When experimenting with microprocessors a serial interface with 3.3 digital voltage level is often needed. A MAX232 can be used, but often also an usb-serial interface is needed since modern computers, like the Apple iMac's do not have a serial interface.

Here is a description to get a very cheap usb-serial interface based on a cellphone USB data cable which uses the Silicon Labs CP210x single-chip USB to UART bridge.

  1. Get a GSM-USB cable at eBay, marktplaats or whatever.
    These cables often uses the Silicon Labs CP210x single-chip USB to UART bridge.
  2. Look for the pinouts at http://pinouts.ru/ to identify the rs-232 TXD, RXD and GND signals.
    For instance pinouts of the commonly available siemens datacable can be found here
  3. This cable's usualy comes with a Windows driver, so after installing this driver Windows users can start using this serial port.
  4. For Mac OS X users it is slightly complicated, since most often no driver is provided with the cable. A few additional steps are needed.
  5. Look in the System Profiler (Aplications/Utilities) under Hardware/USB which type of interface is used for the cable:

    usb data cable:

    Version: 1.00
    Bus Power (mA): 500
    Speed: Up to 12 Mb/sec
    Manufacturer: Silicon Labs
    Product ID: 0x10c5
    Serial Number: 0001
    Vendor ID: 0x10ab

  6. Here we see that this is indeed a Silicon Labs interface.
  7. Download the Virtual COM Port (VCP) driver from the Silicon Labs website (http://www.silabs.com/) The CP210x USB to UART Bridge Virtual COM Port (VCP) drivers are required for device operation as a Virtual COM Port to facilitate host communication with CP210x products.
  8. Install the driver by double clicking the SLAB_USBtoUART Installer
  9. Unfortunately the standard product and vendor ID in the driver are wrong. So you have to put the Product and vendor ID obtained from the System Profiler, as explained above into:
  10. Open a terminal session
  11. type: sudo kextload /System/Library/Extensions/SLAB_USBtoUART.kext
  12. type: touch /System/Library/Extensions
  13. type: ls -al /dev/tty.SLAB*
  14. You should see the comport:
    crw-rw-rw- 1 root wheel 9, 8 Oct 18 08:32 /dev/tty.SLAB_USBtoUART
  15. Now you are ready to use your new serial port.