Passport 2®/Passport 2 LT™ Service Manual
0070-10-0441
2 - 33
Theory of Operation
Nellcor/Interface Board
2.7
Nellcor/Interface Board
Overview
The Nellcor SpO
2
Interface PCB provides the communication path for the two-way data flow
between the Nellcor MP-304 PCB and Passport 2 CPU Control Module. The MP-304
analyzes the analog input from the SpO
2
sensor and outputs the data in digital format via a
serial digital interface. The purpose of this PCB is to control the power supplied by the control
module as well as to reassign pins for two connectors, serving as an adapter between the
two boards.
2.8
Recorder Interface Board (AR-42)
Overview
The function of this board is to provide power filtering for the recorder and suppress the
current spikes. The board also provide fan control and fan tach filtering and conversion to
logic levels that the 860T can accept.
2.8.1
Detailed Description
2.8.1.1
Power Filtering
Power filtering of the recorder logic 5VDC is provided by C5 and C6. This circuit provides a
low pass filter to reduce noise introduced into the system power. Since the load is digital
logic the current peaks are minimal.
The power filtering for the recorder motor and print head is provided by this board. It is this
board that must suppress the high current peaks from affecting the power supply and power
distribution within the system.
The suppression must reduce these peak loads to less than 1 Amp with a maximum voltage
ripple of 0.5VDC.
2.8.1.2
Fan Controller
The digital control provided by the 860T I/O pin operating as a output is conditioned by R5
& C7. This signal is then applied to the gate of Q1 which provides the drive control. The fan
is low line controlled with the fan return being connected to the FET source pin. In series with
the 12V2 is R1, 30 ohm resistor, that limits the fan voltage to approx. 9V and reduces the fan
noise. When fan operates on 12VDC it draws 100mA, typical.
2.8.1.3
Fan Sense
The Tach-generator, TG, generates two cycles of square wave per rotation of the fan motor.
The square wave is then converted to a Digital logic level, which turns on Q2 (FET). Q2’s
output is driven to Digital Logic low. The 860T uses this level to sense if the fan is functional.
If FTACH goes to Digital logic High The 860T will drive the FAN_CNTRL to Digital logic low
turning off Q1 FET removing Digital Ground from the Fan and turning it off.
