Mike's PBX Cookbook

Mac's and serial TTY's

Prolific PL2303 serial-to-USB adapter

Mac's are excellent tools for accessing serial device TTY ports (to console into PBX's, switches, and routers). You just need a serial to USB adapter, the right driver, and some Terminal software.
You can use screen, although Minicom (or a GUI program) offer more features and functionality.


Most Serial-to-USB adapters will work on a Mac with one of the following OS X drivers...
UPDATE: Mavericks (10.9) includes a driver for FTDI-based Serial-to-USB adapters.

Prolific PL2303:PL2303_MacOSX_1_6_0.zip
FTDI USB Serial:FTDIUSBSerialDriver_v2_3.zip
NOTE: It may be necessary to remove any previous driver before installing a newer one,
eg: $ sudo rm -rf /System/Library/Extensions/ProlificUsbSerial.kext

If your adapter doesn't work with either of these, try the following sources:

After installing the correct driver, plug in your USB-Serial adapter, and open a Terminal session (Applications/Utilities).
Enter the command ls /dev/cu.*, and look for something like usbserial (or similar):

$ ls /dev/cu.*
/dev/cu.Bluetooth-Modem		/dev/cu.iPhone-WirelessiAP
/dev/cu.Bluetooth-PDA-Sync	/dev/cu.usbserial

This indicates the USB-Serial driver is working. Select this port name in a terminal program.

Note: Check your USB adapter works after a Mac OS Update, as you may have to re-install the driver.

You might notice that each serial device shows up twice in /dev, once as a tty.* and once as a cu.*. So, what's the difference? Well, TTY devices are for calling into UNIX systems, whereas CU (Call-Up) devices are for calling out from them (eg, modems). We want to call-out from our Mac, so /dev/cu.* is the correct device to use.

The technical difference is that /dev/tty.* devices will wait (or listen) for DCD (data-carrier-detect), eg, someone calling in, before responding. /dev/cu.* devices do not assert DCD, so they will always connect (respond or succeed) immediately.


Having installed the right driver, our USB-Serial adapter will show up in /dev/cu.* (shown above). We now need to install some terminal emulation software before we can connect to anything.

Note: If you can't find a driver for your adapter (eg, Belkin), try Serial which has built-in support for many USB-Serial devices.


It's not actually necessary to download an install extra software, as you can use the Mac OS X built in Terminal and screen. Screen lacks some features, but it does include VT100/ANSI terminal emulation, and can be extremely useful.

  1. Open an OS X terminal session (window)
  2. Find the right TTY device. Type: ls /dev/cu.*
  3. With the USB-Serial adapter plugged in, you'll get a list, including something like this:

    $ ls /dev/cu.*
    /dev/cu.Bluetooth-Modem		/dev/cu.iPhone-WirelessiAP
    /dev/cu.Bluetooth-PDA-Sync	/dev/cu.usbserial
    The 'man screen' page
  4. Then type: screen /dev/cu.usbserial 9600 (in this example).

  5. The 9600 at the end is the baud rate. You can use any standard rate,
    eg, 9600, or 19200 for a Sig Server!

  6. To quit the screen app, type CTRL-A, then CTRL-\.

Type man screen in Terminal for further information on screen. (use 'enter' or 'space' to scroll, and 'q' to quit).


Alternatively, if you'd like a few more features, and a retro feel, you can install minicom 2.2 .
Minicom supports VT100 emulation, which means it sorta kinda works with Meridian Mail (Function keys: fn + f-key).

  1. Download and run the minicom 2.2 package installer.
  2. Then manually edit your $PATH with sudo nano /private/etc/paths.
  3. Add the following line to the bottom of the file: /opt/minicom/2.2/bin, save and exit - then relaunch the terminal.

Run minicom -s first to configure your serial interface device name, and other options. Then, Save setup as dfl (default) and Exit. Your serial interface device name is found with ls /dev/cu.* (as shown above). Always launch minicom, with your selected serial adapter plugged in and available to avoid an error. In the example below, the serial device is /dev/cu.usbserial:

Minicom Configuration Options

Configuration Options

Minicom Connected to Port /dev/cu.usbserial

Connected to Port

Minicom Command Summary

Command Summary

In minicom, commands can be called by CTRL-A <key>, for example, change your serial port settings with CTRL-A P
Press CTRL-A Z for a Command Summary, and help on special keys. Enter CTRL-A X to quit.

HINT: Change your OS X terminal window size to 80x25 (1 line more) so you can see the bottom Minicom status bar.
(Terminal Preferences ► Settings ► Window). Open a new window for this change to take effect.

terminal settings

Note, the minicom installer creates /opt, and its not hidden in the OS X finder. Rectify this with: sudo chflags hidden /opt


With handshaking set to software (xon/xoff) or none, which covers most devices, serial data communication needs just 3 wires: RXD (2), TXD (3), and GND (5). Generally, hitting <enter> should cause a response, or <ctrl-r> should 're-draw' the screen. If you don't get any characters, try adding a Null-Modem cable or adapter, which swaps RXD and TXD (pins 2 and 3). Signal ground is Pin 5 in a DB-9, or Pin 7 in a DB-25.

Pin Functions for RS-232
TXD(3)Serial Data Output
RXD(2)Serial Data Input
GND(5)Signal Ground
RI(9)Ring Indicator
RTS(7)Request to Send
CTS(8)Clear to Send
DSR(6)Data Set Ready
DCD(1)Data Carrier Detect
DTR(4)Data Terminal Ready
DB-9 Male

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