Radio Shack TRS-80 Model III - Port EBH; Port EAH Output Bits with Uart Control Register Disabled; Port EAH Input Bits

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Table of Contents3
List of Illustrations5
List of Tables7
Introduction9
System Overview11
TRS-80 Model III Block Diagram12
Disassembly/Reassembly13
Disassembly15
Case15
CPU Board15
FDC Board15
Video Monitor (CRT) and Video Board16
Reassembly17
RS-232 Board17
Main Power Supply17
Disk Drive17
CPU Circuit Board19
Technical Description21
Video SYNC Circuits21
Processor21
Ram21
Address Decoding22
Memory Map of Model III22
IO Port Description23
Video24
Video Timing Diagram24
Keyboard25
Cassette Interface25
Adjustments and Jumper Positions26
Line Printer and Real Time Clock26
IO Bus26
Model III IO Bus27
Input or Output Cycles28
Input or Output Cycles with Wait States28
IO Bus Timing Diagrams28
Model III Port Bits29
CPU Printed Circuit Board - Component Side31
CPU Printed Circuit Board - Circuit Side32
CPU Module Parts List33
Model III CPU Schematic Diagram - Video and Horizontal Sync Circuits39
Model III CPU Schematic Diagram - Processor Section40
Model III CPU Schematic Diagram - RAM Section41
Model III CPU Schematic Diagram - Address Decoding Section42
Model III CPU Schematic Diagram - Video Section43
Model III CPU Schematic Diagram - Keyboard Section44
Model III CPU Schematic Diagram - Cassette Interface Section45
Model III CPU Schematic Diagram - Line Printer and Real Time Clock Section46
Model III CPU Schematic Diagram - I/O Bus Section47
Model III CPU Schematic Diagram - RS-232-C Interface and FDC Interface Connectors48
Floppy Disk Interface49
Control and Data Buffering51
Nonmaskable Interrupt Logic51
Drive Select Latch and Motor on Logic51
Wait State Generation and Waitimout Logic52
Clock Generation Logic52
Disk Bus Selector Logic52
Write Recompensation and Clock Recovery Logic52
Adjustments and Jumper Options53
Floppy Disk Controller Chip53
Jumper Connections53
FDC Board to CPU Board Signal Description54
Floppy Disk Interface PC Board - Component Side55
Floppy Disk Interface PC Board - Circuit Side56
Floppy Disk Interface PC Board Parts List57
Model III Floppy Disk Interface Schematic59
RS-232C Circuit Board61
RS-232C Technical Description63
BRG Programming Table63
Port and Bit Assignments65
Port E8H65
Port E9H65
Port EAH65
Port EBH66
Port EAH Output Bits with Uart Control Register Disabled66
Port EAH Input Bits66
RS-232 Board to CPU Board Signal Description67
RS-232C PC Board - Component Side68
RS-232C PC Board - Circuit Side69
RS-232C PC Board Parts List70
Model III RS-232C Schematic Diagram71
Power Supply73
Functional Specifications75
Troubleshooting75
Equipment for Test Set-Up75
Visual Inspection75
No Output76
Start-Up76
Load Board Values76
O2 Collector Waveform76
Performance Test77
Voltage and Ripple Specification77
Power Supply Component Values78
Power Supply Schematic Diagram79
Video Monitor81
Video Monitor Printed Circuit Board84
Service Adjustments85
Deflection Yoke Assembly85
Video Monitor Assembly Schematic Diagram86
Illustrated Parts Catalog87
Outer Case Assembly89
Case Top Assembly90
Base Plate Assembly91
Metal Chassis Assembly92
Disk Drive Assembly93
Illustrated Parts Catalog Parts List94
Miscellaneous96
Mini Disk Drive99
General Description101
Physical Description101
Functional Description101
Interface Connections101
Physical Checkout102
PC Board Component Locations102
Logic Board (Component Side)102
Non-Linear Servo Board (Component Side)103
Linear Servo Board (Component Side)103
Mounting the Disk Drive104
Flat Ribbon Cable Assembly104
Resistor Terminator104
Cable Assembly - Connector Pin Removal Chart104
Theory of Operation105
Functional Block Diagram Description105
Index105
Spindle Drive105
Data Electronics106
Functional Block Diagram106
FM Recording106
Data Recording107
Data Reproduction107
Write Timing Diagram108
Read Timing Diagram108
Operation109
Select Lines (DS1-DS4)109
Direction and Step (DIR) (STEP)109
Compensated Write Data (CWD)109
Interface Connector PIN Assignments, J1 (External), J4 (Internal)110
Power Connector PIN Assignment110
Side Select (SDSEL)111
Index Pulse (IP)111
Write Protect (WPRT)111
Read Data (RD)111
Maintenance113
Physical Description of the PC Boards113
Circuit Board Test Points113
Option Select113
Logic Board Test Points and Connector Locations114
Servo Board Connector Locations115
Preventive Maintenance116
Cleaning the Head116
Alignment and Adjustment116
Drive Motor Maintenance116
Head Radial Alignment/Ce Alignment117
"Cat Eyes" Pattern117
Radial Adjustment117
Track Alignment118
Index Sector Adjustment118
Track Stop Adjustment118
Index Adjustment118
Head Amplitude/Compliance Check119
Final Check119
Upper Arm and Carriage119
Disk Controller Board Parts List120
Servo (Motor Control) Board Parts List124
Illustrated Parts Catalog - Exploded View126
Mini-Disk Drive Schematic128
Non-Linear Servo Board130
Linear Servo Board130
Mini-Disk Drive Schematic of New Board131

Radio Shack TRS-80 Model III - Port EBH; Port EAH Output Bits with Uart Control Register Disabled; Port EAH Input Bits

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