2026-09-27 · class-notes, a-plus, comptia, messer, hardware, motherboard, pci, pcie, expansion-slots, buses
Full class notes for A+ Core 1 3.5: buses, PCI vs PCIe, parallel vs serial, PCIe lanes, and installing add-in cards safely.
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.
By the end of these notes you should be able to:
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.
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.
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
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.
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.
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.
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.
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.
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.
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.
Expansion slots are a physical-access story, and the risk is specific to what a card on the bus can reach:
| Term | Meaning |
|---|---|
| Bus | A shared pathway on the motherboard carrying data between components. |
| Trace | A fine copper line on the board forming part of a bus. |
| Expansion slot | A connector on the expansion bus for adding an add-in card. |
| Add-in / adapter card | A card fitted to an expansion slot to add capability. |
| PCI | Peripheral Component Interconnect; older parallel expansion bus. |
| PCIe | PCI Express; modern serial expansion bus organised into lanes. |
| Parallel communication | Bits of one transfer sent at once over many wires. |
| Serial communication | Bits sent one at a time over a single pathway. |
| Lane | A PCIe pathway; one path in each direction (full duplex). |
| x1 / x16 | The lane count of a PCIe interface ("by 1", "by 16"). |
| 32-bit / 64-bit bus | PCI widths; 32 or 64 parallel data lines. |
| Keyway / key | Notch-and-tab that enforces card orientation and type. |
| Retention latch | A clip on the board that holds a PCIe card down. |
| Throughput | The rate of data a bus can move; scales with PCIe lane count. |
| Copper (contacts) | The gold edge contacts; none should show once seated. |