Theory of Operation—2465B/2467B Service
the various devices and circuit functions. The controlling
signals are generated as a result of the Microprocessor
placing specific addresses on the Address Bus. Figure 3-2
illustrates the enables and strobes generated by the
Address Decode circuitry.
bit A15 HI) select one of two read-only memories (ROM),
U2160,
or U2260. When the VMA (Valid Memory Address)
and E (Enable) outputs from the Microprocessor go HI, the
selected ROM is enabled, and the data from the selected
address location is read from the ROM.
Address decoding is performed by a programable array
logic device, a three-line-to-eight-line decoder, and a
four-
line-to-sixteen-line decoder attached to the Address Bus.
The five most significant address bits are decoded by
U2250.
This device initially separates the total
addressable-memory space (64K-bytes) into thirty-two,
2K-byte blocks. Addresses in the top 32K-bytes (address
The programmable array logic device also generates
the OE and WE signals to the random-access memory
(RAM). This RAM can be accessed with addresses 8000
to 9FFF if either PBO, PB1, or PB2 signals are HI. In this
mode ROMS, U2160 and U2260 are not accessible in this
address range.
HEX
ADDRESS
DECODED BY
U2250
HEX
ADDRESS
0000
07FF
0800
0FFF
1000
7FFF
8000
FFFF
RAM-U2460
ADDRESS
DECODING
(U2550)
RESERVED
FOR OPTIONS
R0MS-U2160
AND U2260
RAM-U2460
0800
083F
0840
0B7F
0880
08BF
08C0
08FF
0900
093F
0940
097F
0980
09C0
09FF
0A00
0BFF
0C00
y ODFF
\ 0E00
0FFF
DECODED BY
U2550
HEX
ADDRESS
DMUX2 OFF
DAC MSB CLK (087F)
DAC LSB CLK (0880)
PORT 1 CLK (08C0)
ROS 1 CLK (0900)
ROS 2 CLK (0940)
PORT 2 CLK (0980)
FURTHER ADDRESS
DECODING (U2660)
OVERLAY OF
0800-09FF
OVERLAY OF
0800-09FF
OVERLAY OF
0800-09FF
/ -
/ 09C0
/
09C1
/
09C2
09C3
09C4
09C5
09C6
09C7
09C8
09C9
09CA
09CB
09CC
09CD
09CE
09CF
\ 09D0
09DF
V
09E0
\ 09EF
\ 09F0
V
09FF
DECODED BY
U2660
DMUX2 ON
DMUXO OFF
DMUXO ON
PORT 3 IN
DMUX1 OFF
DMUX1 ON
LED CLK
DISP SEQ CLK
ATN CLK
CH 2 PA CLK
CH 1 PA CLK
B SWP CLK
A SWP CLK
B TRIG CLK
A TRIG CLK
TRIG STAT STRB
OVERLAY OF 09C0-09CF
OVERLAY OF 09C0-09CF
OVERLAY OF 09C0-09CF
6019-09
Figure 3-2. Address decoding.
3-7
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