BBS:      TELESC.NET.BR
Assunto:  Re: serial port signal path
De:       Computer Nerd Kev
Data:     Thu, 29 Jan 2026 09:12:05 +1000
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bp@www.zefox.net wrote:
> Does anybody happen to know how data travels between gpio pins
> 8 and 10 and the system console? In other words, what hardware
> and maybe software facilitate the exchange? For example, the
> internal ethernet adapter is connected to the CPU via USB. Is
> something analogous for the system console?

I know with the BCM2835 (Pi 1 and Zero) both the UARTs (serial
ports) are part of the SoC, as described in the peripherals
documentation (often called the datasheet, though it's not really).
They're "peripherals", but part of the chip so the signals connect
directly - definitely no USB interface in-between. Other RPis
_could_ be different but it's extremely unlikely because at least
one UART peripheral is pretty essential, even just during product
development.

> I'm seeing what look like console failures on older-model Pi2s
> (v1.1) and wondering if some accessory subsystem, (usb comes
> to mind but I don't think that's it specifically) when running
> FreeBSD-current.

Even though it's part of the same SoC as the CPU and other things,
the UART is still something that can fail or be misconfigured even
if there's nothing like USB in-between. Running a program/script
that toggles the power light on/off (or another GPIO signal that
you monitor) would indicate whether the CPU is still running when
the serial console hangs.

> The machine stops without complaint or warning, does not answer
> ping and has to be power cycled.

Since it doesn't respond to a ping either, that suggests that it's
probably crashed completely.

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