DEC VT220 Technical Manual

EK-VT220-TM-001

VT220

Technical Manual

Prepared

by

Educational

Services

of

Digital

Equipment

Corporation

1st

Edition,

November

1984

copyright

0

1984

by

Digital

Equipment

Corporation

All

Rights

Reserved

Printed

in

U.S.A.

The

reproduction

of

this

material,

in

part

or

whole,

is

strictly

prohibited.

For

copy

information,

contact

the

Educational

Services

Department,

Digital

Equipment

Corporation,

Maynard,

Massachusetts

CH754.

The

information

in

this

document

is

subject

to

change

without

notice.

Digital

Equipment

Corporation

assumes

no

responsibility

for

any

errors

that

may

appear

in

this

document.

The

following

are

trademarks

of

Digital

Equipment

Corporation,

Maynard,

Massachusetts.

mamaoma

DEC
DECmate
DECnet
DECSsystem-10

DECSYSTEM-20

DECUS
DECwriter
DIGITAL
LA
MASSBUS

PDP

p/OS

Professional
Rainbow
RSTS
RSX
UNIBUS

VAX
VMS
VT
Work

P.rocessor

This

equipment

has

been

ertified

to

comply

with

the

limits

for

a

Class

B

computing

device,

pursuant

to

Subpart

J

of

Part

15

of

FCC

Rules.

Only

peripherals

(computer

input/output

devices,

terminals,

printers,

etc.)

certified

to

comply

with

the

Class

B

limits

may

be

attached

to

this

computer.

Operation

with

noncertified

peri

erals

is

likely

to

result

in

interference

to

radio

and

television

reception.

It

has

been

type

tested

and

found

to

camp

y

with

the

limits

for

a

Class

B

computing

device

in

accordance

with

the

specifications

in

Subpart

J

of

part

15

of

FCC

Rules,

whic

are

designed

to

provide

reas

nable

protection

against

Gch

interference

in

a

residential

installation.

Howev

r,

ther

is

no

guarantee

that

interference

will

not

occur

in

a

particular

installation.

If

this

equipment

does

cause

interference

to

radio

or

television

reception,

which

can

be

determined

turning

the

equipment

off

and

on,

the

user

is

encouraged

to

try

to

correct

the

difference

by

one

or

more

of

the

fa]

owing

measur

reorient

the

r

eiving

antenna

relocate

the

comput

r

with

respect

to

th

v r

move

the

computer

away

rom

the

receiv

r

plug

the

computer

int

a

different

outlet

so

that

omputer

and

receiver

ar

on

different

branch

circuit

.

If

necessary,

the

user

should

consult

the

d

aler

or

an

experienced

radio/television

technician

for

additional

suggestions.

The

user

may

find

the

following

booklet

prepared

by

the

Federal

Communications

Commission

helpful:

"How

to

Identify

and

Resolve

Radio-TV

Interference

Problems".

This

booklet

is

available

from

Washington,

DC

20402,

Stock

No.

the

US

Government

4-

-v10

45

4.

Printing

Officp,

This

process

repeats

for

all

characters

being

DMA'ed

into

the

line

buffer

RAM,

until

the

first

scan

line

for

all

characters

in

the

screen

row

has

been

processed

(DMA

transaction

completed).

The

remaining

scan

lines

of

the

data

row

are

processed

by

accessing

the

character

address

and

attribute

values

stored

in

the

line

buffer

RAM

during

the

DMA

activity.

Once

all

the

scan

lines

for

the

screen

row

have

been

processed,

a

new

DMA

transaction

will

be

used

to

input

the

next

screen

row

for

display.

The

character

generator

circuits.

(Figure

6-12)

consists

of

three

main

•

Access

mux

•

Line

buffer

•

Character

(CHAR)

output

circuit

TO

ViDEO
CONVERTER

TO/FROM
CPU LOGIC

TO
ATTRIBUTE

CiRCUITS

CHRO ,",-CHR7 H

CHAR

OUTPUT

CIRCUiT

ACCESS

MUX

CONTROLS

o

AO

H-A3

H

~~~M

~

,l..-

..,/

LOGiC

lJ

ADO

H-AD7

H

FROM

iAL

T SEL)

ATTRIBUTE

--------1

C1RCUITS

L..-

__

,j

FROM

,.-----...1'\

9007

SCO

H-SC3

H

FRorv

1

TiMiNG

TIMING

G

EN

1-.----..;;...,--1

FROM

I

QUART \

BNKSO

H-BNKSl

H

I .

I

I

FROM

8051

CPU

'--

__

---,

FROM

ATTRIBUTE

RAM

.....

-----1

FROM
SCREEN

RAM

1..-

-1

Figure

6-12

Character

Generator

Block

Diagram

6-16