Elenco Electronics MO-1251 Manual do Utilizador descarregar pdf (Página 74)

100

CABLE TESTER

continued

from

page 77

switches quickly, all three

devices

will appear to

be on at the same

time.

That

process is called multi-

plexing: it lets each

device share

the common transmission line

part

of the time. Multiplexing is

the basis of modern phone

trans-

mission;

that's how hundreds of

calls are sent over a single pair of

wires.

Our cable

tester

reverses

the

standard multiplexing scheme to

allow testing many wires with

just a single input and a single

output. The principle is shown in

Fig. 2; note

that the

rotary

switches there are not ganged.

place both switches in po-

sition

1, LEDI will light, indicat-

ing continuity. But if we leave Si

position

1 and move S2 to posi-

tion 2, the LED

should

out. If

it doesn't, there must be a con-

nection- intentional or

other-

wise- between lines 1

and

accomplish that type of

switching with a 4051 multiplex -

er/demultiplexer,

shown in Fig.

3. As shown in Table 1, depend-

ing

on the binary code present at

inputs A. B, and C of the 4051, we

can

connect the common

line

any of

the eight transmission

lines. Note that if we apply a high

level to the Inhibit line, all con-

nections are broken between the

common and transmission lines,

regardless of the

state

of the A, B,

and C inputs. Like the rotary

switches in Figures 1 and 2, the

4051

can accommodate AC and

DC signals in both directions.

The circuit

If the PC had a standard inter-

face with

24 input and 24 output

lines, multiplexing would be un-

necessary. It doesn't, so the cable

tester uses the 11 output

lines

the PC's parallel port to control

six 4051 IC's,

and

one

of the port's

five

input lines to sense trans-

mission status.

The complete cir-

cuit is shown in Fig.

select

ICI, we must bring its

Inhibit line low while keeping the

Inhibit

lines of IC2 and IC3 high.

Then, depending

on what

ap-

ply to the A, B, and C

inputs, we

Fig. 1. A

MULTIPLEXED CIRCUIT

allows

a single

cable to

carry

several

different

( Irvinn1s.

Fig. 2. BY DECOUPLING

the

switches

and driving (B1) and sensing (LEDI) the

common lines. it's possible to test various

combinations of

connections.

C 4051

_ 1

COMMON

6 5

INHIBIT

Vss

VFF

Fig. 3. THE CMOS 4051 allows a single

common line to connect to

one

eight

input/output

lines

when

the inhibit line

is low.

can select any of

the eight trans-

mission

lines. The Common

line

(pin 3)

connected

+V,

volts will be transmitted

through

the selected

transmission

line to

the cable

under test

via

J1.

The other end

of the cable

connected

to J2.

There. a

line is

selected

by similar

use of the

In-

hibit and A, B,

and

lines of

IC4,

IC5 and

IC6. The Common

out-

put

line is sensed

by pin

11 of the

printer port. Resistor

R1 serves

as a pull

-down to

prevent

false

readings.

Think of

the combination

ICI, IC2, and

IC3 as

a 24- position

electronically

controlled

rotary

switch.

Similarly,

the

combina-

tion of IC4,

IC5, and IC6

acts like

a second

rotary

switch. Each

"switch"

independently

ad-

justable.

For example.

line 1 can

be selected

as the

input to

the

cable, while

any

of lines 1

through

24 can

be sensed

at the

output of

the cable.

That inde-

pendent

selection allows

check-

ing of all possible

combinations

input

and

output lines. Power

provided by

three 11/2-volt AA

batteries.

Addressing

the 4051's

On the input

side, lines

A. B,

and

C are connected

to printer -

port

pins 4,

3, and 2, respectively.

Those pins

correspond

to the

three

lowest

bits (D2. D1, and DO)

of the

8 -bit parallel

data. lb select

transmission

line 1, we

connect

the A, B, and

C lines to ground;

select

transmission

line 8, we

connect

A, B. and C to

Note

that all three IC's

(ICI IC3)

are addressed;

avoid

contention

releasing

the

Inhibit line

of only one IC at

time.

The Inhibit line for ICI is

pin 1 (STROBE).

For IC2, it

is pin 14

(AUrrOFV),

and for

IC3

is pin 16

rrvrr).

lb select IC1,

send a decimal

5 to port 890 (OUT

890,5). A deci-

mal

6 selects IC2,

and decimal 0

selects IC3.

Addressing

the output side is

complicated. Bits D7 and

(pins 9 and 8, respectively) are

used to generate

the A. B. and C

signals through

IC7 and IC8. lb

select IC4,

set D7 =1 and D6 =1.

Select IC5 with D7 = 1 and D6 = 0.

Finally, select IC6 with

D7 = 0 and

D6 =1. The A,

B. and C lines are

driven by D5, D4, and D3

(pins 7,

6, and

respectively).

So. to se-

lect transmission line

"5"

IC5.

we would

make D7 =1, D6 = 0,

D5 =1. D4 =0

and

=O.

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