Nellcor N-180, Manual

Page: 23 / 83

Section 3: Functional Block and Circuit Analysis
3-19
isolators. Device U10 is a level adjuster. Device U-18 inductors A–H reduce
electromagnetic interference.
LED Current Check Circuits (Sheet 4 of 4)
When power is applied, the monitor housekeeping diagnostics check the
current flowing to the LED s to determine if any LED is malfunctioning in a
way that could lead to the misinterpretation of the LED presentation. For
example, if the top right segment in an LED display failed, you could
interpret an eight as a six. The diagnostic check gives you confidence in the
front panel readings. Here is how the diagnostics work.
Resister R34 is placed in series with the current for LED driver QA1. The
voltage drop across R34 is amplified by U17A, which compares this resulting
value with the processor-controlled reference from VOLUME DAC U3/U4A.
By driving selected segments while checking the current value at
LED_CHECK the processor logic can reveal if a segment is drawing too little
or too much current.
If a current value is out of tolerance the processor displays Err 3 on the LED
display. You can override an Err 3 condition by pressing any front-panel
button, but for obvious reasons, you should only do this as an emergency
measure. The operator manual has specific warning information regarding
Err3 conditions.
LED Drive PCB
Note: Power and Ground connections for the ICs on the CPU board are
shown as “power terminal” on the schematic
Device
U
1
A
and device
U
2
A
on the LED Drive PCB (figure FO10-3) are
decoders. Decoded output from
U
1
A
and
U
2
A
goes to 16 drivers. Eight outputs
from decoder UIA feed drivers OA1A-H; eight outputs from U2A feed drivers
OA2A-H. The 16 drivers output 16 signals named COM1 -15. The state of
signals COM 0-5 and signals COM 8-13 determine whether SATURATION
and RATE LED s light red (8-13) or green (0-5).
The binary input to the decoders U1A and U2A is fed to inputs A, B and C of
the respective decoders. Depending upon the output of the CPU PCB
microprocessor, these three signal lines represent encoded information to
enable readings on SATURATION, RATE, BATTERY IN USE, LOW BATT,
PULSE SEARCH , NEONATAL and the pulse amplitude LED
S
. The actual
segments that light on the respective LED elements depend upon the coding
of DRV0-7 at the time that an LED is enabled. Signals DRV0-7 come from the
CPU PCB I/O controller as signals LED 0-7 (which feed processor device U23,
a driver).
LED PCB
The LED PCB (figure FO10-4) consists of LEDs that provide status
information,