Re: serial console
On Thu, 31 Jul 2025 09:49:37 +0800
jeremy ardley <jeremy.ardley@gmail.com> wrote:
> On 31/7/25 08:08, Steve Dondley wrote:
> >
> >> On Jul 30, 2025, at 7:59 PM, mick.crane <mick.crane@gmail.com>
> >> wrote:
> >>
> >> I've never really known what a serial console is.
> >> Say in relation to a Raspberry Pi or Arduino
> >> I assumed a serial console was a device with a screen and keyboard.
> >> When people talk about a serial console they are talking about
> >> connecting a Debian PC to a device with a parallel, wired cable to
> >> a parallel port or an adaptor on a PC and emulating this console
> >> thing with software?
> >> Like a telnet connection?
> >> mick
> >>
> >
> > Serial means one bit after another. Console is an old school term
> > for a computer control panel on old mainframes. As those died out,
> > the word “console” morphed into meaning your display/keyboard/mouse
> > which can often be serial input/output devices. There are various
> > ports for connecting a serial device to a computer.
> >
>
> The serial interface RS-232 was invented to connect a Data Terminal
> Device (DTE) an a communications device (DCE).
>
> In most incarnations the DTE was a teletype or video terminal, while
> the DCE was usually a modem.
>
> The RS-232 Standard specifies +/- 12V as the nominal signalling level
> (actually above +3V and below -3V).
If you want to be really technical, RS-232 does not specify the
electrical signal parameters, the +-12V is the V-24 standard. RS-232
specifies this:
> The standard also specifies a 25
> pin connector (DB-25) with additional pins for out-of-band
> signalling, such as DTR which is asserted by a DTE to indicate it is
> operating.
>
> The standard is for DTE to DCE but many people bodged it to allow two
> DTE to communicate, using so called 'null modem' cables etc.
>
> The implementation for Pi allows it to act as a DTE or DCE with 4
> wires, Tx, Rx, VCC, GND. An additional pin can used to inhibit
> sending data.
>
> It is important to note that the Pi Serial terminal is not the same
> as the SPI or I2C communications also used on the device.
Which are also serial ports, but suitable only for very short distances
such as within electronic PCBs and using very different protocols. A
modified SPI is used by Canon for communication between cameras and its
AF series of lenses.
>
> The Pi Serial terminal can be used to debug bootup and to communicate
> to other serial devices such as industrial controllers, usually using
> RS-485 rather than RS-232
>
RS-485 and RS-422 use cables with a specified impedance, unlike RS-232,
so can work over much longer distances and/or at higher speeds. I've had
RS-485 working over a kilometre of cable at 9600 Baud. RS-232/V-24 is
only really reliable over about twenty metres at its highest speed.
--
Joe
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