Circuit Descriptions
1502C MTDR Service Manual
5–9
negative pulse at the gate output is supplied to U1023’s reset input, causing the
microprocessor to start at the beginning of its programmed routine each time power
is applied.
The 16-bit address space of Z80 processor U1023 is divided into five primary areas.
They are:
H
Program Memory (EPROM) space
H
RAM space
H
Non-volatile RAM space
H
Display RAM space
H
Enable and Select Signal space
The program memory is stored in 64-kilobyte (kb) EPROM U2020, which is
divided into two 32–kb bank-switched halves. Both halves occupy locations
OOOOH to 7FFFH in the processor’s address space. The most significant address
bit on the EPROM, which determines which bank is addressed, is set by flip-flop
U2030A. This bank-switching flip-flop can be toggled by the processor with two
select lines, decoded in the enable and select signal address space. The select signal
for the EPROM is generated by combined address line A15 with the MREQ signal
in U1045A. Whenever the processor addresses a location where A15 is not set, the
program memory will be selected to place data on the bus.
The first RAM is eight-kilobyte memory U1021, selected by a signal generated by
a 1-of-8 decoder, U1022. This decoder operates on the three most significant address
bits (A
15
, A
14
, A
13
) in combination with MREQ. Each of its decodes represents a
selection of a particular
1
/
8
th of addressable locations. The first four decode signals
are not used because they are located in the program memory space. The fifth decode
is the select signal for the first RAM, occupying locations 8OOOH to 9FFFH.
The second RAM is also an 8-kb memory, U1020, made non-volatile by lithium
battery BT1010 and non-volatile memory controller U1010. The select signal for
this RAM is generated similarly to that for the first RAM with the sixth
1
/
8
th decode
of U1022. This decode occupies AOOOH to BFFFH.
The display RAM is also an 8-kb memory, U1040, located in the display module.
It is selected by the seventh decode of U1022. It occupies locations COOOH to
DFFFH.
The remaining addressable space is used to generate enable, select, or trigger
signals, which read, write, and control other circuits of the instrument. The eighth
1
/
8
th decode signal of U1022 is used to enable four other 1-of-8 decoders: U2021,
U2022, U2024, and U2026. These four decoders are further selected by the four
Address Decoding and
Memory
Program Memory
(EPROM)
RAM
Non-Volatile RAM Space
Display RAM Space
Enable and Select Signal
Space
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