TeleVideo 955 Guide

TELEVIDEO®
955 VIDEO DISPLAY TERMINAL
MAINTENANCE MANUAL
Document l3l968-00-C
September 1987
Copyright
Copyright (c) 1987 by TeleVideo Systems, Inc. All rights
reserved~ No part of this publication may be reproduced:
transmitted, transcribed, stored in a retrieval system, or
translated into any language or computer language, in any form
or by any means, electronic, mechanical, magnetic, optical,
chemical, manual, or otherwise, without the prior written
permission of TeleVideo Systems, Inc., 1170 Morse Avenue,
P.O. Box 3568, Sunnyvale, California 94088-3568.
Disclaimer
TeleVideo Systems, Inc. makes no representations or warranties
with respect to this manual. Further, TeleVideo Systems, Inc.
reserves the right to make changes in the specifications of the
product described within this manual at any time without notice
and without obligation of TeleVideo Systems, Inc. to notify any
person of such revision or changes.
TeleVideo is a registered trademark of TeleVideo Systems, Inc.
TeleVideo Systems,· Inc.,
Sunnyvale, CA 94088-3568
1170 Morse Avenue, P.O. Box 3568,
408/745-7760

TABLE OF CONTENTS
l.
2.
3.
4.
Troubleshooting Guide
Overvi~w
Description of 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 at 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 of Operation
Overview
Gate Array Logic Board Theory of 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 of Operation
Clock Circuit
CPU System and Control
Character Generation
Display Generation
Screen Data Storage
Communications
iii
1-1
1-1
1-2
1-2
1-3
1-3
1-3
1-4
1-4
1-6
1-6
1-8
1-9
1-10
1-10
1-11
1-12
1-12
1-12
1-12
1-13
1-13
1-17
1-22
1-22
1-22
2-1
3-1
3-2
3-5
4-1
4-1
4-1
4-1
4-3
4-4
4-6
4-9
4-10
4-11
4-12
4-13
4-13
4-15
4-19
4-20
4-21
4-22

5 •
Service Information
Regional S~les Offices
6. Operator's Mnnual
7.A.ddenda
Specifications
Bulletins
LIST OF 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
IC Reference (Gate Array Logic Board)
IC Reference (Enhanced Gate Array Logic Board)
955 Terminal Spare Parts List
955 Parts Reference List
Memory Map
Memory Map
LIST OF 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)
iv
5-1
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 in Section 2, and an
Operator's Manual, you can repair most failures quickly. If
difficulty arises, contact your dealer.
STOP! Do not open the case as described in this manual unless
you are a 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, a general-purpose ASCII terminal, is fully code-
compatible with our 925/950 family of terminals. The main element
in the system is a 65C02 CMOS custom microprocessor. For CRT
control, the 955 uses an advanced alphanumeric VLSI CRT
controller chip.
Figures 1-1 and 1-2 show front and rear views of 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 is 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 to
a minimum. To isolate a faulty module, swap a suspected module
with a known-good one.
DESCRIPTION OF THE MODULES
Logic Board
The logic board fits inside a tray that slides into the base of
the terminal. It is easily reached by removing two screws from
the base.
The 65C02 microprocessor-based control circuitry on the logic
board, acting on its own internal system program, stores and
processes data received and data to be transmitted. The
circuitry also generates the video and sync signals needed to
display data on the screen.
1-2

The logic board has four distinct interconnected divisions that
are discussed in this manualls Theory of Operation section.
They are:
Main processor
Display circuitry-processor
Random Access Memory (RAM)
Interface for external input/output
Power Supply
A switching power supply is located in the pedestal below the
CRT. It provides the following voltages for the terminalls
electronic circuitry:
+ 5V for TTL logic
+12V for the video signals and RS-232C communications
-12V for RS-232C communications
- 5V for the optional modem
Two
user-replaceable fuses--Fl on the power supply and a line
fuse in back of the terminal--protect the circuitry.
Video Module (Video Monitor Board)
The video module is located behind the CRT. It contains
horizontal and vertical sweep and video amplification circuitry.
This circuitry produces a television-type noninterlaced raster
display. The video signals generated by the display circuitry
cause pixels to appear at designated positions across scan lines.
These pixels form a character when selectively combined in a 7 x 9
dot rna tr ix.
Keyboard
The 955 has a detachable keyboard connected to the terminal by a
coiled keyboard cable.
The keyboard consists of an array of keyswitches mounted on a
printed circui~ board with an 8049 microcomputer and other leis.
The 8049 microprocessor on the keyboard scans the keyboard
matrix, encodes the data and transmits two bytes per character
requested. Data is sent through a 6-wire coiled cable over an
asynchronous serial channel at 9600 baud to the main logic board.
The keyboard microprocessor also activates an audio transducer
for the bell and the simulated keyclick.
1-3

On the main logic board, the serial data is converted to parallel
data and decoded by 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. As soon as you open the case,
discharge the CRT by connecting one end of a grounding l~ad to
the metal chassis and the other end to a screwdriver with an
insulated handle. Slip the metal end of the screwdriver under
the plastic cap of the anode, as shown in Figure 1-3. Be careful
not to touch the metal portion of the screwdriver or the ground
lead.
Figure 1-3
Discharging Voltages
CRT ANODE
1
INSULATED HANDlE
SCREWDRIVER
METAL FRAME
Use Figure 1-4 as reference for opening the case.
1. Turn off the terminal.
2. Disconnect the power cord and data cable(s) from the back of
the termina17 disconnect the keyboard cable from the front
of the terminal.
3. Turn the terminal so the screen faces away from you.
1-4

4. Remove the three screws securing the monitor rear housing to
the case (remove the screw under tne power cora r1rst, then
remove the two screws near the top of the housing).
NOTE: Remove the screws completely. If necessary, tip the
unit backwards slightly.
5. To release the clamp holding the bezel and monitor rear
housing together (See Figure 1-4):
a. Grasp the housing with your left hand.
b. Sharply tap the right side of the housing with the heel
of your right hand.
6. 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 a potential shock hazard. Follow the
CRT discharge procedure on page 1-4 of this section before
removing and replacing the power supply.
Use Figure 1-5 as reference for removing and replacing the power
supply.
1. Turn off the terminal and open the case (See page 1-4).
2. Turn the terminal so the screen faces away from you.
3. Remove the two Phillips screws securing the bezel to the
case (the screws are located inside the case, one on each
side of the power supply).
4. Disconnect the Ground connector, the power connector (P4)
and the DC connectors (P1, P2 and/or P3) from the power
supply.
NOTE: Only two connectors are attached at PI, P2 and P3.
Pay close attention to the polarity for reference during
reassembly.
5. Turn the unit so the screen faces you. Remove the two
Phillips screws securing the power supply to the case.
6. Slide the power supply toward you to remove it from the
case.
Replacing the Power Supply
1. Turn the unit so the screen faces you. Slide the power
supply into the terminal until it locks into the plastic
support bracket.
2. Reconnect the Ground connector, the power connector (P4) and
the DC connectors (PI, P2 and P3).
NOTE: Any of the three connectors may be used. For
example, you may take the connectors off Pl and P2 and
replace them on P2 and P3.
3. Insert and tighten the two Phillips screws securing the
power supply to the case.
4. Replace the bezel and monitor rear housing and close the
case (See page 1-11).
1-6

Figure 1-5
Removing the Power Supply
•
FRONT VIEW
Pi
P2
POWER SUPPLY
POWER CONNECTOR (P4) __ :::::::::---tii"~"",,!!!!!,;;::."~!!!!!!!!!!!!!~~-:;
BACK VIEW
1-7

Removing the Video Module
STOPI The CRT retains high voltages, even after the power has
been turned off, and poses a potential shock hazard. Follow the
CRT discharge procedure on page 1-4 of this section before
removing and replacing the video module.
Use Figure 1-6 as reference for removing and replacing the video
module.
1. Turn off the terminal and open the case (See page 1-4).
2. Turn the unit so the screen faces away from you.
3. Disconnect the voltage lead at the anode by gently lifting
the rubber cap and unhinging the metal lead.
4. Carefully remove the CRT connector (small PC board at the
back of the CRT).
5. Remove the power connector (red), the video connector
(white) and the yoke connector (blue) from the video board.
6. NOTE: In some 955 terminals the video module is on a base
that slides up and down~ in others, the video module is on a
hinged base.
To determine which model you are working on, check below the
video module. If there are three screws below the module,
go on to the next step. If not, go to step 9.
7. Remove the two Phillips screws (one on each upper support
bracket) holding the video module in place.
8. Remove the middle screw below the module and loosen the
screw on each side of it. This allows the video module to
slide down.
9. If the model you are working on is hinged, you must remove
four screws.
a. Remove the screw on each side of the plate holding
the video module in place.
b. Remove the two screws (one below the other) on the
right side plate.
10. Remove the four Phillips screws holding the video board to
its base~ slide the board out of the terminal.
1-8

Figure 1-6
Removing the Video Module
t+----VIDEO MODULE
Replacing the Video Module
1. Turn the terminal so the screen faces away from you.
2. Slide the video module onto its base (See Figure 1-6) until
the four screw holes in the board line up with the four
screw holes in the base.
3. Insert and tighten the four Phillips screws holding the
video module to its base.
4. Reconnect the power connector (red), the video connector
(white) and the yoke connector (blue) on the video module.
5. Carefully replace the CRT connector (small PCB at the back
of the CRT).
6. Replace the anode cap:
a. Pull back the rubber portion of the anode cap, exposing
the metal leads.
1-9

b. Pinch the two leads together and insert them into the
opening for the anode cap in the CRT. Release the
leads.
c. Secure the rubber portion of the anode cap.
7. Insert and tighten the Phillips screws securing the module
to the case. See step 6 (Removing the Video Module) if you
are replacing the sliding module. See step 9 if you are
replacing the hinged module.
8. Replace the bezel (if removed) and monitor rear housing:
close the case (See page 1-11).
Removing the Logic Board and Shroud
Use Figure 1-7 as reference for removing and replacing the logic
board and shroud.
1. Turn off the terminal. Disconnect the power cord and data
cable(s) from the back of the terminal: disconnect the
keyboard cable from the front of the terminal.
2. Rotate the complete unit so the back faces you. Remove the
two Phillips screws (in the base) holding the logic board
tray in place.
3. Grasp the logic board tray by its center tab~ pull the tray
toward you. Disconnect the red (P5) and white (P2) logic
board connectors to slide the tray completely out of the
terminal.
4. Remove the four brass screws securing the shroud to the
logic board; remove the six Phillips screws securing the
logic board to the tray.
5. Lift the logic board out of the tray.
Replacing the Logic Board and Shroud
1. Align the six screw holes in the logic board with the screw
holes in the tray.
2. Loosely mount the six Phillips screws holding the logic
board in place: insert and tighten the four brass screws
holding the shroud to the logic board, then tighten the six
Phillips screws securing the logic board.
3. Replace the logic board tray in the base:
a. Fit the metal flanges on the logic board tray into the
guides at the back of the terminal. Hold the connector
cables up so t.hey do not interfere as you replace the
tray.
1-10

b. Reattach the red connector (P5) and the white connector
I _ "'" \ _ __ I' __ , _ __..! _ '- _ _ __ ~
\~~J on ~ne LOgl~ UUdLU.
c. Align the keyboard connector with the opening in the
front of the terminal.
4. Insert and tighten the two Phillips screws holding the logic
board tray in the base.
Figure 1-7
Removing the Logic Board and Shroud
•
9
LOGIC BOARD ~J~iiiiii~'ii~~~~~~:::~ '~~
LOGICBOARDTRAY/ " /
'------PHILLIPS SCREW
Closing the Case
1. Replace the bezel:
a. Place the bezel face down in front of the terminal.
Raise the bezel slightly and hook its two bottom tabs
into the openings (one under the contrast adjustor; one
under the ON/OFF switch) in the lower case.
b. Swing the bezel up to frame the screen, matching the
ON/OFF switch and contrast adjustor with the openings
in the bezel.
1-11

c. Carefully turn the screen away from you while holding
the bezel in place. Insert and tighten the two
Phillips screws securing the bezel to the case (these
screws are located inside the case, on on each side of
the power supply).
2. Replace the monitor rear housing:
a. Position the monitor rear housing on the case, as shown
in Figure 1-4.
b. Al ign the screw hole under the power cord wi th the
lower screw hole in the housing.
c. Fi t the four tabs in the hous ing over the edge of the
lower case.
d. Fi t the tab in the top of the beze 1 into the s lot in
the top of the monitor rear housing. Squeeze the bezel
and housing together.
e. Insert and tighten the Phillips screw under the power
cord and the two Phi 11 ips screws near the top of the
housing.
Opening the Keyboard Case
1. Disconnect the coiled keyboard cable from the front of the
terminal.
2. Turn the keyboard upside down and remove the six Phillips
screws.
3. Turn the keyboard rightside up and remove the top cover.
Removing the Keyboard Circuit
1. Remove the four Phillips screws securing the keyboard
circuit.
2. Lift the entire assembly out of the keyboard case.
VISUAL INSPECTION
A thorough visual inspection often makes the difference between
success and failure in a repair attempt. Often a problem can be
located just by close visual examination.
Terminal and Keyboard Exterior
Look for signs of accidental damage, abuse, or neglect. Keyboard
failures are often caused by spilled liquids, sprayed cleaning
solvents, staples, or paperclips.
1-12

Are there any dents or deep scratches on the exterior of the
terminal or keyboard? If so, ask the user how and when the
damage occurred. It may contribute to the problem with the unit.
Terminal and Keyboard Interior
Open the cases and inspect the keyboard and terminal interiors.
Keyboard: Check for signs of spilled liquids, foreign objects,
unplugged devices, defective traces, and signs of overheating and
burning. Check the telephone-style connectors (located on the
back of the keyboard case and on the front of the terminal).
Wiring Harness: Check the condition of the wires and look for
crushed insulation, exposed wires, and loose or broken
connectors. Unplug the connectors and check that the pins are
intact.
Logic Board: Check for loose chips, bent pins on chips,
defective chip sockets, signs of overheating and burning,
defective traces, and poor solder joints. Check that devices are
properly installed.
Power Supply: Check for open fuses, defective components, and
signs of overheating and burning.
Video Module: Check for defective components, signs of
overheating, and defective traces.
Remove all defective modules for closer inspection and repair.
When you finish the repairs, replace the module(s) and test them.
FAULT ISOLATION AT THE MODULE LEVEL
TeleVideo's modular terminal design makes isolating a problem to a
particular module easy. You can either follow Table 1-1, Fault
Isolation Guide, or replace each module in turn until the fault is
corrected.
To use Table 1-1, find the description that resembles the problem
in the terminal. Then, if applicable, refer to the
troubleshooting guide for the suspected module.
Table 1-1
Fault Isolation Guide
Symptom
No beep on power up
1-13
Suspected Module
Logic board
Line fuse
Power supply
Keyboard

Table 1-1 (Continued)
Fault Isolation Guide
Sywaptom.
No video, no cursor
No external communication
Incorrect characters
Incorrect attributes
No keyboard communication
Fails to reset properly
when CTRL RESET is pressed
No baud-rate selection
Power supply fuse blowing
Line fuse blowing
High-pitched whine
Distorted video
Characters missing dots
Display too dim/can't adjust
Video fades
Suspected Module
Logic board
Power supply
Video module
CRT
Cable to host
Logic board
Power supply (-12V)
Logic board
Keyboard
Logic board
Logic board
Keyboard cable
Keyboard (See Service
Bulletin number 65*
Logic board
Video module
Power supply
See Service Bulletin
Number 63*
Video module
Video module
Power supply
Logic board
Logic board
Defective CRT
Video module
Power supply
Check that video module
has been modified as
described in Service
Bulletin Number 67*
*Section 7 (Addenda) contains Service Bulletins applicable to the
955 terminal.
1-14

Table 1-1 (Continued)
Fault Isolation Guide
Symptom
Online communication problem
Garbled date, missing data,
"p3ER" error message (revision
C and D logic board only)
False "p3ER" error when using
main RS232 port (in block mode
and half-duplex with pins
4 and 5 jumpered)
Jittery screen
Wavy screen
Incorrect voltages
No light at cathode filament
Display not equal to key entry
Visible retrace scanlines
Keyboard locked up
Fails
Some keys inoperative
Suspected Module
Logic board
Cable to host
Host
Logic Board (See Service
Bulletin Number 64*)
Logic board (See Service
Bulletin Number 58*)
Video module
Hertz setting
Video module
Power supply
External interference
Power supply
Video module
Power supply
Defective CRT
Logic board
Keyboard cable
Cable to host
Host
Brightness adjustment
Video module
Logic board
Keyboard cable
Keyboard
Logic board
Software command
Logic board
Power supply (+5V)
Keyboard
Keyboard
Logic board
*Section 7 (Addenda) contains Service Bulletins applicable to the
955 terminal.
1-15

Table 1-1 (Continued)
Fault Isolation Guide
Symptom
Horizontal bar across screen
Poor linearity
Cursor moves, no characters
Vertical line across screen
Crackling sound with distorted video
1-16
Suspected Module
Logic board
Video module
Video
module
Logic board
Logic
board
Video module
Yoke
Video module