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

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