BCI Capnocheck Plus - Oximeter Board (LOX) Circuitry; LOX Board Overview and Power; Interface and Reset Circuitry; Microprocessor and ADC Circuitry

Chapter 4: Oximeter Board Circuit Description
Chapter 4: Oximeter Board (LOX) Circuit
Description.
General Description.
LOX Board is an optional board used in the Capnocheck® Plus Monitor when
the Oximetry option is selected by the customer. It takes power from the
internal isolated Power Supply and communicates with the master
microprocessor on the Main board through an optically isolated interface.
Power Supply
The isolation transformer which powers the LOX board is located on the Main
Board. Unregulated +8 VDC and -8VDC come to the LOX board through the
three pin connector J2. Two voltage regulators: positive U3 and negative U4
convert voltages down to +5VDC and -5VDC.
Isolated Interface and Reset Circuitry.
Bi-directional communications between LOX board and the Main board are
provided through an optically isolated interface built around U1 and U2.
Transistor Q1 is used to drive U1’s interna] LED.
The master microprocessor can reset the LOX microprocessor in case of lost
communication caused by an ESD discharge. To do so, it transmits a
continuous sequence of “zeroes”, which cause the open collector output of U1
to stay pulled down for most of the time. Capacitor C54 slowly charges
through RS, and transistor Q6 saturates, thereby, resetting the LOX
microprocessor. At power up C21 provides a “normal” reset.
Microprocessor Circuit and Analog-to-Digital Converter.
The LOX board uses a single chip microcontroller, US 87C51. It controls the
analog front-end Analog-to-Digital Converter (ADC), U6, and communications
to the master microprocessor. The clock frequency necessary for the ADC is
also generated inside US and is equal to 2.4576MHZ. Data to and from the
ADC is transferred in synchronous serial mode. ADC U6 contains an internal
voltage reference (output 4), which is used throughout all circuitry.
Capnocheck Plus Service Manual , 4-1