TeleVideo 955 Guida

TELEVIDEO®

955

VIDEO DISPLAY TERMINAL

MAINTENANCE

MANUAL

Document

l3l968-00-C

September

1987

(c)

1987

TeleVideo

Systems,

Inc.

All

rights

reserved~

part

this

publication

may

reproduced:

transmitted,

transcribed,

stored

retrieval

system,

translated

into

any

language

computer

language,

any

form

any

means,

electronic,

mechanical,

magnetic,

optical,

chemical,

manual,

otherwise,

without

the

prior

written

permission

TeleVideo

Systems,

Inc.,

1170

Morse

Avenue,

P.O.

Box

3568,

Sunnyvale,

California

94088-3568.

Disclaimer

TeleVideo

Systems,

Inc.

makes

representations

warranties

with

respect

this

manual.

Further,

TeleVideo

Systems,

Inc.

reserves

the

right

make

changes

the

specifications

the

product

described

within

this

manual

any

time

without

notice

and

without

obligation

TeleVideo

Systems,

Inc.

notify

any

person

such

revision

changes.

TeleVideo

registered

trademark

TeleVideo

Systems,

Inc.

TeleVideo

Systems,·

Inc.,

Sunnyvale,

94088-3568

1170

Morse

Avenue,

P.O.

Box

3568,

408/745-7760

TABLE OF CONTENTS

Troubleshooting

Guide

Overvi~w

Description

the

Modules

Logic

Board

Power

Supply

Video

Module

(Video

Monitor

Board)

Kp.yboard

Removinq

the

Modules

Ooening

the

Case

Removing

the

Power

Supply

Replacing

the

Power

Supply

Removing

the

Video

Module

Replacing

the

Video

Module

Removing

the

Logic

Board

and

Shroud

Replacing

the

Logic

Board

and

Shroud

Closing

the

Case

Opening

the

Keyboard

Case

Removing

the

Keyboard

Circuit

Visual

Inspection

Terminal

and

Keyboard

Exterior

Terminal

and

Keyboard

Interior

Fault

Isolation

the

Module

Level

Troubleshooting

the

Logic

Board

and

Keyboard

Troubleshooting

the

Video

Module

Visual

Inspection

Adjustments

Schematics

Ordering

Spare

Parts

955

Terminal

Spare

Parts

List

955

Parts

Reference

List

Theory

Operation

Overview

Gate

Array

Logic

Board

Theory

Operation

Clock

Circui.t

Clock

Switching

Clock

Stretch

Timing

CPU

System

and

Control

Character

Generation

Display

Generation

Screen

Data

Storage

Communications

Enhanced

Gate

A.rray

I.Jogic

Board

Theory

Operation

Clock

Circuit

CPU

System

and

Control

Character

Generation

Display

Generation

Screen

Data

Storage

Communications

iii

1-1

1-2

1-3

1-4

1-6

1-8

1-9

1-10

1-11

1-12

1-13

1-17

1-22

2-1

3-1

3-2

3-5

4-1

4-3

4-4

4-6

4-9

4-10

4-11

4-12

4-13

4-15

4-19

4-20

4-21

4-22

5 •

Service

Information

Regional

S~les

Offices

Operator's

Mnnual

7.A.ddenda

Specifications

Bulletins

LIST

TABLES

1-1

1-2

1-3

2-1

2-2

3-1

3-2

4-1

4-2

Fault

Isolation

Guide

Gate

Array

Logic

Board

and

Keyboard

Troubleshooting

Guide

Enhanced

Gate

A.rray

Logic

Board

and

Keyboard

Troubleshooting

Guide

Reference

(Gate

Array

Logic

Board)

Reference

(Enhanced

Gate

Array

Logic

Board)

955

Terminal

Spare

Parts

List

955

Parts

Reference

List

Memory

Map

Memory

Map

LIST

FIGURES

1-1

1-2

1-3

1-4

1-5

1-6

1-7

4-1

4-2

4-3

4-4

955

Terminal,

Front

View

955

Terminal,

Rear

View

Discharging

Voltages

Opening

the

Case

Removing

the

Power

Supply

Removing

the

Video

Module

Removing

the

Logic

Board

and

Shroud

955

Block

Diagram

(Gate

Array

Logic

Board)

Clock

Switchinq

CPU

Clock

Stretch

Timing

955

B10ck

Diagram

(Enhanced

Gate

A.rray

Logic

Board)

5-1

1-13

1-17

1-20

2-1

2-4

3-2

3-5

4-7

4-16

1-1

1-2

1-4

1-5

1-7

1-9

1-11

4-2

4-4

4-5

4-14

TROUBLESHOOTING GUIDE

This

troubleshooting

guide

was

designed

for

qualified

repair

personnel.

Using

this

guide,

the

schematics

Section

and

Operator's

Manual,

you

can

repair

most

failures

quickly.

difficulty

arises,

contact

your

dealer.

STOP!

not

open

the

case

described

this

manual

unless

you

are

qualified

service

technician.

The

components

exposed

during

this

procedure

retain

hazardous

voltages

that

are

present

even

after

the

power

cord

has

been

disconnected.

OVERVIEW

TeleVideo's

955,

general-purpose

ASCII

terminal,

fully

code-

compatible

with

our

925/950

family

terminals.

The

main

element

the

system

65C02

CMOS

custom

microprocessor.

For

CRT

control,

the

955

uses

advanced

alphanumeric

VLSI

CRT

controller

chip.

Figures

1-1

and

1-2

show

front

and

rear

views

the

terminal.

Ports,

switches

and

connectors

are

marked.

Figure

1-1

955

Terminal,

Front

View

CONTRAST

ADJUSTOR

ON/OFF

SWITCH

KEYBOARD

CONNECTOR

1-1

Figure

1-2

955

Terminal,

Rear

View

POWER

CABLE---=:=:::::-_~l""'"

SOCKET

~===-~_

Terminal

hardware

divided

into

four

main

modules:

Main

logic

board

Power

supply

Video

module

(video

monitor

board)

Keyboard

This

design

permits

fast

fault

isolation

and

keeps

repair

time

minimum.

isolate

faulty

module,

swap

suspected

module

with

known-good

one.

DESCRIPTION

THE

MODULES

Logic

Board

The

logic

board

fits

inside

tray

that

slides

into

the

base

the

terminal.

easily

reached

removing

two

screws

from

the

base.

The

65C02

microprocessor-based

control

circuitry

the

logic

board,

acting

its

own

internal

system

program,

stores

and

processes

data

received

and

data

transmitted.

The

circuitry

also

generates

the

video

and

sync

signals

needed

display

data

the

screen.

1-2

The

logic

board

has

four

distinct

interconnected

divisions

that

are

discussed

this

manualls

Theory

Operation

section.

They

are:

Main

processor

Display

circuitry-processor

Random

Access

Memory

(RAM)

Interface

for

external

input/output

Power

Supply

switching

power

supply

located

the

pedestal

below

the

CRT.

provides

the

following

voltages

for

the

terminalls

electronic

circuitry:

for

TTL

logic

+12V

for

the

video

signals

and

RS-232C

communications

-12V

for

RS-232C

communications

for

the

optional

modem

Two

user-replaceable

fuses--Fl

the

power

supply

and

line

fuse

back

the

terminal--protect

the

circuitry.

Video

Module

(Video

Monitor

Board)

The

video

module

located

behind

the

CRT.

contains

horizontal

and

vertical

sweep

and

video

amplification

circuitry.

This

circuitry

produces

television-type

noninterlaced

raster

display.

The

video

signals

generated

the

display

circuitry

cause

pixels

appear

designated

positions

across

scan

lines.

These

pixels

form

character

when

selectively

combined

a 7 x 9

dot

rna

ix.

Keyboard

The

955

has

detachable

keyboard

connected

the

terminal

coiled

keyboard

cable.

The

keyboard

consists

array

keyswitches

mounted

printed

circui~

board

with

8049

microcomputer

and

other

leis.

The

8049

microprocessor

the

keyboard

scans

the

keyboard

matrix,

encodes

the

data

and

transmits

two

bytes

per

character

requested.

Data

sent

through

6-wire

coiled

cable

over

asynchronous

serial

channel

9600

baud

the

main

logic

board.

The

keyboard

microprocessor

also

activates

audio

transducer

for

the

bell

and

the

simulated

keyclick.

1-3

the

main

logic

board,

the

serial

data

converted

parallel

data

and

decoded

the

main

microprocessor

and

its

firmware.

REMOVING

THE

MODULES

Opening

the

Case

STOPl

The

CRT

and

capacitors

retain

high

voltages

even

after

power

has

been

turned

off.

soon

you

open

the

case,

discharge

the

CRT

connecting

one

end

grounding

l~ad

the

metal

chassis

and

the

other

end

screwdriver

with

insulated

handle.

Slip

the

metal

end

the

screwdriver

under

the

plastic

cap

the

anode,

shown

Figure

1-3.

careful

not

touch

the

metal

portion

the

screwdriver

the

ground

lead.

Figure

1-3

Discharging

Voltages

CRT

ANODE

INSULATED

HANDlE

SCREWDRIVER

METAL

FRAME

Use

Figure

1-4

reference

for

opening

the

case.

Turn

off

the

terminal.

Disconnect

the

power

cord

and

data

cable(s)

from

the

back

the

termina17

disconnect

the

keyboard

cable

from

the

front

the

terminal.

Turn

the

terminal

the

screen

faces

away

from

you.

1-4

Remove

the

three

screws

securing

the

monitor

rear

housing

the

case

(remove

the

screw

under

tne

power

cora

r1rst,

then

remove

the

two

screws

near

the

top

the

housing).

NOTE:

Remove

the

screws

completely.

necessary,

tip

the

unit

backwards

slightly.

release

the

clamp

holding

the

bezel

and

monitor

rear

housing

together

(See

Figure

1-4):

Grasp

the

housing

with

your

left

hand.

Sharply

tap

the

right

side

the

housing

with

the

heel

your

right

hand.

Lift

off

the

monitor

rear

housing.

Figure

1-4

Opening

the

Case

PRINTER

PORT

1-5

Removing

the

Power

Supply

STOPl

The

CRT

retains

high

voltages,

even

after

the

power

has

been

turned

off,

and

poses

potential

shock

hazard.

the

CRT

discharge

procedure

page

1-4

this

section

before

removing

and

replacing

the

power

supply.

Use

Figure

1-5

reference

for

removing

and

replacing

the

power

supply.

Turn

off

the

terminal

and

open

the

case

(See

page

1-4).

Turn

the

terminal

the

screen

faces

away

from

you.

Remove

the

two

Phillips

screws

securing

the

bezel

the

case

(the

screws

are

located

inside

the

case,

one

each

side

the

power

supply).

Disconnect

the

Ground

connector,

the

power

connector

(P4)

and

the

connectors

(P1,

and/or

P3)

from

the

power

supply.

NOTE:

Only

two

connectors

are

attached

PI,

and

P3.

Pay

attention

the

polarity

for

reference

during

reassembly.

Turn

the

unit

the

screen

faces

you.

Remove

the

two

Phillips

screws

securing

the

power

supply

the

case.

Slide

the

power

supply

toward

you

remove

from

the

case.

Replacing

the

Power

Supply

Turn

the

unit

the

screen

faces

you.

Slide

the

power

supply

into

the

terminal

until

locks

into

the

plastic

support

bracket.

Reconnect

the

Ground

connector,

the

power

connector

(P4)

and

the

connectors

(PI,

and

P3).

NOTE: Any

the

three

connectors

may

used.

For

example,

you

may

take

the

connectors

off

and

replace

them

and

P3.

Insert

and

tighten

the

two

Phillips

screws

securing

the

power

supply

the

case.

Replace

the

bezel

and

monitor

rear

housing

and

the

case

(See

page

1-11).

1-6

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