Novametrix Medical Systems MARS PO2 TECH 2001, Service Manual

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2775 Main Board
Section 4
Theory of Operation
Rev. 01 Model 2001 Service Manual
19
The RAM (IC20) is activated when its Chip Select line RAMCS* (IC20 pin 20) is brought low,
via IC25 pin 8. When the ME* line at IC25 pin 5 is low, and the inverse of address line A17 at
IC25 pin 4 is low, output pin 6 of IC25 will go low. This in turn will drive IC25 pin 10 low, with
PWRON* low at IC25 pin 9, IC25 pin 8 will be low (RAMCS*). If at that time, OE (IC20 pin 22)
is low, a RAM Read occurs, whereas a RAM Write will occur if WE (IC20 pin 27) is low.
Real Time Clock (RTC)
See sheet 3 of 4 on schematic.
A Real Time Clock (IC24) provides the Model 2001 the ability to time stamp collected (printed)
trend data. The 32.768 kHz crystal, Y3, provides the timing signals for the clock chip, which is
powered from the VBACK supply so that the clock can continue to keep time when the monitor
is turned off (provided the monitor’s 12 volt internal battery is connected and maintains at least
a nominal charge).
The RTC is activated when its Chip Select line (CS0) at IC24 pin 2 is brought low. With the
monitor on PWRON* will be low (IC25 pin 13), the RTC* line will be brought low by the
processor through IC22 pin 14 (see sheet 1 on schematic), these lines drive RTCCS* (Real
Time Clock Chip Select IC25 pin 11) low. If at this time, the RD* (IC24 pin 8) input is low, a RTC
Read occurs, whereas a RTC Write will occur if the WR (IC24 pin 10) input is low. Addressing
is handled by A0-A3 (pins 4-7) and data I/O through D0-D3 (pins 14-11).
Sound generator
See sheet 3 of 4 on schematic.
A programmable tone generator, IC27, is used to drive the monitor’s audio circuit. The tone
generator is clocked by IC39 pin9 from the 1.638 MHz signal, Q1. The tone generator is
enabled by the processor when IC22 pin12 is brought low. While CE is low, WR is brought low
and data bus information including frequency (pitch) and attenuation (volume) is accepted by
the tone generator. The Ready signal (IC27 pin 4) goes low while accepting data and the
processor is put into a Wait state until IC27 finishes its task; then Ready returns high and the
processor continues its operations.
The AUDOUT output at IC27 pin 7 drives the audio amplifier IC26. The amplifier output is
coupled through capacitors C55 and brought to J109 as the SNDOUT (Sound Out) line. The
speaker which is mounted in the chassis is connected to J109, LS1 is not installed on the 2775
Main Board.
Keypanel Interface
See sheet 3 of 4 on schematic.
The 10 keys (switches) on the Model 2001 front panel are connected to the 2775 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