Fluke 90100 manual 2 300-VOLT Electronic Bias Supply, Front Panel Switches, Processor, VFD Display

Theory of Operation	3
Overall Functional Description of the Model 35360A Tracker Display

3.2.2 300-VOLT Electronic Bias Supply

The 300 volt ionization potential is generated internally by an electronic bias supply circuit. The supply eliminates the need for the 35360A TRACKER Display to contain high voltage batteries. As explained below, the ion chamber collector and guard are maintained at the same potential as the chassis of the 35360A TRACKER Display, and the ion chamber bias plates are negatively biased at 300 VDC. A readout of the bias potential is facilitated by a divided bias signal generated within the bias supply block and connected to the A-D converter.

The output of the electronic bias supply is highly regulated and filtered. In addition, the output energy and current have been limited should the operator make direct contact. Since the bias voltage at the detector input connector is automatically discharged at power down, the operator should always turn the instrument off before attempting to make or break a connection involving the detector interface cable. During operation, the Bias Supply output level is continuously measured and verified. Any abnormal level will be indicated by an annunciator on the VFD.

3.2.3 Front Panel Switches

The nine front panel switches are of the momentary contact type, and are continuously monitored by the microcontroller for detection of contact closure. At the detection of any contact closure, the microcontroller scans the switch set to determine which key has been pressed, and then takes the appropriate action. The dedicated function of each key is described in Section 2.

3.2.4 Processor

The Processor block is based on the Motorola 68HC11 microcontroller and includes 256 KBytes of factory programmed memory (EPROM) containing all of the executable code (firmware). The revision date associated with this code is displayed during the power up sequence on the VFD. Also contained within the Processor block is 32 KBytes of RAM memory which is used to hold all volatile variables (those whose values are lost when the instrument is powered down).

A 512 byte block of nonvolatile memory (EEPROM) has also been provided. A portion of the EEPROM is automatically reprogrammed during each power down sequence to hold all of the user's front panel selections. Thus the units, front panel gain, temperature, and pressure settings in effect at power down can be automatically restored at the next power up. The EEPROM is also used to hold all of the internal electrical calibration factors for the unit, as well as all of the customization selections installed by the user over the RS-232 via the Customization program.

Besides its role as a controller of the other hardware blocks, the Processor Block performs several computational functions. It applies a number of correction factors to the displayed dose and dose rate values including the air density correction factor, the ion chamber calibration factor, the display units conversion factor, and the front panel gain settings. It also performs the ratio calculations necessary to display the percentage-of-center measurement values for the peripheral ion chambers.

3.2.5 VFD Display

All display functions are handled by a 4 line by 20 character alpha-numeric VFD module. The module is under the direct control of the microcontroller and is capable of displaying all of the user's front panel selections as well as the numerical measurement results along with their appropriate units. Other informational items including the battery and bias voltage, calibration date, and ion chamber calibration factors may be displayed. The VFD has a distinct advantage over other display technologies, such as LCD, in that it provides good visibility, and a wide viewing angle in all lighting conditions.

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