DESCRIPTION OF OPERATIONS

The AP301EMS is designed to operate in conjunction with an AP202. The heart of the '301 is the microprocessor reading through a novel single slope integrating and sampling analog to digital converter system.

The microprocessor continuously samples a reference input to calculate gain and offset for any other readings. The analog inputs are read in a 'round robin' fashion and averaged to produce a stable display.

The display software drives a serial interface to the first eight digits and a latch for the ninth digit.

Outputs are open drain FETS driven directly from port C of the uP.

Analog to digital conversion is performed by firstly dumping any charge on the integrating capacitor. The input multiplexer selects the required input and the integrating cap begins to charge. The uP reads an 8 bit value every 16 microseconds for 32 samples or until the 8 bit converter overflows. The result is then mathematically averaged, scaled and offset to give a raw value approximately in millivolts. This reading is then scaled and offset once again as needed for the respective input.

The readings are then averaged over time in an inverse exponential fashion to further reject noise.

The values displayed are updated approximately once per second.

The displayed values are checked against the calibrated alarm points and outputs set The FET outputs are updated two seconds after switch on.