BCI Capnocheck Plus - Peripheral Interface Circuits

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Chapter

3:

Capnocheck

Plus

Circuit

Description

External

VO's

External

VO's

include

an

8

bit

input

buffer

and

a

14

bit

output

register.

The

input

buffer

allows

the

microprocessor

to

read

keyboard

signals

and

serial

data

streams

coming

from

the

Real

Time

Clock

(RTC)

IC,

Analog

-

to

-

Digital

Converter

(ADC)

and

CO2

Bench

internal

Electrically

Erasable

Programmable

Read

Only

Memory (EEPROM).

The

output

register

allows

the

microprocessor

to

control

LED’s,

keyboard

strobe

signals

and

all

other

peripheral

devices.

Pump

and

Valve

Control.

The

pump

and

valve

are

controlled

by

the

microprocessor

through

External

YO

output

register.

Dual

MOSFET

Q4

is

used

to

turn

pump

and

valve

on

and

off.

Speaker

Driver.

The

Speaker

Driver

is

built

around

operational

amplifier

U12A.

It

controls

the

speaker

driving

transistor

Q5.

The

input

signal

AUDIO

generated

by

FPGA

is

filtered

by

passive

R-C

filter

R25-C19-R26

to

restore

the

DC

component

of

the

AUDIO

signal,

which

is

then

amplified

by

U12A

and

Q5

circuitry

and

is

passed

to

the

speaker

through

connector

J6.

Analog

Outputs

Controller.

The

Analog

Outputs

Controller

is

built

around

Digital

-

to

-

Analog

converter

(DAC)

U9.

IC

U9

contains

three

independent

8-bit

(256

voltage

steps)

DAC’s

inside,

which

are

used

for

analog

output

channels

A, B

and

C.

U13A,

with

associated

resistors,

converts

the

2.5VDC

voltage

reference

output

into

a

1VDC

output used

as

a

reference

signal

for

analog

outputs.

The

output

of

the

first

internal

DAC

(signal

OUTA)

is

connected

through

an

ESD

protection

network

R33-C27

to

the

output

DB25

connector.

The

outputs

of

two

other

internal

DAC’s

are

not

internally

buffered

like

the

output

of

the

first

internal

DAC.

That

is

why

two

voltage

followers

U12B

and

U12C

are

used

as

buffers

between

DAC

and

output

connector.

The

DAC

is

controlled

by

the

microprocessor

using

synchronous

serial

communications.

RS232

Controller.

The

RS232

Controller

is

built

around

IC

U10,

which

is

used

to

convert

CMOS

voltage

levels

into

RS232

bipolar

voltage

levels.

U10

is

connected

directly

to

the

output

DB25

connector

through

ferrite

beads.

3-6

Capnocheck

Plus

Service

Manual

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