4700 Refrigerated Sampler Service Guide
Section 4 CPU and Power Supply
4-4
battery can be monitored by the CPU through the resistor
divider network R77-78. The CPU can load the battery by
turning on FET Q8 for testing the battery.
This chip also has an unused watchdog function.
4.1.10 Distributor Motor
Drive – U1,
P1/Rotation Sensor –
P4, U9
U1 is the stepper motor driver for the distributor motor, which
connects to P1. The CPU sends step commands to this chip to
move the distributor. When the distributor is to move, the
encoder drive is also turned on by the CPU. This turns on the
LED in the distributor opto. As the encoding wheel moves, pulses
are sent back through Schmidt trigger U9-C, so that the CPU can
count the pulses and determine the distributor position.
4.1.11 LCD – U29, 31, 33, 34,
35, 36, J1
The LCD display operates on 5 volts. The rest of the circuitry on
the board operates on 3.3 volts. Therefore, the signals going to
the display must be converted to 5V signals. This task is handled
by level translators U29 and U31. The control signals for the
display go through U31 and the data bus through U29. The read,
write, and display chip select signals from the processor are com-
bined by U33, U34, and U35 to become the Enable signal to the
display (J1-8). The contrast control voltage comes from a d-to-a
output from the CPU directly to J1-5. The backlight control
signal goes through FET Q10 to switch the backlight on and off.
Power is supplied to the display from linear 5V regulator VR1.
4.1.12 Pump Drive – U17, 18,
19, 20, 21, 22, Q1-8, P9,
P10, P8
The pump motor connects to P10 and the interlock switch in the
pump connects to P9. If the pump band is in place and latched,
the magnet in the band activates the reed switch in the pump
housing and closes the circuit for the interlock. There are
redundant interlock circuits operating from the interlock switch.
One circuit takes the signal from the switch through Schmidt
triggers U9-F and U9-E to the AND gates U21 and U22. With
this signal present at the AND gates, the Pump Forward or
Pump Reverse drive signals can get through to the opto-isolators.
The other lockout circuit takes the signal from the switch
through Schmidt triggers U17-F and U17-E. These drive
opto-isolator U20, which produces a low signal to the gate of Q7.
Q7 turns on and drives the gate of Q8 high, turning it on. Q8 is
then the path to ground for the motor current when the H-bridge
is active to drive the motor.
For the motor to be driven forward, the forward drive signal from
the CPU goes through AND gate U21 and opto-isolator U18. The
low signal from U18 then turns on Q2, one leg of the H-bridge,
which sources current to the motor on P10-1. The low signal from
U18 also turns on Q5. Q5 inverts the signal and provides a high
drive to Q6, another leg of the H-bridge, which sinks the motor
current from P10-2. The current from Q6 then flows through the
interlock FET Q8 to ground.
The reverse circuit is similar, with the Pump Reverse signal
going through gate U22, opto U19, and on to drive Q1 and Q3,
which are the other two legs of the H-bridge.
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