Smiths Medical BCI 3304, Service Manual

Page: 22 / 36

Chapter 2: Oximeter Circuit Description
2-2 3304 Service Manual
Oximeter On/Off Control
When the oximeter is off and the O/I key is pressed U19-5 is shorted to
ground, which causes flip-flop output U19-3 to go low and turn Q7 off
releasing U25 from the shut down mode.
There are two ways the oximeter can be turned off:
When the O/I key is pressed, the microprocessor generates •
/HALT signal.
When the Power Fault Output signal (/PFO), generated by the •
power monitor/watchdog chip U15, goes low.
When either the /HALT signal is generated or the signal /PFO goes low:
Flip-flop output U13-3 goes high and Q7 turns power supply •
converter U25 off.
System Power Supply
System power supply is powered either through diode D4 when the
Charger is plugged in or through D6 when running on the battery.
A low power linear 5 V regulator (U26) is used to supply continuous
power to RAM, when the unit is turned off. This V-RAM supply also
provides power to the ON/OFF circuitry.
The main power supply is built around U25, a pulse width modulated
switch mode fly-back converter. The transformer T1 is used to generate
+5VDC and -5VDC supplies, and isolated V-ISO supply for the printer
interface. Transistor Q8 is used to turn the power supply into a
temporary off state during data acquisitions.
Microprocessor Circuit
A dual microprocessor core is formed by microprocessors U10 and U11,
PROM U12, and RAM U13. Crystal X1, C47, and C48 form the oscillator
circuit. The microprocessor’s PHI output at U10-71 is one-half of X1’s
frequency, and is used as a clock signal for the second microprocessor
U11.