Fluke Biomedical 190I - Manual (Page 21)

Theory of Operation
General Circuit Description
3
3-1
Section 3
Theory of Operation
3.1 General Circuit Description
A block diagram of the circuit of the 190I is shown in Figure 3-1. The CMOS microprocessor is a TI 7000
series microprocessor, capable of addressing 64k bytes of memory. A 32k byte PROM contains the
operating program and an 8k byte RAM provides storage for operational and data logging information.
The microprocessor addresses the display via two display drivers. The microprocessor senses the
keypad directly, with the exception of the ON/OFF switch. The communications is direct from the
microprocessor through its ports, light emitting diodes, and photosensitive transistors. The
microprocessor reads and writes to the internal EEPROM and reads count data directly from the GM tube
or from the counter (when the count rates are too high to handle each count individually). Calibration and
operational information is stored in the internal EEPROM.
Communication with the 190I is accomplished through an infrared two-way adapter that mounts at the top
of the instrument. The infrared receiver and transmitter are viewed through two small holes in the upper
edge of the decal covering the LCD.
3.2 Operational Considerations and Applications
Control and Configuration
The 190I uses a database, stored in the EEPROM on the main circuit board, for operation. The EEPROM
is used to define the operating modes through a set of configuration and control codes. These codes may
be user-modified with the optional Communicator (P/N 190-1A) to tailor the instrument for a specific
application. For example, any or all of the front panel buttons may be disabled or the unit may be
configured to operate in only one Display Mode. Refer to the Instruction Manual provided with the
Communicator for further details.
Display Calculations
The 190I retains all data internally as both counts and counts per minute. All other displays are calculated
from these data sets and stored calibration information in the EEPROMS, allowing the operator the
capability to switch back and forth between the various display modes without affecting the database
used for calculations.
The counts per minute mode does not require a
calibration adjustment because its time base is
based on the highly accurate crystal clock of the
microprocessor. All other displayed values (e.g.,
mR/h and Sv/h) can be derived from the counts per
unit time value if a suitable calibration constant is
used.
Above 384,000 CPM, the counts in a-time slot are counted and the value multiplied by the appropriate
factor to determine the actual count rate, which may cause a reduced accuracy specification on the fifth
and sixth scales.
NOTE