Radio Shack TRS-80 User manual

TRS-BO®
MODELm
MICROCOMPUTER
•
ervlce
Catalog Numbers 26-1061/1062/1063
/
CUSTOM MANUFACTURED IN U.S.A. BY RADIO SHACKl!A DIVISION OF TANDY CORPORATION

SECTION
NUMBER
II
'"
IV
V
VI
TABLE OF CONTENTS
INTRODUCTION ..
System Overview
DISASSEMBLY/REASSEMBLY ..............••...
Disa5Sembly .................•••••...••.•..
Reassembly ...........................••..
CPU CIRCUIT BOARD ..•...•• ~ •..••••••....•..
Technical Description .......•.....•.••••..•..
Video Sync Circuits ....•.•..............••.
Processor ...........••••••••••••....•..
RAM ...............•...........•.••..
Addrcs.s Decoding ........••...............
Memory Map ..........•••••••••.........
10 PORT Description ......•••..•.•••••••..
Video .
Keyboard
....................•.........
Cassette Interface ..................•......
line Printer and Real Time Clock .......••••••.
10 Bus •.............................•.
Adjustments and Jumper Positions .
Model III 10 Bus Detailed Description ....•......
Model III PORT Bits ................•••.•..
CPU Module Parts List .................••••••.
FLOPPY DISK INTERFACE .
Technical Description .
Control and Data Buffering .
Non ·Maskable Interrupt Logic .
Drive Select Latch and Motor
On Logic .
Wait State Generation and WAITIMOUT Logic
Clock Generation Logic .
Disk Bus Selector Logic .
ReadlWrite Data Pulse Shaping Logic .
Disk Bus
Output Drives .
Write Preoompensation and Clock Recovery Logic
Floppy Disk Controller Chip .
Adjustments and Jumper Options .
Floppy Disk Interface
PC Board Parts List , .
RS-232C CIRCUIT
BOARD. . .
RS-232C Technical Description .........•••••.
Pin ·Out Description . .••••....•..•.
Port and Bit Assignments .
RS·232C
PC Board Parts List .....•••••.......
POWER SUPPLY . . . . . . . . . . . . ..••.•.
Functional Specifications .......•••••..••.••••.
Troubleshooting. . . ......•..••....••.•.
Set-Up Procedures .....................••••.
Visuiliinspec.:tion .........................•.
Surt-Up . . . ...................••..
No Output ..............•••••.•••..
Perfonnilnce Test ................••..•..
Power Supply Component ViIIlues ............•••.
PAGE
NUMBER
1
3
5
7
9
11
13
13
13
13
14
"
15
16
17
17
,.
,.
,.
19
21
25
.,
.3
.3
43
43
44
44
44
44
44
44
45
45
49
53
55
56
57
62
65
67
67
67
67
58
58
69
70

SECTION
NUMBER
VII
VIII
IX
TABLE OF CONTENTS (Cont'd)
VIDEO MONITOR . ..........•... _ _ .
Functional Specifications . .................•...
Se...,ice Adjustments .
ILLUSTRATED PARTS CATALOG
Illustrated Parts Catalog Parts list
MINI·DISK DRIVE .
General Description .............•...........
Introduction ...............••...........
Important Notice ........•••..............
Physical Description .......•..•............
Functional Desaiption ........•..•.........
Interface Connections ........•.•••.........
Physical Checkout ...........••.•.........
Mounting the Disk Drive .......••.•.....
Resistor Termination . .........••.•.........
Flat Ribbon Cable Assembly .
Theory of Openrtion .
Introduction .
Organization of the Disk Drive .
Functional Block Diagram Description .
Indell Pulse .
Write Protect .
Track 00 Switch : .
Spindle Drive . ................•.•......
Positioner Control .
Data Electronics . ..........••.•...........
Data Recording .
Data Reproduction .
Operation .
Interface Electronics Specifications ......•......
Input Control Lines .
SELECT Lines IDS'- - DS4-) .
Drive Motor Enable (MOTORQN) .
Direction and Step (DIA-) (STEP-' .
Compensated Write Data (CWO) ..•.....
Write Enable (WG-) .
Side Select (SDSE
L) .
Output Status .
Index Pulse liP') .
TRACK OOITAKO-j . .....•......•.........
Write Protect (WPRT', .
Read Data (RD-) .................•.......
Maintenance .
Physical Oesaiption of the PC Boards .
Circuit Board Test Points .
Option Select .
Input Line Terminations ....••.............
Pr ... entive Maintenance .. _ .......•••..........
Cleaning the Head ............••...........
Alignment anet Adjustment .
Drive Motor Maintenance .
Carriage Movement Check .
Head Radiel Alignment/CE Alignment .
Track 013 Alignment .
Index Sector Adjustment .
Head Amplitude/Compliance Oleck .
PAGE
NUMBER
73
7S
77
79
8.
91
93
93
93
.3
93
93
94
9.
9.
9.
97
97
97
97
97
97
97
97
97
98
..
..
101
101
101
101
101
101
101
101
103
103
103
103
103
103
lOS
lOS
lOS
lOS
lOS
108
108
108
108
108
109
110
110
111

SECTION
NUMBER
TABLE OF CONTENTS (Cont'dl
PAGE
NUMBER
FIGURE
NUMBER
FiNI Check .
Disk Controller Board Parts List .
SalVo Board Parts List (Non-Linear) .......••.
Servo Board Parts list (linear) .. . ....•....••..
Illustrated Parts Catalog Parts Lin .
LIST OF ILLUSTRATIONS
TiTlE
SECTION 1- INTROOUCTION
TAS.aO Model III Block Diagram .
SECTION
III -CPU CIRCUIT BOARD
111
112
118
117
119
PAGE
NUMBER
4
1
2
3
4
4
5A
58
5C
5D
5E
5F
5G
5H
5J
5K
1
2
2
3
CPU Timing Diagram. . . . . . . . . . . . . . . . . . . . . . . . . . . . . • . • . . . . . . 13
Video Timing Diagram. . . . . . . . . . . . . . . . . . . . . . . . . . . .
•• • . . . . . . 16
10 Bus Timing Diagram .. . . . . . . . . . . . . . . . . . . . . . • • . . . • • • • • . . . 20
CPU Printed Circuit Board -Component Side. . . . . • • • . • • . • • • • • • . .. 23
CPU Printed Circuit Board _Circuit Side . . . . . • . . . . . 24
Model III CPU Schlml"tic Diagram -
Video and Horizontal Sync
Circuit!> . . . . . . . . . . . . . . . . . . . . . . • • . .. 31
Model
III CPU SChematic Diagram -Processor Section . . . . . • • . . • • • . .. 32
Model
III CPU Schematic Diagram - RAM Section. . . . . . . . . . . . • • • . .. 33
Model 111 CPU Schematic Di"gram-
Address De"coding Section ............................•.... 34
Model
III CPU Schematic Diagram -Video section... ........••.. .. 35
Model
III CPU Schematic Diagram -Keyboard Section. . . . . . . •••. •. . . 36
Model
III CPU Schematic Diagram-
Cassette Interface Section. . . . . . . . . . . . . . . . . . . . . . . . . . . • . . . . .. 37
Model III CPU Schematic Diagram -
Line Printer and
Real Time Clock Section .. . . . . . . . . . . . • • • • • . • . . . 38
Model 11/ CPU Schematic Diagram -I/O Bus Section .......•.•...... 39
Model III CPU Schematic Diagram -
RS·232C Interface and FDC Interface
Connections. . . . . • • • • • • • • • . .. 40
SECTION IV - flOPPY DISK INTERFACE
FOC Board to
CPU Board Signal Description. . . . . . . . . . • • • • • . . . . . . . 46
Floppy Did< Interface PC Board -Component Side ..........••... 47
Floppy
Dilik Interface PC Board -Circuit Side ...........•••..... 48
Model III Floppy Disk Interface SChematic... ... ....•..•.••.... 51

FIGURE
NUMBER
1
,
,
3
LIST OF ILLUSTRATIONS (Cant'd)
TITLE
SECTION V-R5-232C CIRCUIT BOARD
R5-232C Board
to CPU Board Signal Description
AS·232C PC Board - Component Side
RS·232C
PC Board -Circuit Side .
Model
til RS·232C Schematic Diagram .
SECTION VI -POWER SUPPLY
PAGE
NUMBER
5.
60
61
63
1
,
3
•
Ten Set-Up . . . . . . . . . . . . . . . . . . . . . . . . . • • • . . . . . . . . . . . . . . .. 67
02 Coll&ctor Waveform . . . . . . . . . . . . . . • . . . • • • . • . • • • • • . 68
02 Bue Waveform . . . . . . . . . . . . . . . . • • . • • • • . . • . • • . . . 68
Power Supply 5ct1ematic Diagram ..••• , . . . . . . . 71
SECTION VtI- VIDEO MONITOR
1
,
3
Video Monitor Printed Circuit Board .......................•...
Deflection Yoke Assembly .
Video Monitor Assembly Sd1ematic Diagram
...........••..••.•..
SECTION VIII -ILLUSTRATED PARTS CATALOG
76
77
78
1
,
3
•
5
1
1A
2
3
•
5
6
7
8
8
10
11
12
13
"
15
16
17
18
18
,.
20
20
Outer Case Assembly. ..................... ......... .. 81
Case Top Assembly. ...........................•••••.... .. 82
Base Plate Assembly .......••...••.•••.••••...•........... 83
Metal Chassis Assembly .. . . . . . . . . . . . . . . . . . • . . . . . . . . . . . . . 84
Disk Drive Assembly . . . . . . . . . . . . . . . . . • . . • . . . . . . . . • . . .. 85
SECTION IX -FLOPPY DISK DRIVE
PC Board Component Locations (Logic Board) . . . .
.. ........ .. 94
PC Boord Components Locations (servo Boards)
..............•.... 95
Resistor Termination. . . . . . . . . . . . . . . . . . . . . . . . • • . • • • . . • . . . .. 96
Cable Assembly - Connector Pin Removal Chart ....••.•••••....... 96
Funetional Blode Diagram 98
FM Recording .............................•..•••.•.•. 98
Write Timing Diagram. ... .. . . . .. ............... .• .•• .100
Read Timing Diagram .................... .......
100
Interface Configuration.. . . . . .. . . . . .. . . . . . . . . •. . . . . .. . . . . 101
Logic Board Test Points and Connector Locations ....••.••••••••. 106
Servo Board Comector Locations.. . . . . .. ... .. . . .. . .. .•. .. . . 107
"Cat's Eyes" Panern .. .................................. .. 109
Radial Ad;ustmeot ..................•..•••••........•.... 109
Track
tJ0 Stop Adjustment . • • • • . . . • • • • • • • • . • • • . . • . 110
Index Sector Timing .. . . . . . . . . . . . . . . . . . . • . • . • • . . . . • . . . . . 110
Index
Adjustment. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110
Upper Arm and Carriage
.................••••............. 111
Illustrated Parts Catalog -Exploded View .....•••••••.••.•••.... 118
Mini·Disk Drive Schematic (Sheet
11 ........••.•••••.•••••.•.. 120
Mini-Disk Drille Schematic (Sheet 2) ..........••••............. 121
Servo Board Schematic .....................•.............. 122
Mini·Disk Drive Schematic (new board) ............ ••••. . . 123
Mini·Disk Drive Schematic (new
boardl ....• ,.................. 124

TABLE
NUMBER
LIST OF TABLES
TITLE
SECTION V - R5-232C CIRCUIT BOARD
PAGE
NUMBER
1
2
1
2
BRG Programming Table .
SECTION VI- POWER SUPPLY
Load Board Values. ...... . ............••••..•..•....
Voltage and Ripple Specifications ....
SECTION IX - MINI·DISK DAIVE
Interface Connector Pin Assignments .............•...
Power Connector Pin Assignmcots ...............•.••.
55
58
59
'02
'02

SECTION I
INTRODUCTION
,

SYSTEM OVERVIEW
The Aadio 5tIack TASSO Model III Miaocomputer is an enhanced venion of Aadio 5tIack's popular TRsaO Microcom-
puter (Model n. The TAs-80 Model III is sohware eompatible with the Modell §() thillt ownenof either system can take
adll8ntilge
of the large number of programs available.
Features of the TAS-80 Model III which are common to the TR5-80 Modell include:
•Availability of Level lor Level II BASIC in ROM
• Full size typewriter style keyboard
• A 12-inch video display
•Built·in cassette interface
•Character display
of 16 lines of 64 characters
•Graphics under control
of BASIC (128 H x 48 VI
• UL reoognized construction
In addition. tne Model III has the following standard features:
•12-«.ey numeric keypad for rapid entry of numbers
• Augged cabinet housing keyboard. electronics, video display. and power supply
•Direct drive video monitor for improved resolution
•Internal power supply
•Parallel printer port for
USll with Aadio 5tIack printers
several
other features are available when Level II BASIC is used; features such as: real time clock, upper and lower case
characters,
RAM internally expandable to 48K bytes, I/O l)Or\ for peripheral expansion. and cassette interface available
with
500 aoo 1500 baud rates.
Optional peripherals for the TAS-aO Model
III include disk drives (two built·in, two external) with double density for
increased storage capacity. and ahuilt·in R5-232 serial interface for communications and peripheral interface.
The Block Diagram shows the various internal components and connections of the Model III Microcomputer.
3

LA ~
rf1
POWER
SUPPLY
(optional)
HJ
POWER
SUPPLY
o
o
J~ ;r:::P~~~==~ ~
CH~HJ AC D+' I0 AC o:p
CASSETTE
I/O
I/O BUS
Z·80
4K or 14K ROM
4K, 16K, 32K, or 48K RAM
REAL TIME CLOCK
UPPER/LOWER CASE VIDEO
500/1500 BAUD CASSETTE INT.
KEYBOARD INTERFACE
LINE PRINTER INTERFACE
I/O BUS INTERFACE
LINE
PRINTER
PORT
II
RS·232
I/O
~ ~ , , • I
RS'232C
INTERFACE
(optional)
II
o
FLOPPY
DISK
CONTROLLER
(optional)
TO
EXTERNAL
DISKS
,---r--L--,
~
-1
\",/J
-
[}
[H ,
DISK #0
(optional)
DISK #1
(optional)
ri::1 I
~ dygOAE
a
BRIGHTNESS
AND
CONTRAST
FIGURE
1. TRS·80 MODEL III BLOCK DIAGRAM

--- --- - -------- - ------------- -
SECTION II
DISASSEMBL Y IREASSEMBLY
5

CASE:
DISASSEMBLY
R5-232 BOARD (optional):
1. Remove all cables hom the bottom and rear of the Com·
puter. Position
the Compuler on its rear panel to provide
easy access to the ca~ bottom. Remove the ten screws
from the case bottom. Notice the different types and
lengths
of screws and note their positions. Set them
aside in groups.
2. Position the Computer upright and remuve Ihe:#6 screw
and washer from Ihe top of the back panel of the case.
3. Very carefully remove case
top, lifting striaght up and
setting it aside to the left (if facing video screenL Be
careful not to exoeed Ihe length of the video cable.
4. Remove $Crews from the chassis shield and the ground
connectors and remove the shield .••
CPU BOARD:
1. Remove all cable. connecting thc CPU Board {power
supply cable, video,
keyboard, and cassette cables, and
if applicable,
the RS·232 ,md FDC inter-connect cables).
2. Remove the five screws fastening the PC Board (two in
upper right corner, three on bOltom).
3. Remove the small PCB Mount Bracket and its two
screws from atop the metal chassis bracket .••
4. Make sure all cables to the CPU Board have been discon,
nected then remove tt'le Board. (If your unit uses plastic
spacer mounts to hold the Board, press the small tOOs on
the mounts through the mounting holes in the PCB and
gently pull the CPU Board off.l
FDC BOARD (optional):
NOTE: The CPU Boarr! must be removl! before removing the
the FDC Board.
1. Be sure to disconnect aU cables to the FDC Boord,
2. Remove the screws holding the FDC Board to the metal
chass.is and remove the Board.
•"Not applicable
10 all units.
NOTE:
The CPU Board must be removed before removing
the RS·232 Board.
1. Be sure to remove all cables connecting the RS·232
Bo~rd.
2. R!!move the screws connecting the PC Board to the
chassis and remove the Board.
MAIN POWER SUPPLY:
I , Remove aU interconnect cables to the Boards fastened
on the metal chassis Ixac:ket (power supply cable,
video,
keyboard, and cassette cables, and If applicable,
RS·232 and FDC power connectors and the Disk ribbo ....
cable).
2. Remove the four screws fastening the metal chassis
bracket
to the case bottom.
3. Make sure all inter-eonneeting cables arc removed, then
carefully lift
out chassis bracket and attached Boards.
4. Remove the four screws fastening the Power Supply to
Ihe Bracket (notice where the ground tab is fastened),
then
car.fully remove the power supply,
DISK DRIVE POWER SUPPLY (optional):
1. Disconnect all cables and wires connecting to the Power
Supply.
2. Remove the four screws fa.tcning the Board to the Disk
Mounting Bracket (notice where
the ground tab is fas·
tened/. then carefully remove the Power Supply.
DISK DRIVES (optional):
1.
To remove the Disk Drive in the top position, carefully
remove the FOC inter·connect cable connected to lhe
rear of the Drive.
2. Remo¥e the fOlK screws and washers (two on ealt'l sidel
which connoct
the Drive to the Disk Mounting Bracket.
3, Disconnect the power supply connectOr from the bot·
tom of the top board in the Disk Drive and also remove
the ground wire from the rear of the Drive.
7

4. To remove the Disk Drive in the bottom position, you
must first remove the Disk Drive Power Supply.
5. After removing the Powe, Sul)ply, remove the FDC
inter·connect cable from the rear
of the Drive.
6. Remove the four screws and washers (two on each side)
whidl connect the Drive to the Disk Mounting Bracket.
7. Disconnect the power supply connector from the bot·
tom of the top board in the Disk Drive and also remove
the ground wire from the rear
of the Drive.
VIDEO MONITOR (CRTI AND VIDEO BOARD
1. Disconnect the four color coded wires with spade lugs
from the CRT yoke. {Be sure to note their positions.)
2. Disconnect the cOflnector on the rear of the CRT neck.
WARNING
There may be ahigh voltage ctIirge on Ihe high voltage
anode. To discharge, connect one
end of awire to a
known good ground end connect the
other end of the
wire to the blade of acommon screwdriver. Insert the
screwdriver blade under the suction cup and touch it
to the clip holding the wire to the CRT.
B
3. Insert acommon screwdriver under the suction cup on
the high·voltage anode wire on the side of the CRT. Use
the screwdriver 10 compress the clip holding the wire to
the tube and pull the wire
hee.
4. Remove the ground wire fastened directly to the Video
Board.
5. Remove the upper right and I't>wer reft nun and washers
which hold the CRT in place.
CAUTION
Udropped, the CRT may implode. To avoid this kind of
accident. support the CRT while performing the next
step.
6. Remove the remaining l~w,r right and upper leh nun.
and washers and carefully remove the CRT.
7. Disconnect Ihe CPU cable connector from the Video
Board.
8. Remove the two screws fastening the VidllO Board to the
Case Top and carefully lift out Board.

RS-232 BOARD (optionol):
REASSEMBLY
6. Reconnect the FOC inter·connect cable to Ihe rear of
the Drive.
1. Install tne
PC Board using :#6 )( 318" screws. If appli·
cable, press tne
PC Board onto tnc plastic spacer mounts
then fasten with the screws.
2. Reconnect
all cables to tne RS·232 Board.
FDC BOARD loptionall:
1. Install tne PC Board u5ing :i;6 )( 3/8" screws and the
plastic spacer mounts,
if used.
2. Reconnect all cables to the FDC Board.
CPU BOARD:
1.
Make sure good inwlating wa5hef"S are attached to the
CPU Board IRev. Fboards only) tnen fasten the Board
u~n<;l #6)( 1/4" screws.
2. Reconnect all cables
to the CPU Board {power supply
cable, video. keyboard, and cassette cables. and ifappli·
cable. RS·232 and FDC inter-eonnect cables!.
3. Attach the small PCB Mount Bracket (if used) to the
metal chassis brncket with two 1;6 x 1/4" screws.
MAIN POWER SUPPLY:
I. Fasten the Power Supply to the metal chassis bracket
using four #6 )( 1/4" screo.\ls. Be sure the ground tab is
fastened back in place.
2. Install the chassis bracket (with all Boards
in place) in
the case bottom using four #6 x 1/4" screws.
3. Reconnect all cables (power supply, video, keyboard,
and cassette cables. and AS·232 and FOC
pO'Ner cables
and
Disk ribbon cable it necessary).
DISK DRIVE (optional);
1. Place the
Disk Drive in the bottom position and reoon·
nect the ground wire and power supply connector.
2. Fasten the Drive with four
#6 x1/2" screws and four
flat
wash81'S (two on each side).
3. Reoonnect the FDC inter<onne<:t cable to the rear of
the Drive.
4. Position
the second Disk Drive in the top position and
reconnect the ground wire and power supply connector.
5. Fasten with four screws and washers (two on each side).
DISK DRIVE POWER SUPPLY (optional):
,. Before installing thc Power Supply, be s.ure that the
bottom Disk Driva is mounted in place and the Disk
Shield is ill positon on the Disk Mounting Bracket.
2. Aeoonnect all cables and wires
10 the Power SuppIV.
3. Fanen the Power Supply with four :?6 x 3/8" screws.
Be sure the ground tab is fastened back in place.
VIDEO MONITOR (CRT) AND VIDEO BOARD;
1. Positron tne CRT
in the Case Top and install the upper
left and lower rignt ;::10 w<lshers and nuts.
2. Install the upper right and lower left :::!10 washers and
nuts. Be sure to reconnect the ground wire from the
CPU cable. It will require two nut5 to fasten it.
3. Install the Video Board into the Ctse and fasten with
two #6 x
1/4" screws.
4. Connect the ground wire with solder lug back. to the
Video Board.
5. Install tne plug on the rear of the CRT neck.
6. Innall the four color ooded wires with spade lugs
to
their a5S0ciated terminals (as determined by acolored
dot on the yoke neor each terminal).
7. Install the high·voltage anode wire on the side of the
CRT. Use ascrewdriver to compres.s the clip and insert
it into the CAT. Press down on the suction cup to
secure.
CASE:
1. Double'check to be sure
all wires are connected cor·
rectly and
all Boards are properly fastened,
2. Attach the chassis shield (if used) with #6 x 1/4"
screws ltnd reconnect the ground connectors.
3. Carefully pillce the
Case Top over the Case Bottom.
Do not hit the CRT neck. It could implode or break
off.
4. Install the #fj x 3/8" sneet metal screw and flat washer
in the top rear panel of the Case.
5. Carefully rest the Computer on its rear panel and replace
the ten #8 saews; five 1" sheet metal toward rear, three
718" machine head afong front. and two 1" machine
head
in remaining positions.
9

SECTION III
CPU
CIRCUIT BOARD
11

TECHNICAL DESCRIPTION
The techniul description of the Model III Computer CPU
circuit board will be broken down inlo nine sections. These
ilIre:
1. Processor
2. RAM
3. Adress Decoding
4. Video
5. Video Sync Circuits
6. Keyboard
7. Cilssette Interfaces
8. Line Printer and Real Time Clock
9_ 1/0 Bus
This breakdown, which follows the partitioning of the sche-
maticdiagrams, will allow
ea'iY explanation and referencing.
VIDEO SYNC CIRCUITS
The video sync circuits are required
10 meet the pulse width
end polarity requilements
of the video monitor. It alo;o allows
adjustment
in both the horizontal and vertical planes in dis·
crete increments. The verticol sync pulse VSYNC
is active
low and approximately
693J,LSec in duration, The frame late
is either 50 or 60 Hertl. The horizontal rate is 15.840 Hz and
the signal HSYNC is active high
with a duration of approxi-
mately 8psec. The vertical plane adjustml!rlt covers atotal
of eight rows and is adjustillble in increments of one row
{R1. R2, R4l. The horizonUI plane adjustment increments
by two charac'lers for a totilll coverage of 16 character posi-
tions. Both adjustments lire ac<;omplished with the use of
wired "AND" gates U22 and U38 {LS266J.
PROCESSOR
The
CPU chip is aZ.sO that runs at aclock speed of 2.02752
MHz. The CPU clock is derived by dividing the basic video
clock (10.1376
MHz) by five. U62 performs this function
and the dock is non~ymmetrical with a 40 percent high duty
cycle. The dock signal POC is run through an active pull-up
and becomes PCLOCK which has afull 5''1011 swing with fan
rise and fall times. The reset switch. which is located on the
keyboard. is ORed with power on reset (R7. C54. U15) to
provide aSystem RESET"' signal. The Reset pin on the Z.g()
is driven by RESET". The MIdress lines are buffered by
LS244's (U91, U92), the
data lines by an LS245 (USO). and
control lines buffered
by an lS367 (U76). The buffered
control lines are combined in Ua9 and US6 to form memory
and 10 port controls
(RD·, WR·. IN·, OUT·).
See the CPU Timing Diagram for exact time relationships.
U75
is used to S>N tch the data bus buffer during INT AK·
cycle or any read operation (10 01 Memory). Whenever one
of the ROM's IS being acceued, the data bus buffer is dis·
abled
by U108 since Ihe ROM's dala lines are located on the
CPU dala bus, BUSRO', HALT", BUSAK". and RFSH' are
not used. The Model III has three sockets for the ROMs
U104, Ul05, U106). They are 64K bit,32K t>it and 16K bil
ROMs, reslJectillcly.
AAM
The RAM consists of an array of up to twenly-four 16K
dynamic memory chips
with damping resinors located on all
address and control lines (RP1, RP2. RP3). Chips U710 U14
are addressed at
4000 to 7FFF. U25 to U32 are addressed at
8000 to BFFF and U43 to U50 are addressed at Cooo 10
FFFF. The memory data is interfaced to the data bus by
1\'\/0 8T26s (U63, U64). Normally the data is driven Into the
memory array. However. on aread cycle U19 switd1es the
direction of
the transceivers and drives memory dala onto
the data bus. The· 5volts required by the RAMs is supplied
by a3·terminal regulator (MC7905Cl which takes -12 volts
as its input. See the CPU Timing Diagram for the relationship
between RAS". MUX,
CAS·,
_.
"""",._ .
.u_._ .
"
.. n
""
""" ..
~ .,.._ ." ..... a.. ,. ,, ..... ~""'. ~, .,~ ". : .... _
___ i. ..-
.. _...- 1JUU'.nJ1.J1fUlJ1.iUl/UUl.fiJU1JlIlJ1n.n.nnnnnnIU1IU\.N'\J1J\J1IlIlJl..n.r\.il...
. '''...,.~~~~~~
.. '.. ::x I I ------=y x::::=
_o·~ ---F1 \I \.... I
"0" ~ ...J _----1
FIGURE 1. CPU TIMING DIAGRAM
13