Mask standards often require that the pulse under test be an isolated pulse - that is, a positive or negative one preceded and surrounded by zeroes. An isolated one pulse is a pulse that is not affected by inter-symbol interference or noise from adjacent pulses. To test a device with realistic traffic signals, an oscilloscope must be able to find and trigger on the specific bit pattern before performing the mask test. Pulse mask templates contained in a scope's library often make use of the scope's isolated one triggering, so it will not be necessary to stimulate the communication device with a known, fixed pattern. In such a case, the scope automatically sets up a trigger to capture an isolated one pattern, as defined by the particular ANSI or ITU standard.
Many standards require testing of both the positive and negative polarity pulses.
This makes inverted mask testing a requirement for compliance testing. Most scope communication kits will allow for the display, testing, and triggering on both positive and negative masks, and in some cases, this is accomplished by simply setting the pulse polarity to negative or positive. Connectivity is critical Connecting the DUT to the oscilloscope is an important issue for communications customers. Adapters are needed to match the input impedance of the scope to the impedance of the device under test. For differential communication signals, the signal must be converted to a single-ended voltage to measure with the oscilloscope.