Radio Shack TRS-80 Model 4 Product manual

TERMS AND CONDITIONS OF SALE AND LICENSE OF RADIOSHACK COMPUTER EQUIPMENT AND SOFTWARE
PURCHASED FROM A RADIO SHACKCOMPANY-OWNED COMPUTER CENTER, RETAIL STORE OR FROMA
RADIO SHACKFRANCHISEE OR DEALER AT ITSAUTHORIZED LOCATION
LIMITED WARRANTY
I. CUSTOMER OBLIGATIONS
A. CUSTOMER assumes full responsibility that this Radio Shack computer hardware purchased (the “Equipment”), and any copies of Radio Shack
software included with the Equipment or licensed separately (the “Software”) meets the specifications, capacity, capabilities, versatility, and other
requirements of CUSTOMER.
B. CUSTOMERassumes full responsibilityfor the condition and effectiveness of the operating environment in which the Equipment and Software are to
function, and for its installation.
II. RADIO SHACKLIMITED WARRANTIES ANDCONDITIONS OF SALE
A. For a period of ninety (90) calendar days from the date of the Radio Shack sales document received upon purchase of the Equipment, RADIO
SHACK warrants to the original CUSTOMER that the Equipment and the medium upon which the Software is stored is free from manufacturing
defects. THIS WARRANTY IS ONLY APPLICABLE TO PURCHASES OF RADIO SHACK EQUIPMENT BY THE ORIGINAL CUSTOMER FROM
RADIO SHACK COMPANY-OWNED COMPUTER CENTERS, RETAIL STORES AND FROM RADIO SHACK FRANCHISEES AND DEALERS AT
ITS AUTHORIZED LOCATION. The warranty is void if the Equipment’s case or cabinethas been opened, or if the Equipment or Software has been
subjected to improper or abnormaluse. If a manufacturing defect is discovered during the stated warranty period, the defective Equipment must be
returned to a Radio Shack ComputerCenter, a RadioShackretailstore,participatingRadio Shackfranchisee or Radio Shack dealer for repair, along
with a copy of the sales document or lease agreement. The original CUSTOMER’S sole and exclusive remedy in the event of a defect is limited to
the correction of the defect by repair, replacement, or refund of the purchase price, atRADIO SHACK’S election and sole expense. RADIO SHACK
has no obligation to replace orrepairexpendable items.
B. RADIO SHACKmakes no warranty as to the design, capability, capacity, or suitabilityfor use of the Software, except as provided in thisparagraph.
Software is licensed on an ‘‘AS IS’’ basis, without warranty. The original CUSTOMER’S exclusive remedy, in the event of a Software manufacturing
defect, is its repair or replacement within thirty (30) calendar days of the date of the Radio Shack sales document received upon license of the
Software. The defective Software shall be returned to a Radio Shack Computer Center, a Radio Shack retail store, participating Radio Shack
franchisee or Radio Shackdealer along with the salesdocument.
C. Except as provided herein no employee, agent, franchisee, dealer or other person is authorized to give any warranties of any nature on behalf of
RADIO SHACK.
D. Except as provided herein, RADIO SHACK MAKES NO WARRANTIES, INCLUDING WARRANTIES OF MERCHANTABILITY OR FITNESS FOR
A PARTICULAR PURPOSE.
E. Some states do not allow limitations on how long an impliedwarranty lasts, so the above limitation(s) maynot apply toCUSTOMER.
III. LIMITATION OF LIABILITY
A. EXCEPT AS PROVIDED HEREIN, RADIO SHACK SHALL HAVE NO LIABILITY OR RESPONSIBILITY TO CUSTOMER OR ANY OTHER
PERSON OR ENTITY WITH RESPECT TO ANY LIABILITY, LOSS OR DAMAGE CAUSED OR ALLEGED TO BE CAUSED DIRECTLY OR
INDIRECTLY BY “EQUIPMENT” OR “SOFTWARE” SOLD LEASED, LICENSED OR FURNISHED. BY RADIO SHACK, INCLUDING, BUT NOT
LIMITED TO, ANY INTERRUPTION OF SERVICE, LOSS OF BUSINESS OR ANTICIPATORY PROFITS OR CONSEQUENTIAL DAMAGES
RESULTING FROM THE USE OR OPERATION OF THE “EQUIPMENT” OR “SOFTWARE”. IN NO EVENT SHALL RADIO SHACK BE LIABLE
FOR LOSS OF PROFITS, OR ANY INDIRECT, SPECIAL, OR CONSEQUENTIAL DAMAGES ARISING OUT OF ANY BREACH OF THIS
WARRANTY OR IN ANY MANNER ARISING OUT OF OR CONNECTED WITH THE SALE, LEASE, LICENSE, USE OR ANTICIPATED USE OF
THE “EQUIPMENT” OR “SOFTWARE”.
NOTWITHSTANDING THE ABOVE LIMITATIONS AND WARRANTIES, RADIO SHACK’S LIABILITY HEREUNDER FOR DAMAGES INCURRED
BY CUSTOMER OR OTHERS SHALL NOT EXCEED THE AMOUNT PAID BY CUSTOMER FOR THE PARTICULAR “EQUIPMENT” OR
“SOFTWARE” INVOLVED.
B. RADIO SHACK shallnot be liable for any damagescaused bydelay in delivering orfurnishing Equipment and/or Software.
C. No action arising out of any claimed breach of this Warranty or transactions under this Warranty may be brought more than two (2) years after the
causeof actionhas accrued or more than four (4) years after the date of the Radio Shack sales document for the Equipment or Software, whichever
first occurs.
D. Some statesdo not allow the limitation or exclusion ofincidentalor consequential damages, so the above limitation(s) or exclusion(s) may not apply
to CUSTOMER.
IV. RADIO SHACK SOFTWARE LICENSE
RADIO SHACK grants to CUSTOMER a non-exclusive, paid-up license to use the RADIO SHACK Software on one computer, subject to the following
provisions:
A. Except asotherwise provided in this Software License, applicable copyright lawsshallapply to the Software.
B. Title to the medium on which the Software is recorded (cassette and/or diskette) or stored (ROM) is transferred to CUSTOMER, but not title to the
Software.
C. CUSTOMER may useSoftwareon one host computer and accessthat Software throughone ormore terminalsif the Software permits thisfunction.
D. CUSTOMER shall not use, make, manufacture, or reproduce copies of Software except for use on one computer and as is specifically provided in
thisSoftware License.Customeris expressly prohibited fromdisassembling the Software.
E. CUSTOMER is permitted to make additional copies of the Software only for backup or archival purposes or if additional copies are required in the
operation ofone computerwith the Software, but only to the extentthe Software allowsa backup copy to be made. However, for TRSDOS Software,
CUSTOMER ispermitted to make a limitednumber of additional copies forCUSTOMER’S own use.
F. CUSTOMERmayresellor distributeunmodified copiesof the Software providedCUSTOMER haspurchased one copy ofthe Software for each one
sold or distributed. The provisions of this Software License shall also be applicable to third parties receiving copies of the Software from
CUSTOMER.
G. All copyright notices shall be retained on all copies of the Software.
V. APPLICABILITY OF WARRANTY
A. The terms and conditions of this Warranty are applicable as between RADIO SHACK and CUSTOMER to either a sale of the Equipment and/or
Software License to CUSTOMER or to a transaction whereby RADIO SHACK sells or conveys such Equipment to a third party for lease to
CUSTOMER.
B. The limitations of liability and Warranty provisions herein shall inure to the benefit of RADIO SHACK, the author, owner and/or licensor of the
Software and anymanufacturerof the Equipment sold byRADIO SHACK.
VI. STATE LAW RIGHTS
The warranties granted herein give the original CUSTOMER specific legal rights, and the original CUSTOMER may have other rights which vary from
state to state.

TRS-80®
MODEL 4
TECHNICAL
REFERENCE
MANUAL
A DIVISION OF TANDY CORPORATION
FORT WORTH, TEXAS 76102

TRSDOS®Version 6 Operating System:
© 1983 Logical Systems.
Licensed to Tandy Corporation.
All Rights Reserved.
Model 4 Technical Reference Manual; Hardware Part:
© 1983 Tandy Corporation.
All Rights Reserved.
Model 4 Technical Reference Manual; Software Part:
© 1983 Tandy Corporation and Logical Systems.
All Rights Reserved.
Reproduction or use, without express written permission from Tandy Corporation of any
portion of this manual is prohibited. While reasonable efforts have been taken in the
preparation of this manual to assure its accuracy, Tandy Corporation assumes no
liability resulting from any errors or omissions in this manual, or from the use of the
information contained herein.
TRSDOS is a registered trademark of Tandy Corporation.
10 9 8 7 6 5 4 3 2 1

IMPORTANT NOTICE
This Technical Reference Manual is written for owners of the TRS-80 Model 4 Computer, who
have a thorough understanding of electronics and computer circuitry. It is not written to the
beginner’s level of comprehension.
This manual contains detailed schematics and theories of operation for each major part of the
Model 4. These tools will aid you with designing interfaces for your computer, repairing your own
computer after its warranty has expired, or simply obtaining practical knowledge of your TRS-80
Model 4 operation.
Radio Shack will not be liable for any damage caused, or alleged to be caused, by the customer
or any other person using this technical manual to repair, modify, or alter the TRS-80 Model 4
Computer in any manner.
Many parts of the computer electronics are very sensitive and can be easily damaged by
improper servicing. We strongly suggest that for proper servicing, the computer be returned to
Radio Shack. Because of the sensitivity of computer equipment and the potential problems
which can result from improper servicing, the following limitations apply to services offered by
Radio Shack:
1. If any of the warranty seals on any Radio Shack computer product are broken, Radio Shack
reserves the right to refuse to service the equipment or to void any remaining warranty on
the equipment.
2. If any Radio Shack computer product has been modified so that it is not within
manufacturer’s specifications, including, but not limited to the installation of any non-Radio
Shack parts, components, or replacement boards, then Radio Shack reserves the right to
refuse to service the equipment, void any remaining warranty, remove and replace any non-
Radio Shack part found in the equipment, and perform whatever modifications are necessary
to return the equipment to original factory specifications.
3. The cost for the labor and parts required to return the Radio Shack computer equipment to
its original specifications will be charged to the customer in addition to the normal repair
charges.

i
TABLE OF CONTENTS
Part 1 / Hardware
SECTION I Introduction.....................................................................................................................................1
1.1 SYSTEM OVERVIEW ......................................................................................................................3
1.2 BLOCK DIAGRAM............................................................................................................................3
1.3 JUMPER OPTIONS..........................................................................................................................5
SECTION II Disassembly / Assembly................................................................................................................7
2.1 Disassembly .....................................................................................................................................9
2.1.1 Case .................................................................................................................................................9
2.1.2 CPU Board........................................................................................................................................9
2.1.3 FDC Board (optional)........................................................................................................................9
2.1.4 RS-232 Board (optional)...................................................................................................................9
2.1.5 Main Power Supplies........................................................................................................................9
2.1.6 Disk Drives (optional)........................................................................................................................9
2.1.7 Video Monitor (CRT) and Video Board...........................................................................................10
2.2 Assembly........................................................................................................................................11
2.2.1 RS-232 Board (optional).................................................................................................................11
2.2.2 FDC Board (optional)......................................................................................................................11
2.2.3 CPU Board......................................................................................................................................11
2.2.4 Power Supply..................................................................................................................................11
2.2.5 Disk Drive (optional) .......................................................................................................................11
2.2.6 Disk Drive Power Supply (optional)................................................................................................11
2.2.7 Video Monitor (CRT) and Video Board...........................................................................................11
2.2.8 Case ...............................................................................................................................................11
SECTION III CPU Circuit Board ........................................................................................................................13
3.1 Model 4 Theory of Operation..........................................................................................................15
3.1.1 Introduction.....................................................................................................................................15
3.1.2 CPU and Timing .............................................................................................................................15
3.1.3 Buffering .........................................................................................................................................15
3.1.4 Address Decoding ..........................................................................................................................15
3.1.5 ROM ...............................................................................................................................................19
3.1.6 RAM................................................................................................................................................19
3.1.7 Keyboard ........................................................................................................................................19
3.1.8 Video...............................................................................................................................................19
3.1.9 Real Time Clock .............................................................................................................................21
3.1.10 Cassette Circuitry ...........................................................................................................................21
3.1.11 Printer Circuitry...............................................................................................................................21
3.1.12 I/O Connectors................................................................................................................................21
3.1.13 Sound Option..................................................................................................................................21
3.2 Model 4 I/O Bus..............................................................................................................................26
3.3 Model 4 Port Bits ............................................................................................................................28
Schematic 8000173, 64K Version CPU PCB 8858090..................................................................31
CPU Component Layout.................................................................................................................43
Parts List, CPU PCB #6700104AA3...............................................................................................45
Parts List, CPU PCB 8858090........................................................................................................49
SECTION IV Floppy Disk Interface ...................................................................................................................53
4.1 Model 4 FDC PCB #8858060.........................................................................................................55
4.1.1 Control and Data Buffering.............................................................................................................55

ii
4.1.2 Nonmaskable Interrupt Logic......................................................................................................... 55
4.1.3 Drive Select Latch and Motor On Logic......................................................................................... 55
4.1.4 Wait State Generation and WAITIMOUT Logic............................................................................. 56
4.1.5 Clock Generation Logic ................................................................................................................. 56
4.1.6 Disk Bus Selector Logic................................................................................................................. 56
4.1.7 Read/Write Data Pulse Shaping Logic .......................................................................................... 56
4.1.8 Disk Bus Output Drivers ................................................................................................................ 56
4.1.9 Write Precompensation and Clock Recovery Logic ...................................................................... 56
4.1.10 Floppy Disk Controller Chip........................................................................................................... 57
4.1.11 Adjustments and Jumper Options.................................................................................................. 57
FDC Board to CPU Board Signal Description ............................................................................... 58
SCHEMATIC 8000095, PCB CONTROLLER #8858060 .............................................................. 59
Parts List, FDC PCB #8858060..................................................................................................... 61
4.2 Model 4 FDC PCB #8858160 ........................................................................................................ 63
4.2.1 Control and Data Buffering ............................................................................................................ 63
4.2.2 Nonmaskable Interrupt Logic......................................................................................................... 63
4.2.3 Drive Select Latch and Motor ON Logic ........................................................................................ 63
4.2.4 Wait State Generation and WAITIMOUT Logic............................................................................. 63
4.2.5 Clock Generation Logic ................................................................................................................. 64
4.2.6 Disk Bus Selector Logic................................................................................................................. 64
4.2.7 Disk Bus Output Drivers ................................................................................................................ 64
4.2.8 Write Precompensation and Write Data Pulse Shaping Logic ...................................................... 64
4.2.9 Clock and Read Data Recovery Logic........................................................................................... 66
4.2.10 Floppy Disk Controller Chip........................................................................................................... 66
Schematic 8000268, FDC PCB #8858160.................................................................................... 67
Component Layout, FDC PCB #8858160...................................................................................... 69
Parts List, FDC PCB #8858160..................................................................................................... 71
SECTION V Mini-Disk Drive (not available).................................................................................................... 73
SECTION VI Power Supplies (not available)................................................................................................... 75
SECTION VII Video Monitor (not available)...................................................................................................... 77
SECTION VIII Exploded View / Parts List.......................................................................................................... 79
Exploded View, Model 4 Computer, Catalog #26-1067/8/9........................................................... 81
Parts List, Model 4 Computer, #26-1067/8/9................................................................................. 83
SECTION IX RS232C Circuit Board.................................................................................................................. 85
9.1 RS-232C Technical Description..................................................................................................... 87
9.2 Pinout Listing ................................................................................................................................. 88
9.3 Port and Bit Assignments .............................................................................................................. 89
RS-232 Board to CPU Board Signal Description........................................................................... 91
Schematic #8000076, RS232C Printed Circuit Board................................................................... 93
Parts List, RS232C PC Board........................................................................................................ 95

iii
LIST OF FIGURES
Fig. No. Description Page No.
1-1 TRS-80 Model 4 Interconnection Diagram 4
3-1 Model 4 Block Diagram 16
3-2 RAM Memory 20
3-3 Timing of U3 & CPU 23
3-4 Timing of U4 24
3-5 CPU Video Access Timing 25
3-6 I/O Bus Timing Diagram 27
4-1 Write Precompensation Timing 65

1
PART I
HARDWARE
SECTION I
INTRODUCTION

3
INTRODUCTION
1.1 SYSTEM OVERVIEW
The Radio Shack TRS-80 Model 4 Microcomputer is an enhanced version of Radio Shack’s popular TRS-80 Model
Ill Microcomputer. The TRS-80 Model 4 is software compatible with the Model Ill so that owners of either system
can take advantage of the large number of programs available.
Features of the TRS-80 Model 4 which are common to the TRS-80 Model III include:
•Availability of Level I or 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 V)
•UL recognized construction
•12-key numeric keypad for rapid entry of numbers
•Rugged cabinet housing keyboard, electronics, video display, and power supply
•Direct drive video monitor for improved resolution
•Internal power supply
•Parallel printer port for use with Radio Shack printers
Other features available when Level II BASIC is used are: real time clock, upper and lower case characters, RAM
internally expandable to 128K bytes, I/O port for peripheral expansion, and cassette interface available with 500
and 1500 baud rates.
Optional peripherals for the TRS-80 Model 4 include disk drives (two built-in, two external) with double density for
increased storage capacity, and a built-in RS-232 serial interface for communications and peripheral interface.
1.2 BLOCK DIAGRAM
The Block Diagram (Figure 1-1) shows the various internal components and connections of the Model 4
Microcomputer.

4
FIGURE 1-1. TRS-80 MODEL 4 INTERCONNECTION DIAGRAM

5
1.3 JUMPER OPTIONS
1.3.1 16K to 64K
1. Remove U77-U84
2. Move E5-E6 to E5-E4
Move E1-E2 to E2-E3
Move E12-E13 to E12-E11
3. Add E7-E8 and eight Tandy #8040665 ICs in
locations U77-U84.
4. Remove capacitors 068, 072, 076, 080, 084, 088,
092 and 096.
1.3.2 64K to 128K
1. Remove shunt at U72 and add IC Tandy
#8075468 in its place.
2. Add eight Tandy #8040665 ICs in locations U85-
U92.
1.3.3 Graphics Board
1. Remove E14-E15 (and screws near U65 and
U71).
2. Plug in Graphics Board
3. Replace mounting screws at U65 and U71.
1.3.4 E9-E10 are not used at this time

7
SECTION II
DISASSEMBLY / ASSEMBLY