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Circuit Theory
3–14
1740A/1750A/1760 – Series Service Manual
Diagram
6
Microprocessor
The Microprocessor controls the functions of the 1740A/1750A/1760–Series. It
has a 32-bit internal architecture and operates with a 16-bit data bus and a 24-bit
address bus.
U18 is the Microprocessor. It is crystal controlled, with Y1 as the active element
of the clock oscillator. DS1 is an indicator that turns on and holds when the 5V
supply stabilizes during turn on. U7 senses the 5V supply and pulls the RESET
line low if the 5V supply goes low.
SP1 is a permanent magnet-type speaker for audible feedback that is driven by
Q3. CR2 is an inductive compensation for the speaker voice coil.
U13 is a Read Only Memory (ROM) with 18 addresses; it outputs the 8 Most
Significant Bits (MSB) to the data bus.
U5, U8, and U12 are the address buffers for the 24-bit address bus. The bus is
enabled by ADDR EN2 from the decoder. The DIR control line is held high
allowing the processor to write to the buffer whenever the ADDR EN2 is pulled
down. The buffered address bus selects addresses in the NOVRAM, RAM, and
FLASH EPROM.
The Data Buffers, U15 and U19, are bidirectional. When the DIR control line is
low data from the NOVRAM, RAM, and FLASH EPROM is read into the
Microprocessor data bus on the DATA EN2. When DIR is pulled high, and
DATA EN2 is pulled down (by the Decoder), the Microprocessor writes to the
Data Buffers on the data bus.
The NOVRAM (U14) stores all of the constants used by the Microprocessor.
The Microprocessor writes the 8 MSBs into the NOVRAM when both CE and
WE are pulled low. RW from the Microprocessor pulls down WE. CE is pulled
low by NOVRAM which is decoded by the address decoder. Content of the
NOVRAM is read back out to the Microprocessor, through the Data Buffer
(U19) when RW goes high and the CE and OE are pulled low by NOVRAM.
System level code is loaded into RAM (for reading by the Microprocessor) from
the Flash EPROM, where it is stored. Unless VPP is high (for programming
purposes) the Flash EPROM, U10 & U16, functions as a 256k X 8 Read Only
Memory (ROM). (Write instructions are ignored.) U10 stores the lower 8 bits
and U16 the upper 8 bits. It is read out when FLASH and RD LO and RD HI are
pulled low.
Microprocessor
Data and Address Buffers
NOVRAM, RAM, & Flash
EPROM
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Содержание 1740A Series
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