Mitsubishi Electric MUX-25TV - E1 User Manual download pdf (Page 68)

HARDWARE HACKER

baud

rates include 300

baud

for modems

and

9600 baud

for printers.

You can

roughly relate

baud rate to cps

(characters

per second) by

dividing by

10.

Thus, a

300 -baud

modem link

mazes out at 30 cps.

Most UART

circuits

need

input refer-

ence

frequencies

that are

far higher than

the communication

baud

rate.

Among

other

reasons,

this

lets the

receiver circuit

sample

only the

very center

of a

received

data bit

for minimum

noise. Often, but

not always,

the system

baud

rate reference

will be 16 times

the actual

transmission

and

reception

baud

rate.

While eight data

bits are

usually the

norm, five or seven

are sometimes

used.

It

is also possible

to use

an optional

error -

checking parity

bit, although

this

is not

done

too much

in local microcomputer

use.

The parity bit

forces all of the

ones in

a

word

to

an even number;

if an odd

num-

ber

was received there

was an error. Odd

parity can also

be selected.

It is

extremely

important that the

baud

rate, number of

data

words, number of

stop bits,

and parity (if

any) agree between

sending

and

receiving ends.

Since

both

baud

rates must agree

to better

than one-

percent accuracy,

crystal

control

is nor-

mally used at both

ends.

Show

me some

UART

circuits.

The Intersil

IM6402

is

a good

choice

for a

modern

hardware

UART.

This

one

is

available

from

Digi -Key $6.90,

(part

no.

NT5000 -ND)

among

other

dealers.

The

IM6402

is pin -compatible

with earlier

UARTs,

but

needs only a

single + 5 -volt

power

supply.

As with earlier

hardware

UARTs,

the

IM6402

needs an

external

16 x baud

rate

that

is input

to the

transmitter

on pin

40

and input

to the

receiver on

pin

17.

The

number of

data, parity,

and

stop

bits

is

programmed

by jumpering

pins 34

through

39 to ground

or + 5

volts.

Making

all of these

+ 5

volts gives

you eight

data

bits,

no parity,

and two

stop

bits. Consult

the data

sheet

for any

of the

other com-

binations

if you

need

them.

Naturally,

you should

never

use any

in-

tegrated

circuit

without the

data

sheet

and

any

available

ap notes

on

hand and

these

22 pf

22 pf

1.23 MHz

--gl

+5V

WIPOWINP

O

OO©000

0 0© 0

O O©

O O m O

O

Parallel

Data

Inputs

8

7

6

5

4

3

2

1

oo

0

0

Send

11

+5V

2.2 Meg

74HC04

z ,

0.1pF.

XTL

EPE BWt BW2 SB

PI

CEN

TD8 TD7 TD6 TD5

TD4 TD3 TD2

TD1 TX TRE TBL

TBE MR

IM6403 UART

(top view)

s+V DIV GND RRD

RD8 RD7 ROM

RD5 RD4 RD3

RD2 RD1 PE

FE

OE

SFD XTL

DRR DR RAI

4)`©4)

OOOO000

0

®4F,0 0

mffiOmc

+5V

4800 Baud

Serial

Data

Output

Fig. 3.

Typical UART

transmitter could be

used for a laptop

keyboard.

should be thoroughly

understood

before

you

do use a given

UART.

In general,

the "top half"

of a hardware

UART

is used

to transmit,

and the "bot-

tom

half" to

receive.

The

IM6403

is

a

variation

on the

IM6402 that

has some interesting

advan-

tages.

Instead of

a pair of

16 x baud

rate

inputs, you can

hang

a

crystal

directly on

the chip

and generate your

own

internal

baud

rate. Unfortunately,

many popular

baud

rates need higher-

frequency crystals

that require

higher supply

voltages,

preferably

+ 9 or +

10 volts, or the

baud

rate

generator

will not

oscillate

reliably.

Figure 2

shows

a UART transmitter

that

is set to

4800 baud, uses a cheap

crystal,

and

runs off a + 5 -volt

supply.

A transmit-

ter accepts

parallel

data in and outputs

serial

data.

Pins

34

through 39

are shown

programmed for

eight data bits,

no parity,

and two

stop bits.

The inverter on the right

22 pf

:1;

22 pf

1.23 MHz

+5V

+5V

2.2

Meg

74HC04

2

XTL EPE 8W1

BW2 SB

PI

CEN

TD8 TD7 TD6

TD5 TD4 TD3

TD2 TD1 TX TRE

TBL TBE MR

IM6403 UART

(top view)

V

DIV

GND

RRD

RD8 RD7 RD6

RD5 RD4 RD3 RD2 RD1

PE FE OE SFD

XTL DRR DR RRI

O.1 µF

4800

Baud

Serial

Data

Input

+5V

1N914

74HC04

33K

3

8

7

6

5

4

3

2

1

if

Parallel

Data Outputs

Data

Ready

0.01

µF

Fig.

4. A UART

receiver used

to

drive

a parallel

dot

-matrix printer.

80

/ MODERN

ELECTRONICS

/

April 1986

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