Tandy 6000 HD, Service Manual

Page: 51 / 416

Service Manual
TANDY 6000/6000-HD
TAN D Y COMPUTER PRODUCTS --------------------------------------------------------------------
6/ Troubleshooting Procedures
6.1 TANDY 6000/6000-HD I/O Processor System
This section of the manual will guide service personnel
through the system checkout procedure. The troubleshooting
steps are organized in a flowchart manner. Following these
steps will guide you to the faulty component or components.
This procedure is intended to isolate only to the field
replaceable units such as the power supply, i/o processor
board, socketed ICs, etc. The following sections describe
the theory of operation and troubleshootng for the I/O
Processor System. The 16-bit CPU and Memory Boards are
covered in section 6.2.
6.1.1 Setup
First, disconnect the unit from the AC power source. Next,
separate the Top Cover Assembly from the Base Assembly. See
Section 3 for complete instructions on disassembly. Verify
the monitor remains electrically connected to the
Video/Keyboard PCB and the Power Supply. Verify there are
no disk(s) inserted in the drive(s). Verify that the power
switch on the underside of the left lip of the Base assembly
is OFF, then connect the unit to the AC source and continue.
6.1.2 Power-on Diagnostics
When the power switch on the TANDY 6000/6000-HD is turned
ON, the microprocessor starts executing the program in the
BOOTSTRAP ROM. This program is divided into several
sections that are executed in order if the previous section
passed. These diagnostics are of a stair-step nature in
that individual functions are tested before being put
together to perform the complete task. This will be
detailed in the following paragraphs.
1. Power ON. System RESET furnished by the hardware is
applied. This causes the Z80 to begin executing program
code from the BOOTSTRAP ROM at address Q000H.
2. The initialization of the system begins by disabling
interrupts and deselecting all RAM memory but Page 0.
3. Initialize CRTC (CRT Controller). Specify horizontal
sync, vertical sync, enable video, select 80 character
mode, white-out the entire screen.
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