Mindray Passport 5-Lead, Руководство по обслуживанию

Страница: 233 / 544

Passport 5-Lead, 5L, LT, XG Service Manual 0070-00-0420 2 - 31
Theory of Operation Block Diagrams
Sequencer
The primary function of the sequencer is to synchronize the LED drive signals with those that
control the sequential demodulation of the photodetector’s amplified output.
The sequencer also directly controls the switches U3A, U3B, and U3C, which set the current
to voltage preamp gain. Since the gain required for the transmitted red light may differ from
that required for the transmitted infrared light, synchronization of the gain control switches
with the drive signals is also required.
Finally the last function of the sequencer is to cancel the residual signal and any low
frequency signal noise. The sequential demodulation process for each wave-length subtracts
from the detector’s signal when the LED is activated, the residual signal that exists when the
LED is not activated. This cancellation process works only for residual signals at frequencies
significantly lower than the drive frequencies of the LED’s (about 1.3 kHz). The interfering
signals it works against are primarily:
The sequencer is composed of an EPROM U24, a counter U23 and a latch U25. The eight
bits of the counter are fed to the eight least significant address bits of the EPROM. The
microprocessor provides the five most significant address bits trough the control register.
The five control signals are changed infrequently but the counter increments every 16 s at the
falling edge of the
CLKSEQ
clock. Thus the EPROM may be thought of as containing 32
(25) sequencer programs. The one chosen by the five microprocessor signals is cycled in a
continuous loop.
The program length would be constrained to be 256 steps if not for the counter reset logic
(U26A and U26D). Counter is reset whenever address bit 5 and red drive are both active.
Reset logic allows flexibility in the choice of cycle length.
The outputs of the EPROM are the control signals for the LED drivers (
RED DRV
and
IR
DRV
), sequential demodulation process in the preamp (S3, S4, S5, and S6), and the gain
setting for the current to voltage preamp (S1 and S2). They are latched by the rising edge of
the CLKSEQ clock signal for precise timing relationships.
LED Drive
LED Drive circuit interfaces TTL LED drive signals ( IR DRV and RED DRV ) to red and infrared
LEDs. It consists of two circuits: LED voltage source and LED driver switch.
LED voltage source provides voltage to anodes of both LEDs and acts as intermittent short
circuit protection device. Under normal operation Q1 is “ON” and Q4 is “OFF”. When a
short circuit condition occurs higher current will cause larger voltage drop across the resistor
R24. This will turn Q4 “ON” and Q1 “OFF”, cutting off the voltage supply to the sensor.
D6 DCG0 DC GAIN
D7 DCG1 DC GAIN
STOBE NAME APOUT2