5800 Refrigerated Sampler Service Guide - Isco

5800 Refrigerated Sampler Service Guide
Section 4 CPU and Power Supply
LED in the distributor opto. As the encoding wheel moves, pulses
are sent back through Schmidt trigger U32-C, so that the CPU
can count the pulses and determine the distributor position.
4.1.11 LCD – U2, 3, 5, 6, 8 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 U2 and U8. The control signals for the
display go through U2 and the data bus through U8. The read,
write, and display chip select signals from the processor are combined
by U5, U6, and U3 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 Q1 to switch the backlight on and off.
Power is supplied to the display from linear 5V regulator VR1.
4.1.12 Pump Drive – U20, 21,
22, 23, 24, 25, Q8, Q14,
Q16, P8-10
The pump motor connects to P7 and the interlock switch in the
pump connects to P8. 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 U32-F and U32-E to the AND gates U20 and U21. 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 U25-F and U25-E. These drive
opto-isolator U24, which produces a low signal to the gate of Q10.
Q10 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 U20 and opto-isolator U22. The
low signal from U22 then turns on Q14, one leg of the H-bridge,
which sources current to the motor on P7-1. The low signal from
U22 also turns on Q11, which inverts the signal and provides a
high drive to Q9, another leg of the H-bridge, which sinks the
motor current from P7-2. The current from Q9 then flows
through the interlock FET Q8 to ground.
The reverse circuit is similar, with the Pump Reverse signal
going through gate U21, opto U23, and on to drive Q16 and Q12,
which are the other two legs of the H-bridge.
When the pump is called into operation, the Pump Encoder Drive
signal goes out through U31 to P9, thus turning on the LED in
the pump opto. Pump pulses come back in as the pump turns, go
through Schmidt trigger U32-d, and into the CPU.
4.1.13 Case Heater – U33 The compartment containing the CPU is heated to keep water
from condensing on the board and eliminate the need for desiccant
in this area. The heater is R133, which is a 20 ohm, 10
watt resistor. It is driven by U33, a solid-state relay. It is on 20%
of the time and delivers about 1.5 W of heat.
4-4