On Sat, 1 Feb 2025, Paul Koning wrote: > >> 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.
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. > 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
