Olympic 50 - Analog to Digital Converter; Software Overview

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Analog

to

Digital

Converter

Refer

to

the

Main

Board

schematics,

Figs.

4-16,

4-17

and

4-18.

Ull,

the

A/D

converter,

is

an

integrating,

charge

balanc-

ing

type.

Each

conversion

cycle

takes

about

400

msec.

It

has

a

dynamic

range

of

+

32,768

counts

(15

bits

plus

sign;

a

total

of

65,536

counts).

The

input

signal

to

the

A/D

is

about

48.82

HV

per

gram.

The

system

is

designed

to

internally

resolve

1.25

grams

for

each

count

of

the

A/D

(0.8

counts/gram).

The

gain

of

the

A/D

(and

the

system)

is

controlled

by

R7

and

R8

which

provide

its

reference

voltage.

This

reference

voltage

should

be

1/2

of

the

A/D

range

of

+

32,768

counts

or:

1x

48.82uV

x

1.25g

x

32,768cnt

=

1.000

V (Typ

at

TP3)

2 g

cnt

This

may

vary

depending

on

the

exact

sensitivity

of

the

load

cells,

This

is

fully

ratiometric.

For

example,

ifthe

+5

V

excitation

to

the

load

cells

decreases

slightly,

then

the

load

cells’

signal

decreases

proportionately

for

the

same

weight;

however,

the

A/D

reference

voltage

will

also

decrease

thereby

increasing

the

A/D’s

sensitivity

(gain).

The

result-

ant

digital

output

will

remain

the

same.

To

utilize

the

full

range

of

the

A/D,

the

system’s

zero

offset

is

such

that

12.50

kg

gives

a

zero

input

voltage

to

the

A/D.

25.00

kg

will

equal

+0.610

volts

and

zero

kg

equals

-0.610

volts.

These

voltages

can

be

measured

at

TP2.

NOTE:

The

zero

offset

voltage

at

TP1

will

be

lower

than

-0.610

V

because

it

must

also

cancel

out

the

weight

of

the

platform.

Voltage

at

TP1

is

around

-1.17

volts.

This

may

vary

somewhat

depending

on

the

exact

weight

of

the

scale’s

platform

and

the

particular

degree

of

zero

offset

of

the

load

cells.

The

pC

monitors

the

Data

Valid

line

and,

when

valid,

reads

the

A/D

binary

output.

The

A/D

presents

the

high

order

bits

or

low

order

bits

depending

on

the

request

by

the

pC

via

the

High

Byte

or

Low

Byte

Enable

lines.

Note

:

All

voltages

given

are

nominal.

Each

system

will

vary

depending

on

its

specific

load

cell

sensitivity,

zero

ofíset

and

tray

weight.

Microcomputer

Refer

to

the

Main

Board

schematic,

Fig.

4-18.

The

microcomputer,

Ul2,

is

an

80C39,

a

member

of

the

8048

family.

It

has

RAM

inside

and

uses

an

external

EPROM,

U13.

U15

is

an

input/output

expander.

The

function

of

the

pC

is

controlled

by

firmware

(software).

The

software

occupies

4K

bytes

of

the

EPROM.

Page

4-9

Software

overview

The

computer

performs

housekeeping

chores

such

as

initialization

to

thePound-Ounce

modeand

auto-zeroupon

'

power

up.

It

performs

zero

tracking

for

slow

shifts

less

than

+10

grams.

If

weight

is

applied,

it

computes

weight

in

proper

units.

Also,

it

performs

averaging

and

locks

the

best

approxima-

tion

of

the

weight

on

the

display

if

requested

by

the

Lock/

Unlock

button.

The

last

locked

weightis

stored

in

the

non-volatile

external

RAM.

This

data

is

retrieved

in

the

Recall

mode.

The

non-

volatile

RAM

has

a

serial

interface.

The

front

panel

switches

are

continually

scanned.

The

pC

ㆍ

interfaces

to

the

A/D

directly

but

otherwise

interfacing

is

done

by

U15.

Data

is

clocked

to

each

of

the

two

display

drivers

(on

the

Display

Card)

serially

with

a

common

clock.

The

pC

does

7

segment

decoding

and

formatting

to

illuminate

the

correct

segments

and

annunciators.

The

quad

DIP

switch,

S1

in

Fig.

4-3,

can

be

used

to

aid

in

trouble-shooting

and

calibration.

S1-1

is

the

Memory

Bank

Select

switch.

It

can

select

either

the

upper

4K

or

lower

4K

bytes

of

the

EPROM.

If

S1-1

is

up, the

Scale

powers

up

in

thePounds-Ounces

mode.

IfS1-1

is

down,

the

Scale

powers

up

in

theKilograms

mode.

Otherwise,

the

code

in

both

sides

of

the

EPROM

is

identical.

As

mentioned,

the

A/D

has

1.25

gm/count

internal

reso-

lution.

However,

due

to

noise,

the

A/D

data

is

divided

by ^

2

by

the

pC

to

give

an

actual

internal

resolution

of

2.5

grams

per

count.

If

S1-2

is

turned

on,

the

display

will

show

these

counts

from

O

to

32,768.

The

system

is

biased

(with

the

zero

adjustment)

so

that

11,384 counts

is

zero

weight

(that

will

be

a

signal

voltage

of

about

-0.610

volts

at

TP2,

Fig.

8).

12.50

Kg

will

be

a

reading

of

16,384

counts

(a

signal

voltage

of

0

volts)

and 25.00

Kg

will

give

a

reading

of

21,384

”

counts

(a

signal

input

of

+0.610

volts).

*

Voltages

will

vary

depending

on

the

specific

sensitivity

of

individual

load

cells

and

whether

the

system

is

calibrated.

In

the

calibration

mode,

the

Zero

button

alternately

zeros

and

unzeros

the

display.

Ifthe

display

test

switch,

5

1-3,

is

activated

(prior

to

power-

up),

then

a

display

test

routine

is

activated

which

shows

segments

and

annunciators

sequentially

and

then

turns

them

all

on.

This

is

convenient

for

verifying

that

no

segments

are

open

or

shorted

to

another

segment.

Also,

it

is

used

to

adjust

the

segment

brightness.

Ifthe

Scale

is

turned

on

with

the

On/Off

switch

while

either

the

Zero,

Lb/Kg,

or

Lock/Unlock

switch

are

also

held

down,

then

all

segments

and

annunciators

light

up

(re-

gardless

of

what

dip

switches

are

activated)

until

the:

switches

are

released.

SS50-N1-220V(B09)