Atari 1200XL

Atari 1200XL

Atari 1200XL home computer
Specifications
Manufacturer	Atari, Inc.
Type	Home Computer
Released	December 1982 (announced); March 1983 (shipped)
Discontinued	June 1983
Intro price	US$899 (launch), reduced to US$299
CPU	MOS Technology 6502C (SALLY) @ 1.79 MHz (NTSC only)
Memory	64 KB RAM, 16 KB ROM (two 8 KB × 8 ROMs)
Storage	External cassette tape (Atari 1010), optional 5.25" floppy disk drive (Atari 1050)
Display	320×192 pixels (max), 40×24 text, 16 colours (ANTIC/GTIA)
Sound	POKEY: 4 voices, 8-bit mono
Dimensions	47 cm × 25.4 cm × 7.6 cm (18.5" × 10" × 3")
Weight	2.7 kg (6 lbs)
OS / Firmware	Atari OS Rev. 10 / Rev. 11 (no built-in BASIC)
Predecessor	Atari 800
Successor	Atari 800XL
Codename	Sweet-16 (derived)
Model no.	CX5-1200

The Atari 1200XL is a home computer manufactured by Atari, Inc., announced on December 13, 1982, and shipped in March 1983. It was the first model in Atari's XL series, intended to replace the Atari 800. The 1200XL introduced a redesigned low-profile case styled by industrial designer Regan Cheng, four programmable function keys (F1–F4), a dedicated HELP key, two status indicator LEDs, and a built-in self-test diagnostic.

Despite its improved aesthetics, the 1200XL was commercially unsuccessful. Software incompatibilities caused by the revised Rev. 10 operating system ROM, the reduction from four to two joystick ports, the lack of a Parallel Bus Interface (PBI), and the absence of built-in BASIC all drew criticism. The +12V pin on the SIO port was disconnected, preventing some older peripherals from operating. The chroma signal was not connected to the monitor jack, limiting composite monitor output quality.

The 1200XL was sold only in North America (NTSC); no PAL version was produced. After poor sales, the price dropped from US$899 to US$299 over the course of 1983. Atari discontinued the 1200XL in June 1983 and replaced it with the Atari 600XL and Atari 800XL.

Architecture and Processor

The 1200XL uses the MOS Technology 6502C (SALLY variant) microprocessor, running at 1.79 MHz (NTSC). The SALLY variant includes a HALT input (pin 35) that allows the ANTIC display processor to halt the CPU and take control of the address and data buses for direct memory access during display generation.

The CPU is supported by four custom Atari chips:

• ANTIC (C012296) – Alphanumeric Television Interface Controller. A programmable display microprocessor with its own instruction set for graphics generation. Controls bus access, RAM refresh, and generates display lists.
• GTIA (C014805) – Graphics Television Interface Adaptor. Generates colour and luminance signals from display data received from ANTIC. Handles player/missile (sprite) graphics, collision detection, console switches, and keyboard click output.
• POKEY (C012294) – POT Keyboard Integrated Circuit. Provides four-channel audio generation, serial I/O (SIO) control, paddle controller scanning, random number generation, and keyboard matrix scanning.
• PIA (6520-equivalent) – Peripheral Interface Adaptor. General-purpose parallel I/O chip handling joystick port inputs, LED control outputs, motor control, and command signals.

Memory and Storage

The 1200XL contains:

• 64 KB RAM – organised as eight 64K×1 DRAM chips (4164-type), one per data line (U1–U9, with U7 as address decoding). 48 KB is directly available to programs, with the remaining 16 KB shared with ROM and I/O address space, accessible via bank switching controlled by the MMU.
• 16 KB ROM – two 8K×8 ROM chips (U12, U13) containing the operating system. Atari OS Rev. 10 shipped initially; some later units received Rev. 11 with partial compatibility fixes.

BASIC is not included in ROM. Users must load BASIC from a cartridge, cassette, or disk.

Storage options:

• Atari 1010 Program Recorder – cassette tape drive, connected via SIO.
• Atari 1050 Floppy Disk Drive – external 5.25" floppy drive, connected via SIO.
• Cartridge slot – single left-side slot (30-pin edge connector) for ROM cartridges up to 16 KB.

Memory Management

Address decoding uses a Hard Array Logic (HAL) chip, a 3-to-8 decoder, and supporting gates. The inputs are address lines A8–A15 and control signals including ROM enable (from PIA) and Refresh. The outputs provide chip selects for GTIA, POKEY, PIA, OS ROM, RAM, and cartridge, plus COMM A, COMM B, and COMM C signals.

Display and Graphics

Graphics are handled by the ANTIC and GTIA chips:

• Resolution: up to 320×192 pixels (bitmap mode)
• Text: 40 columns × 24 rows (default)
• Colours: up to 16 on screen, from a palette of 256 hues across 16 luminance levels
• Player/Missile Graphics: 4 players (sprites) and 4 missiles, with collision detection and priority control
• Graphics modes 12–15: GTIA-specific modes available via BASIC on the 1200XL
• Smooth scrolling and fine control over display lists, enabling split-screen and mixed-mode graphics

The 1200XL's video output circuit provides enhanced chroma for a more colourful RF image. However, the chroma line is not connected to the 5-pin DIN monitor jack, limiting monitor output to composite video and luminance only. The monitor connector pinout is:

Pin	Signal
1	Composite luminance
2	Ground
3	Audio output
4	Composite video
5	Ground

Sound Capabilities

Audio is generated by the POKEY chip (C012294):

• 4 independent voices (channels), each with 8-bit resolution
• Programmable frequency, volume, and distortion per channel
• White noise generation for percussion and sound effects
• Audio output is routed to the RF modulator and to the monitor jack (pin 3)
• Keyboard click sound is output through the TV/monitor speaker (via GTIA S3 line), unlike the Atari 800 which used an internal speaker

Input/Output and Expansion

The 1200XL provides the following interfaces:

Serial I/O (SIO) Port

A 13-pin connector for daisy-chaining peripherals (disk drives, printers, modems, cassette drives). Maximum baud rate: 19,200. The +12V pin (pin 12) is not connected on the 1200XL, which prevents RS-232 adapter devices that relied on this voltage from functioning without an external power source.

Controller Jacks

Two 9-pin D-type connectors (reduced from four on the Atari 800). Compatible with Atari joysticks and paddle controllers. Each port provides four digital directional inputs, a trigger input, and two analogue (pot) inputs.

Cartridge Slot

A single 30-pin edge connector on the left side of the case (moved from the top on the 800). Supports 8 KB and 16 KB ROM cartridges.

Monitor Jack

5-pin DIN connector providing composite video, luminance, audio, and ground. Chroma is not connected (see Display section).

RF Output

Switchable between channel 2 and channel 3 via a rear-panel slide switch (SW3). RF cable is detachable.

Absent Interfaces

The 1200XL lacks:

• Parallel Bus Interface (PBI) – present on the later 600XL and 800XL
• Composite chroma output on the monitor jack
• +12V on the SIO bus

Design and Features

The 1200XL's case was designed by Regan Cheng and uses a low-profile aluminium and smoked plastic construction. It contains a single motherboard (replacing the multi-board design of the 400/800), with upper and lower RF shields secured by push rivets.

Keyboard

A full-stroke 62-key keyboard with mylar membrane contact layer. The keyboard connects to the motherboard via a 15-conductor ribbon cable with a Molex connector. Keys include standard alphanumeric characters, special characters, and controls.

Function Keys

Eleven function keys are arranged above the keyboard on brushed aluminium nameplates:

• RESET – interrupts and restarts the operating system or cartridge
• START – starts a game or program
• SELECT – selects different games or options
• OPTION – chooses among program variations
• F1–F4 – four user-programmable function keys (default: cursor movement in BASIC)
• HELP – displays the built-in self-test options
• INVERSE VIDEO (labelled "S") – toggles inverse video
• BREAK – program interrupt

Status LEDs

Three LEDs behind a translucent plexiglass strip above the function keys:

• POWER – lit when the unit is powered on
• L1 – lights when the keyboard is disabled (F1 function)
• L2 – lights when the international character set is selected (F3 function)

The LED board connects to the motherboard via a 5-conductor ribbon cable.

Built-in Self Test

Accessed by pressing HELP during the power-on logo display. Tests include:

• Memory Test – tests ROM (two 8K bars) and RAM (48 blocks of 1 KB each). Green = pass, Red = fail.
• Audio Visual Test – plays a six-note sequence across all four POKEY channels, with musical staff display.
• Keyboard Test – displays a keyboard graphic; pressed keys highlight in inverse video.
• All Tests – runs all three tests in sequence.

Easter Egg

During the keyboard self-test in "All Tests" mode, the test spells out "Michael Colburn," the programmer's name. This was removed in the later 600XL and 800XL.

Power Supply

The 1200XL uses a linear power supply with the regulator electronics on the motherboard. An external step-down transformer (the "ingot" power adaptor, part CA017964) converts 115 VAC mains to 9 VAC, which enters through a rear-panel power jack. On the motherboard, the 9 VAC is rectified by a bridge rectifier (CR8, MDA 990-2) and regulated by two 78M05 voltage regulators (A1, A2) to produce a single +5 VDC rail at 1.7 A maximum.

The DC output specification is +5V ±5% (4.75V–5.25V). Input power consumption is 31 VA.

Unlike the Atari 800 (which provided +5V and +12V), the 1200XL generates only +5V.

General Maintenance

For detailed maintenance practices including cleaning, connector care, and common failure points, refer to the dedicated Atari 1200XL General Maintenance page.

Troubleshooting

A comprehensive troubleshooting guide addressing power, video, RAM, and keyboard issues with component-level diagnostics is available on the Atari 1200XL Troubleshooting Guide page.

Capacitor Replacement Guide

Detailed capacitor replacement guidelines with a full component list and post-recap voltage verification procedures can be found on the Atari 1200XL Capacitor Replacement Guide page.

Technical Details

System Architecture at a Glance

Sub-system	Specification (Atari 1200XL, 1983)
CPU	MOS Technology 6502C (SALLY) @ 1.79 MHz
Bus width	8-bit data • 16-bit address (64 KB addressable)
ROM	16 KB (two 8K × 8 ROMs, OS Rev. 10/11)
RAM	64 KB (eight 4164 64K×1 DRAMs)
Video	ANTIC C012296 + GTIA C014805, up to 320×192, 256-colour palette
Sound	POKEY C012294, 4 voices, 8-bit mono
I/O	PIA (6520), HAL (MMU)
Storage	External SIO (cassette, floppy), cartridge slot
Ports	SIO × 1, Joystick × 2, Cartridge × 1, Monitor (DIN-5), RF out
Power	+5 VDC only (1.7 A max), 9 VAC input via external transformer

Key IC Locations

Atari 1200XL Motherboard IC Map

Location	Component	Function
U1–U9 (excl. U7)	4164 DRAM	64 KB RAM (one chip per data line, D0–D7)
U12, U13	8K × 8 ROM	Operating System
U14	HAL	Memory Management / Address Decoding
U19	GTIA (C014805)	Graphics / Colour Generation
U20	ANTIC (C012296)	Display Processor
U22	6502C (SALLY)	CPU
U23	PIA (6520)	Peripheral Interface
U24	POKEY (C012294)	Audio / Keyboard / Serial I/O
A1, A2	78M05	+5V Voltage Regulators
A3	RF Modulator	TV Signal Output
CR8	MDA 990-2	Bridge Rectifier

Related Pages

• Atari 1200XL General Maintenance
• Atari 1200XL Troubleshooting Guide
• Atari 1200XL Capacitor Replacement Guide
• Atari 800XL
• Atari 800
• Atari 8-bit Family Keyboard Repair