Connect to a switch or router console from a Mac with a USB serial cable

What you need

• A console cable that suits the device: USB-A or USB-C to RJ45 for a classic console port, or a USB cable straight into a mini-USB or USB-C console port.
• The device powered on. A USB console port on a switch usually only appears on the Mac once the switch has power.
• SheepTerm for Mac. (The same menu entries exist on Windows; see the end of this page.)

Step 0: check that the Mac sees the cable

Plug the cable in, then list the serial devices in Terminal:

ls /dev/cu.*

A new entry such as cu.usbserial-1420 or cu.usbmodem14101 means macOS has a driver for the cable. SheepTerm offers exactly this list (every /dev/cu.* device except the Bluetooth one), so if the entry is missing here it will be missing in SheepTerm too.

Driver notes. We cannot test every cable, so treat this as a starting point:

• Many cables and device-side USB console ports show up as cu.usbmodem* and need no driver. An Aruba CX USB-C console port, for example, appears as cu.usbmodem* once the switch is powered.
• FTDI-based cables use a driver that ships with macOS, so they normally just appear as cu.usbserial-*.
• Prolific PL2303 and some other chips are the usual source of trouble. Plugable documents a Mac App Store app (“PL2303 Serial”) that installs the driver extension; see their guide. Counterfeit chips are a known problem with this family.
• If nothing appears: try another USB port, avoid hubs, and check that the cable carries data (a charge-only cable will not work). Vendor drivers for the same chip can conflict with the one macOS ships, so remove old ones before installing another.

Connect with SheepTerm

1. Open the serial formChoose File › New Serial Console…. (It has no keyboard shortcut.)
2. Pick the deviceThe Device menu lists the cu.* names. If you plugged the cable in after opening the form, click the circular-arrow button beside it to rescan. Its tooltip reminds you to plug the console cable into a powered switch first.
3. Choose the baud rateThe list is 9600, 19200, 38400, 57600, 115200 and 230400, and it starts at 9600. Many switches and routers ship with a 9600 console, others use 115200; check the device’s documentation. SheepTerm always uses 8 data bits, no parity, 1 stop bit and no flow control (8N1).
4. Set the device family (optional)Pick the vendor so highlighting matches, or leave it on Auto. See the highlighting guide.
5. Click ConnectThe tab opens and the session info shows the port and speed, for example serial · cu.usbserial-1420 · 9600 8N1.
6. Wake the promptPress Return. SheepTerm never sends anything to a device on its own, so a quiet console stays quiet until you type.

Send a Break (ROMMON, boot loader, password recovery)

Some devices only stop their boot sequence when they see a serial break condition. With the serial tab selected, choose Edit › Send Break or press ⌃⌥B (Control-Option-B).

• The item is greyed out unless the selected tab is a serial console; on an SSH tab or a local shell there is no line to put a break on.
• SheepTerm sends the break ahead of anything still queued, including a long paste, and prints break sent (or break failed with the reason) in the tab. macOS holds the break for roughly 0.4 seconds.
• Whether the device reacts depends on the device and on the cable’s driver. Some older drivers ignore a break without reporting an error, so if nothing happens, try again at the moment the device prompts for it, or try a different cable.

When it does not work

• Blank screen. The device may simply be silent. Press Return once or twice.
• Garbage characters. The baud rate does not match. Close the tab and reconnect at the device’s speed.
• The device is not in the list. Rescan, check Step 0, and make sure the device has power.
• The port is busy. A serial port can be open in only one program at a time. Close screen, minicom or another SheepTerm tab that holds it. (For the same reason, Split Right and Split Down are refused on a serial tab.)
• You need other settings. If your equipment needs parity, 7 data bits or hardware flow control, SheepTerm cannot set them today; use another tool for that device.
• The device reboots. If the console drops when the device restarts, SheepTerm tries to reconnect on its own after a short wait, as long as the session had connected before.

Logging a console session

Serial sessions are logged to plain-text files like SSH sessions (turn that off under File › Log Sessions to File, or per session with “Don’t log this session” in the form). A console log starts out named after the cable; after the device shows its prompt a couple of times, SheepTerm renames the log and the tab to include the device name, for example Core-SW (cu.usbserial-1420). More in the logging guide.

On Windows

SheepTerm for Windows has the same File › New Serial Console… and Edit › Send Break (Ctrl+Alt+B) with a COM port picker instead of /dev names. Be aware that the author has not yet confirmed serial on a real COM port; it has been exercised with a simulated port and automated tests.

Related

• SSH through a jump host
• Log and search network device sessions
• Vendor highlighting for CLI output
• Compare SheepTerm with other terminal clients