Memory (ROM and RAM)
The 1200XL operating system is resident to two 8K X 8 ROMs. The ROMs input CPU address
lines AO through A12 and the chip selects from the address decoding circuitry and generates
data on CPU data lines DO through D7. RAM is organized as 8-64K x 1 chips, one for each data
line. 48K RAM is directly available.
Memory Management Unit (MMU)
The Memory Management (Address Decoding) circuitry consists of a Hard Array Logic
(HAL), one 3 to 8 decoder and a few gates. The input to this circuitry are the address lines A8
thru A15 and control signals like ROM enable (From PIA) and Refresh. The outputs of this
circuitry are GTIA chip select, POKEY chip select, PIA chip select, O.S. ROM selects, RAM
selects, cartridge selects and COMM A, COMM B, COMM C.
The RF Modulator
The RF Modulator inputs the composite video from the video summation circuitry and the
monoaural audio signals and produces a modulated signal suitable for the television.
The modulated signal will have the following characteristics with a 75Ω termination:
Maximum Voltage: 2 mV
Minimum Voltage: 1 mV
Audio Sound Carrier Frequency: 4.5 MHz
Frequency Response:
Ch. 3 61.25 MHz ± 0.5
Ch. 2 55.25 MHz
Power Supply
The 1200XL has a linear power supply. The power supply electronics is resident on the
motherboard. The power supply accepts a 9VAC input through the power jack. An external
step-down transformer accepts 115 VAC from the power line and steps it down to 9VAC. The
9VAC input is rectified by a bridge rectifier and then regulated power outputs from the
regulators are bussed on separate power rails to power distinct sections of the motherboard. All
power rails have a common ground.
The DC. output of each regulator is 5V ±5% (1.7A Max). Input voltage is 9 VAC. Input power
is 31 VA.
SYSTEM INTERFACE
The 1200XL provides the following interfaces:
• The Serial Input/Output (SIO) interface
• The keyboard interface
• The controller jack interface
• The cartridge interface
• The monitor interface
• The LED board interface
1200XL Home Computer
Field Service Manual 1-11
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