The 800 Console Motherboard contains the POKEY chip (A101), the PIA
chip (A102), the Keyboard Key-In/Key-Out analog multiplexers (Z103
and Z104), the I/O Decoder (Z101), Bidirectional Data Buffers (Z105
and Z106), the
controller jacks (J101 through J104), the Keyboard connector jack
(J106), the left and right Program Cartridge jacks (J108 and J109),
the CPU connector jack (J101), the ROM Personality connector jack
(J107), and the
RAM Memory connector jacks (J102, J103, and J104). See Figures 4-7
and 4-8.
Refer to the descriptions provided in paragraph 4.3.2, Console
Motherboard (400), for items similar in both the 400 and 800 Console
Motherboards. The following paragraphs will describe those items
unique to the 800 Console Motherboard.
4.3.3.1 Bidirectional Data Buffer
The Bidirectional Data Buffers (Z105 and Z106) are 74LS243 quad
transceivers, commonly referred to as tristate buffers. Tristate
refers to the three states that can occur in the device; the first
state, pin 1
going high and pin 13 staying low, allows data to flow through the
data buffer in the direction of the CPU; the second state, pin 13
going high and pin 1 staying low, allows data to flow through the
data buffer in the direction of the POKEY chip, the PIA chip and the
Personality pcb; the third state, with pin 1 and pin 13 both staying
low, creates a high impedance condition in the data buffers and
allows other devices to transmit data on the Data Bus lines. This
high impedance state also isolates the CPU and the RAM pcbs from
noise generated from a device using the Data Bus lines.
4.3.3.2 I/O Decoder
The I/O Decoder integrated circuit (Z1010) is a one-of-eight
decoder/demultiplexer. Refer to the description provided in Section
4.3.2. The I/O Decoder is the same in the 400 and 800, but is labeled
Z101 in the 800 console and Z105 in the 400 Console.
4.3.3.3 Composite Video
The 400 and 800 Motherboards route the Composite video signals (COMP
CHROMA, COMP LUM, MOD, and COMP VIDEO) to the Power Supply to build
the RF video output.
4.3.4 ROM Personality Board
The ROM Personality printed circuit board plugs into connector jack
J107 and occupies memory Iocations D800 through FFFF. The Personality
board contains the operating system on three ROM chips, two 4K chips
and one 2K chip ( See Figures 4-9 and 4-10). The two 4K ROM chips
4-14 System Service Manual
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