5-22 170 Series Monitor Revision C
2000947-004
Theory of Operation: Main Board Theory of Operation
Display Interface Module
The display interface consists of a data buffer (U14) for D0–D7 which interfaces to
the ICM7228A seven-segment LED driver IC (U1 on the display board), and D flip-
flop U3 which drives the ±1 UA segments through Q1–Q4 transistors. Data lines
have 100 Ω series resistors (R524–R531) which help to reduce emissions. A diode
clamp device (U18) on the data bus is a used to protect against ESD to the display
area. Figure 5-13 provides a block diagram of the display interface module.
Figure 5-13. Display Module Interface Block Diagram
Status/Switch Input Module
The status/switch input module consists of buffers U15 and U16 and series/parallel
elements for protection against ESD. The monitor membrane switch panel provides
11 switch closures to ground which are fed to 100 kΩ pull up resistors and through
10 kΩ series elements. Buffers U14 and U15 are further protected by capacitors to
ground and diode clamp devices U17 and U22. Two LEDs are present on the
membrane switch panel: the power LED is driven by +5 V through resistor R350;
the recorder LED is driven from PAL U4 through buffer U6 and resistor R351.
Figure 5-14 provides a block diagram of the status/switch input module.
Figure 5-14. Status/Switch Input Module Block Diagram
Display Board
Transistor Drivers
for Pressure
± Display
4
to Data Bus
Latch
Data Buffer
10
to Data Bus
Chip Select and Address
Data
Buffers
Membrane Switch Inputs
Recorder Status Inputs
to Data Bus
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