Welch Allyn LC101 CO2 Module - User Manual - Manual (Page 12)

LC101 CO
2
Module OEM Implementation Manual
Page 12 Confidential Welch Allyn OEM Technologies
Reset Circuit
The first section of a 556 timer is connected to a serial receive data line such that the
microprocessor is forced into reset if the data line is held in a break condition for greater than 10
msec. A second timer section is used to determine if the break condition is maintained for more
than 500 msec.
Holding the receive data line in a break condition for a period of time greater than 10 msec and
less than 500 msec is referred to as a reset.
A hard reset occurs when the break condition is held longer than 500 msec. This condition
forces the microprocessor to enter the boot mode after the break is released.
Power ON, reset and low voltage lockouts are performed by a reset controller. Undervoltage
lockout is set to approximately 4.5 V. On power up, the microprocessor is held in reset for about
100 msec after the supply rises above 4.5 V.
Primary Power Supplies
All primary power supplies run off a single voltage power source and are converted on the Main
Board to various levels. The power source is derived from the (+5 V or +5.75 to +14.5 Vdc) Vin
power supplied by the host system.
A/D Converter
The 8 channel 12 bit serial A/D converter converts the following analog signals into a binary data
stream for the microprocessor: analog waveform from the optical bench, temperature of optical
bench, barometric pressure and pneumatic flow rate.
D/A Converter
The 12 bit serial interface D/A converter provides bench source current selection and bench
detector bias circuit control.
Source Hybrid
The optical bench requires that the IR source be current-regulated. The current-sensing circuitry
is located on a ceramic hybrid module, or source hybrid. A regulator is used that adjusts the
output voltage to maintain a constant current.
Pressure Transducers
The pressure circuitry monitors pressure via transducers in order to compensate for the effect of
pressure variation on the CO
2
measurement. The transducers include
• an absolute pressure transducer which initially provides an ambient pressure
measurement and then continuously monitors internal bench pressures during pump
operation.
• a differential pressure transducer which is used for pump flow regulation.
Due to the use of a single absolute pressure transducer, the ambient pressure
measurement is only updated at power on and watertrap replacement.