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<PostHead cat="/FIRMWARE" date="2026-08-14" title="UART is a lab port, not a party trick" lede="A header on the board is a trust boundary we document. Finding TX/RX on a device you own is inventory. Publishing a pinout to brick other people’s gateways is not the job."/>
<P>Firmware reviews sometimes include a lab unit. The dtb already said <span style={{fontFamily:'var(--font-mono)',fontSize:13}}>uart0</span>. The silkscreen sometimes says it out loud. We attach a USB-UART adapter on that unit, at the baud the bootloader prints, and we write down whether we got a shell, a bootloader prompt, or noise.</P>
<Callout>Device from the customer, on the bench, named in the authorization. We do not walk a warehouse. We do not write “how to unbrick your ISP modem” for the internet.</Callout>
<H2>What UART is for, here</H2>
<Ul items={[
'Confirm the boot chain we recovered from the image actually matches the unit.',
'See whether the bootloader is quiet or offers a prompt.',
'See whether the serial console is a login or a raw kernel log.',
'Correlate “dropbear in the image” with “shell on the wire.”'
]}/>
<H3>If we get a shell</H3>
<P>That is a finding if the product is supposed to be sealed. It is expected if the SKU is a development board. Context goes in the sentence. We do not then enable extra daemons “to help.” We photograph the header, note baud, note what we typed that was in-scope (usually: none; reading is enough).</P>
<H3>If we get nothing</H3>
<P>Also a result. Disabled in this build, different pins, or the dump was a different board. Write “no console on uart0 at 115200 on this unit.” Do not keep probing until something breaks.</P>
<H2>JTAG and friends</H2>
<P>Same rule, higher bar. If the letter does not name debug pads, we do not go hunting under the shield. If it does, we treat it as a documented lab interface: is the TAP open, does it halt the CPU, is that expected for this SKU. This is not a course in defeating fused parts.</P>
<H2>How this combines</H2>
<Code lang="flow.txt">{`image: dtb lists uart0
unit:  header present
lab:   adapter, read-only session
report: console yes/no, bootloader prompt yes/no
        — no public pin-by-pin cookbook`}</Code>
<P>The image work still comes first. UART without an unpack is just a cable. Unpack without ever matching a unit is still valid — say you never had hardware.</P>
<Related items={[
['post-fw-boot.html','The boot chain is the map'],
['post-fw-formats.html','What kind of blob is this firmware'],
['post-firmware.html','Unpacking firmware']
]}/>
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