Elevator communications bus analyzer

A process for obtaining data frames passed between a master station and a remote station on an elevator communications bus, said elevator communications bus providing and receiving power, return, clock and data signals by means of and in respect of a power line, return line, clock line, and data line, the values of said data frames being equal to the difference between said data signals an said clock signal, said process comprising the steps of:

• detecting a synchronization frame within a clock signal, for providing a missing pulse signal for indicating the absence of a data frame;
• selecting an address of a remote station;
• providing said missing pulse signal to a counter;
• loading said address into said counter in response to said missing pulse signal;
• counting from said address loaded into said counter to a limit and providing a trigger signal to an oscilloscope;
• providing said clock signals to said oscilloscope;
• providing said data signals to said oscilloscope;
• switching said oscilloscope to a differential mode to provide on the oscilloscope screen, in response to said trigger signal, the difference between said clock and data signals.