The Advanced
®
Model 3250/4250 Service Manual
26
perform timekeeping. The battery also pre-
serves the contents of internal memory.
Dip Switch: The three-position or four-
position dip switch allows the user to down-
load new software via one switch. The other
two switches are available for selecting differ-
ent 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, V
PP
.
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 instrument. The board includes
voltage supplies, indicators, analog-to-digital
converters, drive circuitry, parallel ports, seri-
al ports, and various switches.
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.
I/O Ports: Circuitry for the two serial ports
(RS-232 and barcode) and the parallel printer
is present on the application board, with
cabling to the actual connectors on the back
panel and the internal printer.
Display & Keypad: The display and keypad
are interfaced to the processor via program-
mable logic devices (PLDs). The keypad is
polled in a row/column process which identi-
fies the key pressed to the microprocessor.
The application logic, in response to proces-
sor commands, controls the LEDs located on
the keypad.
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 head
motor controls, and red LEDs are used to
indicate that the head up and down optical
sensors have been tripped.
Application Logic: The application logic is
made up mainly of two programmable logic
devices (PLDs). These provide software
accessible hardware registers, enabling the
software to read the keypad, sensors, and ana-
log-to-digital converters. These also allow
manipulation of the display, external compo-
nents such as the fan, head motor, etc., and
interface to the RS-232 ports. The printer is
controlled 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.
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