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Breaking into a cable

Sometimes it is necessary to cut a cable and directly access the wires inside.  Here's how it's done.

You'll need:
  • The cable to cut
  • Wire cutters / wire strippers
  • A manual of the device to which you want access, including a pin diagram (sometimes not available)
  • An analog or digital multimeter (some people call it an ohmmeter)
  • Some tape and a Sharpie (permanent marker) for labeling the loose wires (or a label maker capable of printing small tape labels)
  • Optional: an oscilloscope or digital acquisition device to examine signals from the cable
The steps:
  1. Decide how much length of the cable you will need from the device, and cut with scissors or wire cutters at the appropriate place.

  2. The cable has an outer insulation that surrounds an internal foil "shield" that protects the individual wires against electromagnetic noise. This "shield" is usually electrically connected to the connector metal (not the pins, but the metal surrounding the pins). Inside this shield will be several small, individually insulated wires (usually with different color insulation)  Cut along the outermost insulator to expose a fair amount of the shield, and then peel back the shield foil to reveal some length (maybe 3-5 cm) of the individual wires.

  3. In a log book, write down a diagram of the device remaining cable connector (note whether you are representing the cable connector as you are looking at it, or as it is plugged into the device; these are mirror images of each other, so be sure to get this right in your notes!).
    Example:  in your notes you might write and draw:
    When looking at the female connector on the device, the pins will be numbered as follows:

    This means when looking straight at the male connector on the cable, the pins are numbered as follows:

  4. Next, we will identify the mapping between each cable and the pin on the connector.  To do this, use a multimeter set to measure resistance.  Press the 2 leads of the multimeter together to make sure you measure approximately 0 ohms (verifies the meter is working and has power/batteries).
  5. For each individual wire, strip about 1cm of insulation off the end.  The "individual" wire will probably be further composed of many small metal wires all in contact with one another.  Gently twist these together a couple of turns so they are a bit sturdier than just a mass of tiny tires.  Then, with one hand, firmly press one of the multimeter probes to the metal wire. With the other hand, carefully touch each of the pins on the connector; be sure to touch only ONE pin at a time, and check a few times to make sure you are hitting the pin you think you are.  When the resistance goes to 0, you know you have found the pin that is connected to that wire.
  6. For each of these found connections, cut a small piece of tape, use a sharpie to write the pin number on it, and attach it to the wire near the end.  Here is an example of this on another cable, different from the one used in the photos above: 
  7. When you have mapped all the channels, you are done.
  8. Now you can plug in the cable, and examine the signals with an oscilloscope or other acquisition device.  If you need help connecting to a BNC cable, you might check out this link.