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For a typical desktop build, four motherboard headers can look like unnecessary leftovers: COM, SPI TPM, thermal sensor, and Thunderbolt add-in-card headers. Each has a real use, but none is a must-connect feature for every PC. They are documented together on the ASUS PRIME B550-PLUS, which makes them a useful set of examples—not a definitive list of four headers found on all motherboards.
Why do motherboards still have headers most builders never use?
Motherboards serve more than one kind of user. An optional header can support legacy equipment, a particular security setup, extra temperature monitoring, or an expansion card. That does not mean every board has these connections or that a header is useful to every owner.
The contrast is especially clear with specialized hardware: ASUS’s H610A-IM-A industrial motherboard documents interfaces such as serial, parallel, GPIO, I2C, PS/2, debug, and TPM connections. Those options serve requirements beyond an ordinary desktop build; their presence does not establish why any manufacturer chose a particular consumer-board layout.
What does each header do?
| Header | Job | What else you need | When it may be useful |
|---|---|---|---|
| COM / serial | Provides a connection for a serial port module. | A compatible COM module, installed in a chassis slot opening. | When legacy or specialized equipment needs a serial connection. |
| SPI TPM | Connects a discrete Trusted Platform Module for security functions. | A TPM module supported by the exact motherboard. | When the system needs a discrete TPM and firmware TPM is not the chosen or available option. |
| Thermal sensor | Reads temperature from a thermistor cable placed at a chosen point. | A compatible thermistor cable and software or firmware support for reading it. | When an extra measurement of a cooling setup or hot spot is useful. |
| Thunderbolt | Connects a compatible Thunderbolt add-in I/O card. | A supported add-in card and any required PCIe slot specified for the board. | When a compatible expansion-card configuration is needed for Thunderbolt I/O. |
COM: a way to add a serial port
A COM header is for a COM port module, not a direct connection for plugging a serial cable into the motherboard. ASUS’s Z790-AYW WIFI W manual says to connect the module cable to the header and install the module at a chassis slot opening. This is useful if equipment you own requires serial I/O; most ordinary desktop builds do not need to add it. Check the motherboard manual and module compatibility before buying.
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SPI TPM: an optional discrete security module
A Trusted Platform Module can securely store items such as keys, digital certificates, and passwords, and can help protect digital identity and platform integrity. An SPI TPM header accepts a compatible discrete TPM. However, MSI notes that recent processors include firmware TPM 2.0, so many users do not need to buy a separate module. Check the firmware TPM settings and the exact board’s documentation before purchasing one.
Thermal sensor: one more temperature measurement
A thermal sensor header accepts a thermistor cable, which can measure the temperature of a selected object or area. That extra reading may help someone monitoring a custom cooling setup or a particular hot spot. It is an optional monitoring point, not a required connection for a basic PC.
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- Connectivity: PCIe 5.0, 3x M.2 Slots, USB-C, Sensor Panel Link
Thunderbolt: a route for a compatible add-in card
A Thunderbolt header links the motherboard to a compatible Thunderbolt add-in I/O card; the header alone does not provide a Thunderbolt port. ASUS’s Z790-AYW WIFI W manual describes an add-in card arrangement and specifies a required PCIe slot for that particular board. MSI also documents a JTBT5 header for its Thunderbolt 5 expansion card. These examples do not establish universal card compatibility: check the motherboard’s supported configuration and the card maker’s compatibility information.
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- Connectivity: PCIe 5.0, 4x M.2 Slots, Dual USB4, Front and Rear USB-C, Sensor Panel Link
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How to decide whether to use one
- Identify the exact board. Find its model name and consult its manual; a similar header label on another board does not guarantee the same pinout or accessory support.
- Match the header to a real need. Consider COM for serial equipment, a discrete TPM only if firmware TPM does not meet your needs, a thermal sensor for an extra temperature reading, or a Thunderbolt header for a supported add-in-card setup.
- Confirm the complete accessory configuration. Check the manual for the required module or cable, supported card, and any required slot. Do not buy based on the header name alone.
- Leave it unconnected if you have no use for it. These are optional connections; their presence does not mean the PC is incomplete.
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