23
The Advanced
®
3D3/4D3 Service Manual
perform timekeeping. The battery also pre-
serves the contents of internal memory.
Dip Switch: The three-position dip switch
allows the user to download new software via
one switch. The other two switches are avail-
able for selecting different operating modes.
These operating modes and the process for
downloading software are described later in
this manual.
Voltages: Three voltages are present on the
CPU board: +5 VDC, +12 VDC, and V
PP.
+5 VDC powers all the logic. +12 VDC is
switched on and off to create the flash memo-
ry’s programming voltage, VPP.
Glue Logic: The glue logic performs the
functions of creating the RAM memory chip
selects, the Flash memory write signals, the
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 applica-
tion PCB uses a subset of the available sig-
nals.
PCB605 Application Board
The application PCB contains all circuitry
specific to the 3D3/4D3 instruments. The
board includes voltage supplies, indicators,
analog-to-digital converters, drive circuitry,
parallel ports, serial ports, and various switch-
es.
Analog-to-Digital Converters: Two analog-
to-digital converters are provided; one for the
sample thermistor probe, and one for the
block thermistor probe. The thermistors vary
in resistance from approximately 2 kilo-ohms
at room temperature, to approximately 10
kilo-ohms at -12ºC. Typically, a 0.6 ohm
change in the thermistor’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 ana-
log-to-digital converter uses a sigma-delta
conversion technique with on-chip filtering
and a 6.25 VDC reference voltage.
Drive Circuitry: Drive circuitry is provided
to turn on and off the four high current loads
such as the stir wire, the head motor, the ther-
moelectric cooler, and the fan.
The stir wire coil is controlled by the rear
panel-mounted darlington transistor (Q2B).
The drive circuit consists of a programmable
timer, D/A converter, and interface op-amps.
The timer provides a square wave of approxi-
mately 71Hz, while the D/A converter con-
trols the output amplitude to the darlington
transistor.
The head motor is controlled by two relays
located at the rear of the chassis that interface
the 120 VAC motor to the DC logic. The
upper relay raises the head, while the lower
relay lowers the head. The relays get their
commands from the application board via two
FETs and the application logic. LEDs are
provided on the application board for moni-
toring the drive signals. LEDs are also pro-
vided to monitor the signals from the head
sensor board, described below.
The thermoelectric cooler is controlled by the
FET driver transistor mounted on the rear
panel (Q1B). The FET is, in turn, controlled
by the microprocessor through the PLD appli-
cation logic. The processor varies the duty
cycle square wave in response to software
commands and block probe resistance.
The fan is controlled by a FET transistor on
the application board, that is interfaced to the
processor through the application logic.
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