Amiga 500 - Technical Description

Technical Description

The Amiga 500 utilizes a Motorola 68000 microprocessor running at 7.15909 MHz (NTSC) or 7.09379 MHz (PAL). The CPU is 32-bit internally, but uses a 16-bit data bus and 24-bit address bus, providing a maximum of 16 MB of address space.

The earliest Amiga 500 models used nearly the same OCS chipset as the Amiga 1000, so graphics can be displayed in multiple resolutions and color depths, even on the same screen. Resolutions vary from 320x200 (up to 32 colors) to 640x200 (up to 16 colors) for NTSC (704x484 overscan) and 320x256 to 640x256 for PAL (704x576 overscan.) The system uses planar graphics, with up to five bitplanes (four in hires) allowing 2, 4, 8, 16, and 32-color screens, from a palette of 4096 colors. Two special graphics modes are also available: Extra HalfBrite, which uses a 6th bitplane as a mask to cut the brightness of any pixel in half, and Hold And Modify (HAM) which allows all 4096 colors to be used on screen simultaneously (though only one color component – red, green, or blue – can be changed in the next pixel in this mode.) Later revisions of the chipset are PAL/NTSC switchable in software.

The sound chip produces four hardware-mixed channels, two to the left and two to the right, of 8-bit PCM at a sampling frequency of up to 28 kHz. Each hardware channel has its own independent volume level and sampling rate, and can be designated to another channel where it can modulate both volume and frequency using its own output. With DMA disabled it's possible to output with a sampling frequency up to 56 kHz. There's a widely known trick to output sound with 14-bit precision too that can be combined to output 14-bit 56 kHz sound.

The stock system comes with AmigaOS version 1.2 or 1.3 and 512 kB of Chip RAM (150 ns access time), one built-in double-density floppy disk drive that is completely programmable and can read 720 kB IBM PC disks, 880 kB standard Amiga disks, and up to 984 kB using custom formatting drivers. The Amiga 500 houses the keyboard and CPU in one shell, similar to the design of the Commodore 64 and unlike the Amiga 1000.

Despite the lack of Amiga 2000-compatible internal expansion slots, there are many ports and expansion options. There are two Atari 2600 DE9M sockets for joysticks or mice, stereo audio (RCA connectors 1 V p-p). There is a floppy drive port for daisy-chaining up to 3 extra floppy disk drives via an DB23F connector. The then-standard RS-232 serial port (DB25M) and Centronics parallel port (DB25F) are also included. The power supply is (+5V, +/-12V). The system displays video in analog RGB 50 Hz PAL and 60 Hz NTSC using a proprietary DB23M video connector and outputs video at 15 750 Hz (television and CVBS/RGB video compatible, out of range for most VGA-compatible monitors) HSync for standard video modes while a Multisync monitor is required for some higher resolutions. This connection can also be genlocked to an external video signal. The system was bundled with an RF adapter to provide output on televisions with a coaxial input, while monochrome composite video is available via an RCA connector. There is also a Zorro II bus expansion on the left side (behind a plastic cover). Peripherals such as a hard drive can be added via the expansion slot and are configured automatically by the Amiga's AutoConfig standard, so that multiple devices do not conflict with each other. Up to 8 MB of “fast" RAM can be added using the side expansion slot.

The Amiga 500 has a "trapdoor" slot on the underside for an upgrade of 512 kB of RAM. The extra RAM is classified as "fast" RAM, but is sometimes referred to as "slow" RAM since due to the design of the expansion bus it is actually on the chipset bus. Such upgrades usually include a battery-backed real-time clock. All versions of the A500 can have the additional RAM configured as Chip RAM by a simple hardware modification, which involves fitting a later model (8372A) Agnus chip. Likewise, all versions of the A500 can be upgraded to 2 MB Chip RAM by fitting the 8372B Agnus chip and adding additional memory.

The Amiga 500 also sports an unusual feature for a budget machine, socketed chips, which allow easy replacement of defective chips. The CPU can be directly upgraded to a 68010 or to a 68020, 68030, or 68040 via the side expansion slot. In fact, all the custom chips can be upgraded to the Amiga Enhanced Chip Set (ECS) versions.

The case is made from ABS plastics which may become brown with time. This can be reversed by using the public domain chemical mix "Retr0bright".

Whenever the computer is powered on a self diagnostic test is run that will show any failure with a specific colour where Green means no Chip RAM found or is damaged, Red means bad kickstart-ROM, Yellow means mostly a bad CPU (no trap routine) or a bad Zorro expansion card. Blue means custom chip problem (Denise Paula Agnus), Light Green means CIA problem, Light Grey means if stops at Grey that the CIA can be defective, Black/stripes means ROM or CIA problem, Black (no video) means really basic problem. The keyboard LED uses blink codes where 1-blink means the keyboard ROM has a checksum error, 2-blinks means RAM failure, 3-blinks means watchdog timer failure. Using caps lock key and getting a response means CIA and the CPU works.