2 Feb
2025
2 Feb
'25
6:08 p.m.
On Sat, 1 Feb 2025, Paul Koning wrote:
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.
That makes sense to me since you can then choose what "phy" to attach to
it: RS-232, RS-422, IrDA, etc. It's been done since forever, for example
I think all DEC Alpha machines had their CPU's debug UART wired to a pin
header, but it was up to you to add a line driver if you wanted to make it
a real serial port.
More recently e.g. the SiFive HiFive Unmatched RISC-V development board
has this arrangement for UART #1 (UART #0 is the console port, wired to an
onboard dual FTDI USB device already; the other FTDI port being used to
carry JTAG over USB) and I had to wire my own line driver along with a
DE-9 connector to make it a serial port.
But I wouldn't call a bare UART a serial port or use it for external
connections: it's just a UART you need to wire to make it a serial port.
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? 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.
This makes sense to me too: depending on application you can use an FTDI
device which is just a UART with a USB interface (as with the console port
for the RISC-V device mentioned above) or one that actually implements a
serial port with a USB interface, which I'd expect to see with say a USB
RS-232 dongle the contents of which you want to keep to the minimum, so a
single ASIC with probably just a bunch of external passive components fits
perfectly.
Just my point of view, thanks for sharing yours!
Maciej