Half the control signals on a laptop board are held in place by a single resistor. A pull-up or pull-down decides what a line reads when nothing is actively driving it, and when that resistor, its supply rail or the line itself has a problem, the board can refuse to power on for no visible reason. Recognising these resistors in a schematic makes many "missing signal" faults quick to solve.
What they do
A digital input needs a defined level. If nothing drives it, it floats, picking up noise and reading high or low at random — a floating input. A resistor fixes the default:
- Pull-up — a resistor from the signal to a supply rail. The line reads high unless something pulls it low.
- Pull-down — a resistor from the signal to ground. The line reads low unless something drives it high.
The resistor is weak on purpose. Typical values on laptop boards range from about 1 kΩ to a few hundred kΩ, with 10 kΩ and 100 kΩ very common. A chip can override it easily, but when the chip lets go, the resistor sets the level.
Where you meet them on a laptop board
Open-drain outputs: power good, reset, alerts
Many outputs can only pull low; they cannot drive high. These open-drain output outputs need a pull-up to produce a high level. Power-good outputs of regulators, reset lines and alert lines are almost always open-drain. Several open-drain outputs can share one line with one pull-up, so the line goes high only when every source releases it — a simple way to combine power-good signals.
Enable pins
Regulator enable pins often have a pull-down so the regulator stays off until a controller drives the enable high. Sometimes the enable is pulled up directly from the input rail through a resistor divider, so the regulator starts as soon as input power arrives; the always-on 3.3 V and 5 V regulators are frequently enabled this way.
SMBus and I2C
Clock and data lines on SMBus and I2C are open-drain by design and need pull-ups, usually to an always-on or system 3.3 V rail. A battery that is not detected, a charger that does not respond or a touchpad that disappears can all come down to a missing pull-up rail on these lines.
Strap pins
Some chips read certain pins once at power-up to decide how to configure themselves. A pull-up or pull-down on a strap pin selects an option — boot source, voltage level, debug mode. On many Intel platforms, for example, one audio-bus pin doubles as a flash descriptor override strap. Strap resistors are often marked as optional in the schematic, with one of two positions fitted.
ID resistors
Resistor dividers also encode information: adapter wattage on the adapter ID line, board or panel identification on some designs. These are not pull-ups in the digital sense, but they look similar on the page.
Reading them in the schematic
A pull-up appears as a resistor with one end on the signal and the other on a named rail. The rail name matters as much as the resistor: a PCH_PWROK pulled up to +3VS can only go high once +3VS exists, while one pulled up to +3VALW can go high in standby.
| Pattern | Meaning | What to check when the signal is wrong |
|---|---|---|
Resistor from signal to +3VALW / +3V_S5 | Pull-up available in standby | Rail present? Resistor intact? Something pulling low? |
Resistor from signal to +3VS / run rail | Pull-up only when the system is on | Expect the line low in standby — that is normal |
| Resistor from signal to GND | Pull-down, default low | Is something supposed to drive it high? |
| Two resistors, rail to signal to GND | Divider: sets a voltage or threshold | Both values correct? Divider ratio matches? |
Resistor marked @, NC or DNI | Optional strap position | Which position is fitted on your board? |
Diagnosing a signal stuck low
A line that should be high but measures near 0 V has three possible causes. Work through them in order:
- The pull-up rail is missing. Measure the rail at the far end of the pull-up resistor. No rail, no high level — and the real fault is upstream.
- The pull-up resistor is open or missing. Measure across it with the board unpowered, or compare the voltage on both ends with power applied. A rail on one end and 0 V on the other with no load means the resistor is open.
- Something is pulling the line low. Every chip on the line can sink it. Use the boardview to list every pin on the net, then measure the resistance from the line to ground with the board unpowered. A reading far lower than the pull-up value suggests a leaky or damaged chip, or corrosion between the line and ground.
To find which device is pulling it down, isolate the line where the schematic allows: lift a series resistor, separate a zero-ohm link, or remove the suspect chip's connection if you have no other option. When the line comes up after isolating one device, you have found it.
Diagnosing a signal stuck high
A line that should go low but stays high means its driver is not pulling it down:
- Check that the driving chip is powered and out of reset.
- Check whether the driving chip is supposed to act at this stage of the power sequence.
- Look for a broken trace or an open series resistor between the driver and the point you are measuring.
For power-good and enable lines specifically, see Power Good Signals and Why They Matter and Enable Signals in Laptop Power Sequencing.
Measuring without disturbing the line
Pull-ups are weak, so your measuring instrument matters:
- A digital multimeter in voltage mode has a high input impedance and will not disturb a 100 kΩ pull-up noticeably.
- An analog meter or a low-impedance test mode can pull a weak line down and give a false reading.
- Oscilloscope probes in 10× mode are fine; in 1× mode they load the line more.
- Never short a pulled-up line to ground "to see what happens" unless you know the line is meant to be pulled low there, and never connect it to a different rail.
Liquid damage and leakage
Corrosion often creates unintended pull-downs: a conductive residue between a signal pad and a ground pad acts like a resistor of a few kilohms. The line then sits at an intermediate voltage — neither clean high nor clean low — and the chip reading it may interpret it either way. If you measure an odd voltage such as 1.5 V on a 3.3 V logic line, clean the area thoroughly and measure again.
FAQ
How do I know whether a resistor is a pull-up or just a series resistor?
Look at its other end. A pull-up connects the signal to a rail; a pull-down to ground. A series resistor sits in the path between two parts, with the signal continuing on its other side, often under a slightly different net name.
Can I replace a pull-up with a different value?
In an emergency a close value usually works, but the designer chose it for a reason — speed on a bus, leakage, power consumption. Use the value from the schematic whenever you can.
Why does a power-good line read about half the rail voltage?
Either the pull-up is fighting something partially pulling the line down, such as a leaky chip or corrosion, or two different rails are involved. Measure resistance to ground with power off and inspect the area.
Are pull-ups shown in the boardview?
The resistors are, as two-pin parts on the net. Clicking the signal net highlights the pull-up along with everything else on it, which is the quickest way to find a safe probe point.
Bottom line
When a control signal is wrong, find its pull-up or pull-down in the schematic first. Check the rail it pulls to, the resistor itself and everything that can drag the line the other way. Most stuck signals turn out to be a missing rail or a leaky device rather than a failed controller.