Case Heater - U25; General Outputs - U4, 5, P3; Analog Input - U7, 8, 17, 30, P3; Vref - U2 - Teledyne 4700 Serivce Manual

4.1.13 Case Heater – U25
4.1.14 General Outputs – U4,
5, P3
4.1.15 Analog Input – U7, 8,
17, 30, P3
4.1.16 Vref – U2
When the pump is called into operation, the Pump Encoder Drive
signal goes out through U37 to P8, thus turning on the LED in
the pump opto. Pump pulses come back in as the pump turns, go
through Schmidt trigger U9-D, and into the CPU.
The compartment containing the CPU is heated to keep water
from condensing on the board and eliminate the need for des-
iccant in this area. The heater is R89, which is a 20 ohm, 10 watt
resistor. It is driven by U25, a solid-state relay. It is on 20% of the
time and delivers about 1.5 W of heat.
There are four programmable outputs that come out on P3.
These outputs provide logic signals at 0 or 5 volts. U4 and U5
provide one of these two levels, depending on the signal input to
them. The signals go through current limiting PTCs R14-17 and
protection circuits.
P3 also has the analog input (4-20 mA) which can come from a
flow meter and provide flow data for flow proportional sampling.
This input is electrically isolated from the other circuitry on the
CPU. The current coming in flows through zener diode CR11 and
the 10 ohm resistor R46. The zener drops 4.7 volts to provide a
local power source for the isolated circuitry. The rest of the
voltage drops across R46, which is the sensing element. U7 is a
voltage-to-frequency converter that runs on the isolated 4.7 volts
and outputs an AC signal based on the voltage across R46. The
pulses from U7 go through opto-isolator U8, which delivers them
to Schmidt trigger U17-B and the U30. The pulses are then
counted by the CPU.
U2 is a 2.5 volt reference. It provides Vref to the CPU for the
a-to-d converter. It also provides power to the two refrigerator
temperature sensors.
4700 Refrigerated Sampler Service Guide
Section 4 CPU and Power Supply
4-5