Nellcor NPB-40 Service Manual page 41

S3.2.3 Standard User Controls
S3.2.4 Power Supply/Power Control Circuitry
The program that the CPU runs is stored in U5. U5 is a 16-bit wide output,
one-time programmable (OTP) EPROM. During 16-bit wide bus accesses, the
CPU uses address line A0 for low/high byte selection, and is not used as a
normal address line. The CPU can only address 64K x 8 bytes or 32K x 16
bytes. Pin A15 of U5 is tied low, always selecting the lower half of the
EPROM. Signal ROMEN-L is then used to enable the EPROM for the proper
memory area.
The output port external to the CPU consists of an octal D latch, U9. The
output of U9 is always enabled. The output bits of U9 control the beeper
output, the LCD backlight, and the LCD display drive signals.
The standard user controls consist of four momentary push-button switches
on the front panel keypad. These keys are metal dome membrane contact
switches.
The front panel switches Store Data/Print, Display Light/Beep On/Off, and
Shift are connected to the microprocessor, U4, input lines P2.2, P2.3, and
P2.4 and are normally pulled to the high state by R10, R8, and R18.
Whenever one of these keys is pressed, the U4 input line is pulled low. The
switch contacts are debounced in software.
The fourth front panel switch, On/Off, is connected to the power switch
described Power Supply/Power Control Circuitry.
The power supply and power control circuitry consists of the following
primary elements.
• Batteries — Four 1.5 V alkaline "AA" size batteries provide 3.6-6 Vdc
power.
• Power entry circuit — Components protect the NPB-40 from damage
if batteries are inserted incorrectly and provide reverse current limiting
and over-voltage or spike protection. In addition, a self-resetting fuse
protects the power supply from excessive current draw. The power
supply is also protected against electrostatic discharge and
electromagnetic interference.
• Power switch circuitry — This circuit controls power applied to the
power supply circuits. Power control circuitry is connected to the
batteries. It senses any press of the front panel keypad On/Off key and
switches the power supply circuit on or off. A control signal to this
circuit from the CPU will also shut off the NPB-40 when battery voltage
drops to an unusable level.
• Power supply circuits —consist of a power generation circuit that
provides +10Vdc, +5Vdc, and -5Vdc.
Technical Supplement
S-9