> On Jan 31, 2025, at 10:37 PM, Maciej W. Rozycki <ma...@orcam.me.uk> wrote:
>
> On Fri, 31 Jan 2025, Paul Koning wrote:
>
>>> FWIW I was able to get reliable serial communication under Linux of up to
>>> 3.5Mbps with off-the-shelf proper PCIe UART hardware clocked at 62.5MHz
>>> despite that line drivers used with said hardware (soldered onboard) were
>>> spec'd for up to 1MHz only[1]. This was with plain PIO interrupt-driven
>>> operation, but then the UARTs used had decent FIFO sizes of 128 characters
>>> and the FIFO trigger level for the interrupt was reasonably set.
>>>
>>> Finally at 4.0Mbps data corruption reproducibly triggered, but it was
>>> garbled rather than lost characters, so I conclude the reason was either
>>> line drivers finally giving up or the transmission frequency hitting the
>>> bandwidth limit of the serial communication cable used.
>>
>> Was that with an actual RS232 port, i.e., a device using RS232 signal
>> levels, or a "TTL" logic level serial port? I'm guessing the latter.
>
> I'm not sure what you mean by 'a "TTL" logic level serial port', please
> elaborate. Do you mean signalling used between the UART and line drivers
> by any chance, such as with a serial connection made between UARTs without
> actual line drivers in between?
What I meant is that a lot of modern computer modules come with serial ports
that are not RS232 but rather using standard logic levels (TTL 0 and 5 volts,
or perhaps lower voltages such as 0 and 3.3 volts) for their signaling. Those
basically just expose the logic level I/O of the UART or the embedded serial
port.
Vendors like FTDI make adapters for this. You can get their UART to USB
adapter with actual RS232 interfacing, but also with 5 volt or 3.3 volt logic
levels. That last one is what you'd use to plug into the console port of a
Beaglebone Black microcomputer board, for example.
paul
