Novametrix Medical Systems OXYPLETH 520A, Service Manual

Page: 40 / 133

Section 9
Electronic Theory of Operation
30
Model 520A Service Manual 5693-90-01
PRELIMINARY
Keypanel Interface
9.3.18
The12 keys (switches) on the Model 520A front panel are connected to the 2471
Main Board through a ribbon cable at J104. Each key (except
POWER
) is connected
to an 8-bit latch (either IC14 or IC15). When any of these keys is pressed, the
corresponding latch input is brought low. The processor continually reads the status
of these latch outputs, the RDKEY* enables IC14 when low and the RDKEY_2*
line enables IC15 when brought low.
The
POWER
key ON/OFF signal is sent through J104 pin 15. The AC Line Status
signal, LINEST, is generated by the power supply, and is high (+5 volts) when the
monitor is connected to the AC Line (Mains) and the rear panel switch is set to “|”.
This +5 volt level is sent to LED D2 (the green
indicator) on the keypanel via
J104 pin 16. The LINEST signal is also input to the latch at IC14 pin 8 so that the
CPU can detect if the unit is operating on AC line power (IC14 pin 8 high) or on
battery power (IC14 pin 8 low). Diode D9 keeps current from backfeeding into
IC14 when the monitor is turned off but still connected to the AC line.
The 2MIN (D3), AUDIO OFF (D4), ALERT (D5), and LOW BAT (D1) LEDs are
driven by the 8-bit latch IC13. When each of the corresponding signals is driven
high the appropriate LED on the keypanel is illuminated. See
I/O Device Controller
on page 31.
Display Interface
9.3.19
The display is connected to the 2471 Main Board at J107. It is controlled by the
processor using the RD* (Read), WR* (Write), and DISPCS* (Display Chip Select)
lines. Data bits D0-D7 are used as input/output lines and A0 is used in conjunction
with the RD* and WR* lines to distinguish between read and write operations as
listed below.
The CNTRST (Contrast) line is controlled by pressing the front panel key. When
depressed and held the processor controls the digital to analog converter IC29, line
A0 is brought low along with WR* and DACCS*, the data on D0-D7 controls the
voltage at IC30 pin 7 which can vary from 0-4 volts. The output of IC30 pin 7, along
with 4VREF feeds a summing amplifier (IC30 pins 1, 2, 3). The output of the
summing amplifier IC30 pin 3 controls the base of Q10 which in turn controls the
contrast of the display through a variable negative voltage.
A0
RD*
Low WR*
Low
High Status Read
Command
Write
Low Data Read Data Write
Table 1. Display Command/Data table.