Teledyne 4700, Serivce Manual

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4700 Refrigerated Sampler Service Guide
Section 4 CPU and Power Supply
4-3
U12 is another switching regulator that produces 5 volts. This
voltage is used by the display backlight and the general outputs.
Vp goes through a linear regulator (VR1) to become 5 volts for
the circuitry of the LCD display and its associated circuitry.
U15 converts the analog 3.3 volts into –3.3 volts for use by the
analog circuitry of the liquid detector.
4.1.5 Serial Port – U32 The serial data is handled by the UARTs built into the CPU. The
Tx and Rx lines go through U32, which converts between logic
level and RS232 voltage levels. This chip generates its own neg-
ative voltage to supply the negative levels needed for RS232.
Channel 0 goes through the flow meter connector P2. The receive
line shares the inhibit input from the flow meter. Channel 1 is
not used at this time, but is brought to TP16 and TP17.
4.1.6 Flow Meter Interface –
U6, 17, P2
The flow meter connections come in through P2. All of the 4
signal lines are protected by transzorbs and filter capacitors.
Flow pulses and Inhibit from the flow meter go through Schmidt
triggers in U17. Serial input (RS232) shares the Inhibit line. The
Event Mark signal comes from the port on the CPU to U6, which
is a solid-state relay that outputs as a 12V signal.
4.1.7 Liquid Detector – U3,
10, P7
The voltage generated by the piezoelectric film liquid detector
comes in on P7. Op amp U3 acts as a high impedance load for the
detector and buffers the signal delivered to the discrete multi-
stage RC filter. This filter removes the higher frequency compo-
nents of the signal. U10A is a buffer after the filter. U10B and
the diode on its output rectify the AC signal to a positive DC
signal that goes to an a-to-d input on the CPU.
4.1.8 Temperature Inputs –
U16, P5
U16 measures the temperature in the electronics compartment
and outputs a voltage proportional to the temperature. This
voltage goes into one of the a-to-d inputs on the CPU.
The refrigeration air temperature sensor, which is an NTC
thermistor, connects to P5. It is fed a constant voltage of 2.5 volts
as Vref. The voltage appearing across series resistor R8 (6.25K)
goes into one of the a-to-d inputs on the CPU. This is a non-linear
function, so the software must linearize and compute the temper-
ature.
The evaporator temperature sensor circuit is identical to the air
temperature circuit. The sensor also connects through P5.
4.1.9 Microprocessor
Supervision – U23
U23 is a supervision circuit. It monitors the 12 volt power on pin
PFI. When this pin gets down to 1.3 volts, the power fail output
goes to the CPU to start the power-down sequence. This is when
the 12 volt supply falls to about 8 volts. Also, when the 3.3V
supply falls to 2.9 volts, this chip issues the Reset signal that
stops the processor.
This chip also does the switchover to battery power for the RAM
when the 3.3V supply falls below the battery voltage. The battery
supply is from the on-board lithium battery. The voltage of this