Amiga 500

Releases

In October 1989, the Amiga 500 dropped its price from £499 GBP to £399 and was bundled with the Batman Pack in the United Kingdom. This price drop helped Commodore to sell more than 1 million Amiga 500s in 1989.

In late 1991, an enhanced model known as the Amiga 500+ replaced the original 500 in some markets, it was bundled with the Cartoon Classics pack in the United Kingdom at 399 GBP.

The Amiga 500 series was discontinued in mid-1992 and replaced by the similarly specified and priced Amiga 600, although this new machine had originally been intended as a much cheaper model, which would have been the A300. In late 1992, Commodore released the “next-generation” Amiga 1200, a machine closer in concept to the original Amiga 500, but featuring significant technical improvements. Despite this, neither the A1200 nor the A600 replicated the commercial success of its predecessor, as by this time, the popular market was definitively shifting from the home computer platforms of the past to commodity Wintel PCs and the new "low-cost" Macintosh Classic, LC and IIsi models.

Description

Outwardly resembling the Commodore 128, the Amiga 500 houses the keyboard and CPU in one shell, unlike the Amiga 1000. It 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 use nearly the same Original Amiga chipset as the Amiga 1000. So graphics can be displayed in multiple resolutions and color depths, even on the same screen. Resolutions vary from 320×200 (up to 32 colors) to 640×200 (up to 16 colors) for NTSC (704×484 overscan) and 320×256 to 640×256 for PAL (704×576 overscan.) The system uses planar graphics, with up to five bitplanes (four in high resolution) 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 (resulting in 32 arbitrary colors plus 32 more colors set at half the value of the first 32), and Hold And Modify (HAM) which allows all 4096 colors to be used on screen simultaneously. 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 common trick to output sound with 14-bit precision 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 KiB of chip RAM (150 ns access time), one built-in double-density standard floppy disk drive that is completely programmable and can read 720 KiB IBM PC disks, 880 KiB standard Amiga disks, and up to 984 KiB using custom-formatting drivers.

Despite the lack of Amiga 2000-compatible internal expansion slots, there are many ports and expansion options. There are two DE9M Atari joystick ports for joysticks or mice, stereo audio (RCA connectors 1 V p-p). There is a floppy drive port for daisy-chaining up to three 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 or 60 Hz NTSC through a proprietary DB23M connector and in NTSC mode the line frequency is 15,750 Hz HSync for standard video modes, which is compatible with NTSC television and CVBS/RGB video, but out of range for most VGA-compatible monitors, while a multisync monitor is required for some of the 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 RF input, while monochrome composite video is available via an RCA connector (also coaxial). There is also a Zorro II bus expansion on the left side (behind a plastic cover). Peripherals such as a hard disk 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 "trap-door" slot on the underside for an upgrade of 512 KiB 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, or by removing the CPU and plugging a CPU expansion card in the CPU socket. (Though the latter required opening the computer and voiding any remaining warranty). 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 may be reversed by using the public domain chemical mix "Retr0bright", though without a clearcoat to block oxygen, the brown colouring will return.

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, or Agnus),    Light Green means CIA problem,    Light Grey means (if it stops at Grey) that the CIA might be defective,     Black/stripes means ROM or CIA problem,    Black (no video) means there is no video output. The keyboard LED uses blink codes: one blink means the keyboard ROM has a checksum error, two blinks means RAM failure, three blinks means watchdog timer failure. Using Caps Lock key and getting a response means CIA and the CPU works.

Technical specifications

Graphics

Max 6 bpp. The Amiga could show multiple resolution modes at the same time, splitting the screen vertically. An additional mode called Hold-And-Modify (HAM) makes it possible to utilize 12 bpp over a 3 pixels wide span. This works by letting each pixel position use the previous RGB value and modify one of the red, green or blue values to a new 4-bit value. This will cause some negligible colour artifacts however.

Connectors

Expansions

Trap-door expansion 501

A popular expansion for the Amiga 500 was the Amiga 501 circuit board that can be installed underneath the computer behind a plastic cover. It contains 512 KiB RAM configured by default as "Slow RAM" or "trap-door RAM" and a battery-backed real-time clock (RTC). However, the RAM is pseudo-fast RAM, only accessible by the processor, but still as slow as chip RAM. The motherboard can be modified to relocate the trap-door RAM to the chip memory pool, provided a compatible Agnus chip is fitted on the motherboard.

Notable uses