June 15, 2005 6815854H01-A
3-8 Theory of Operation: Main Board
All GCAI Outputs are configured to be at nominal 3.3V High with 15KΩ pull-ups on GPIO lines and
2.2KΩ pullup on ONE_WIRE line while UART is pullup with 100KΩ. USB is configured to comply
with international USB standard.
The one-wire detection will provide the radio with information on what accessory has been plugged
in and will configure the radio correctly to allow smooth operation of smart accessories such as the
keypad microphone and the RS232 programming cable.
Please refer to section 3.14.11, "Flash Programming," on page 3-75 for details on Flash
programming using GCAI’s RS232 UART.
3.7 Main Board
This section provides a detailed circuit description of the XTL 1500 radio main board for VHF/UHF
Range 1/UHF Range 2/700–800 MHz models. The main board contains the following major
sections:
•Radio Power (page 3-15)
• Receiver Front-End (page 3-16)
• Receiver Back-End (page 3-23)
• Transmitter (page 3-29)
• Frequency Generation Unit (page 3-45)
• Controller (page 3-62)
When reading the theory of operation, refer to your appropriate schematic and component location
diagrams located in “Chapter 7. Schematics, Component Location Diagrams, and Parts Lists”. This
detailed Theory of Operation will help isolate the problem. However, first use the ASTRO Digital
XTL 1500 VHF/UHF Range 1/UHF Range 2/700–800 MHz Mobile Radio Basic Service Manual
(6815853H01) to troubleshoot the problem to a particular board.
3.8 Main Board Major Sections
This section contains the main board layouts for each radio frequency band.
3.8.1 VHF (136–174 MHz) Band
The illustrations (Figure 3-6 on page 3-9 to Figure 3-7 on page 3-10) and their accompanying tables
(Table 3-3 on page 3-9 to Table 3-4 on page 3-10) identify the location of the major sections of the
main board.
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