21
V
PP
control signal, the watchdog control sig-
nal, and accessing the dip switch. The CPU
board has two software accessible hardware
registers to read the dip switch and to control
watchdog and V
PP
.
Connectors: A 64-pin connector provides
address, data and interrupt signals. A 16-pin
connector provides general chip selects and
serial receive/transmit signals. The application
PCB uses a subset of the available signals.
PCB115 Application Board
The application PCB contains all circuitry
specific to the 3320 instrument. The board
includes voltage supplies, indicators, analog-
to-digital converters, drive circuitry, parallel
ports, serial ports, and various switches.
Analog-to-Digital Converters: Two analog-
to-digital, 24-bit sigma-delta converters are
provided; one for the sample thermistor probe,
and one for the block thermistor probe. The
thermistors vary in resistance from approxi-
mately 2 kilo ohms at room temperature, to
approximately 10 kilo ohms at -12ºC.
Typically, a 0.6 ohm change in the thermis-
tor’s resistance equates to a 1 mOsm/1.86mºC
change. A separate Wheatstone bridge circuit
is used to measure the voltage across each
thermistor probe. This voltage is first filtered
and then sampled by the analog-to-digital
converter. The analog-to-digital converter
uses a sigma-delta conversion technique with
on-chip filtering and a 2.5 VDC reference
voltage.
Drive Circuitry: Drive circuitry is provided
to turn on and off the three high current loads
such as the impactor, the thermoelectric cool-
er, and the fan.
The impactor coil is controlled by the FET
transistor Q3 on PCB115, that is interfaced to
the processor through the PLD application
logic.
The thermoelectric cooler is controlled by the
FET driver transistor (Q4) on PCB115. The
FET is, in turn, controlled by the microproces-
sor through the PLD application logic. The
processor varies the duty cycle square wave in
response to software commands and block
probe resistance.
The fan is controlled by FET transistor Q2 on
PCB115, that is interfaced to the processor
through the PLD application logic.
Back Panel I/O Board Description -
Part No. PCB140
Circuitry for the two serial ports (RS-232 and
barcode), the parallel printer port, and a
remote programming switch are present on
the I/O board mounted to the rear of the chas-
sis, with cabling back to the application
board.
Display & Keypad - Part No. 202511: The
display and keypad are interfaced to the
processor via programmable logic devices
(PLDs). The keypad provides a two-button
interface between the user and the instrument.
The second line of the display is used to indi-
cate the function of each key. The software
changes the button description on the second
line of the display.
Light Emitting Diodes (LED): LEDs are
provided to indicate when a high current load
is active, a sensor is tripped, or a voltage sup-
ply is active. Green LEDs are used on the
power supply voltages, yellow LEDs are used
on driver signals such as the fan and impactor,
and red LEDs are used for factory diagnostics.
Application Logic: The application logic is
made up mainly of one programmable logic
device (PLD). This provides software acces-
sible hardware registers, enabling the software
to read the keypad, sensors, and analog-to-
digital converters. This also allows manipula-
tion of the display, external components such
as the fan, impactor, etc., and interface to the
RS-232 ports. The external printer is con-
trolled directly from the microprocessor
through an 8-pin D-latch.
Connectors: Connectors are supplied for the
CPU PCB, display, keypad, power, drives,
probes, and back panel I/O ports.
The Advanced
®
Osmometer Model 3320 Service Manual
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