pathway that carries data between the components of the motherboard. It is the “road” tying diverse parts into one system.
2. Memory bus (CPU to RAM) and expansion bus (CPU to add-in cards). Each pathway is dedicated to its role.
3. Parallel; the 32 bits travel side by side on 32 separate wires and arrive together. Width equals the number of simultaneous data lines.
4. 32-bit and 64-bit. Same signalling method, twice the lines.
5. It is longer, adding a further run of contacts with its own keyway for the extra 32 bits. The keyway marks it as 64-bit.
6. They set the card’s orientation/type and physically stop the wrong card seating. Compatibility is enforced mechanically.
7. Serial — one bit at a time down a pathway. No wide parallel bundle.
8. A lane is a PCIe pathway with one path each direction (full duplex); “PCIe x4” is read “PCIe by 4”. The number is the lane count.
9. Roughly four times the throughput of x1, because PCIe scales by adding serial lanes rather than widening the slot. More lanes, more bandwidth.
10. PCIe notches sit closer to the board edge (PCI’s are set further back), and PCIe slots come in different lengths from short x1 to long x16. Latch presence is a third cue.
11. The keying does not match — it is the wrong card/slot. Do not force it. Forcing risks breaking the card or board.
12. Undo the case bracket screw and release the retention latch at the far end. A latched card levered out by force can damage the slot.