Serial ports and telnet

Last updated: 2026-10-29

Besides SSH, a host can be a serial port — a board's or a router's console on a USB adapter — or a telnet session, for a console server, a network lab or a device without SSH. Both are hosts of your book: add one with New Host and choose Telnet or Serial Port at the top of the editor.

A serial port#

A serial port is held by the keeper of the machine its cable is plugged into — your Mac, or a Linux host such as a Raspberry Pi — as a shell is. So:

The console stays open when you close SpectaShell, and what the board printed meanwhile is there when you open it again.
Your other devices open it, or watch it, from Running, as any kept shell (see Shells that keep running). An iPad on the sofa sees the console of a board on your desk.

In the editor:

Runs On is the machine the port is on: This Mac, or a host of your book. On an iPad or iPhone, a port made on a Mac opens through the host of your book that is that Mac; add the Mac as a host, with Remote Login on, and connect to it once.
The port: on a Mac, choose one of the ports plugged in from the menu beside the field, or type it — /dev/cu.usbserial-110. For a port on another host, the button beside the field lists that host's ports (it logs in there, and may ask for its password). On Linux a port is /dev/ttyUSB0, /dev/ttyACM0, or a Raspberry Pi's /dev/serial0.
Speed, Data, Parity, Stop (8N1 unless the device's manual says otherwise) and Flow Control.

Open it with a click on its card, or Open Serial Console in its menu. Its tab and its card in Running are named after the port, cu.usbserial-110. While it is open, the pane's ⋯ menu has:

Release Port: the port is let go, so another program — a tool that flashes the board, esptool or idf.py — can open it. The console and its output stay; what you type goes nowhere.
Reopen Port: takes the port back.
Send Break: a break on the line, which a router's ROM monitor or a password recovery asks for.

A Mac stays awake while one of its serial ports is open, so the board's output is not lost while nobody touches it; closing the lid still puts it to sleep.

When the adapter is unplugged, the console says so and ends; Reconnect opens it again once the adapter is back.

Telnet#

A telnet host has an address and a port, 23 unless you give another. Runs On chooses where the session is made:

This Mac, kept running: in your Mac's keeper, as a serial port is, so it outlives the app and your other devices can open or watch it.
A host of your book: in that host's keeper — the way into a lab network only that host reaches.
On an iPad or iPhone, This device: SpectaShell connects itself. Nothing keeps the session: it ends with the connection, as a plain SSH shell does, and Reconnect starts a new one.

Telnet is not encrypted. What you type, passwords included, crosses the network as it is. The host's card says so, and SpectaShell never types a saved password into a telnet session.

Good to know#

Opening a serial port on Linux needs the dialout group. If the console says so, run sudo usermod -aG dialout $USER on that host and log in again.
A port another program holds cannot be opened: the console says it is in use.
A host the console runs on needs SpectaShell's keeper there; it is put there at the first connection, as for shells kept running.
A USB adapter plugged straight into an iPad is not supported yet (see What SpectaShell does not do).