The LINK 480Z was an 8-bit microcomputer produced by Research Machines Limited in Oxford, England, during the early 1980s.
The 480Z used a Z80 microprocessor with up to 256 KB of bank-switched RAM. The system could be used as a stand-alone unit with cassette-based storage and the BASIC programming language run from ROM, or it could boot CP/NOS (a network version of CP/M) over a local area network from a file server. When fitted with an optional external floppy disk drive the system could boot the CP/M operating system directly.
The 480Z was sold mainly to the UK educational market as cassette-based system or as a diskless node which could be connected, via the proprietary CHAIN Network, to a Research Machines 380Z acting as a file server.
Hardware
Main unit and processor
The LINK 480Z was packaged as an integrated keyboard and system unit. Early systems were supplied with a black sheet-metal case, however this was quickly replaced by a cream-coloured plastic housing. The optional 5¼-inch floppy disk drive unit was external.
The only microprocessor offered was a 4 MHz Z80A.
Memory
Although some early systems had only 32 KB (32 × 1024 bytes) of RAM, most 480Zs were fitted with at least 64 KB of memory. Bank switching allowed memory to be extended to 128 KB on the main board, and up to 256 KB by using the option board (which also included the high resolution graphics hardware). As the Z80 processor could directly address only 64 KB without software support the additional memory was typically used as a RAM disk, specifically the Silicon Disk System. The silicon disk could be automatically loaded with software and data when the 480Z booted to the network. This saved considerable time in a classroom setting, where software could be loaded in advance of the children arriving. The silicon disc retained its contents if the system underwent a soft reset.
The 480Z was also fitted with up to 32 KB of firmware that could also be bank-switched out of the normal address space when not in use, leaving a total of 58 KB of RAM directly available to the user, with 2 KB of RAM reserved for system use and 4 KB inaccessible because of the firmware ROM.
Video
All 480Zs, with the exception of some very early units, were fitted with software-switchable 40×24 or 80×24 character text-only monochrome video hardware. Composite video output was provided for an external monitor, and an internal RF modulator provided a separate output to drive a television set. The text-mode display had its own dedicated memory.
In addition to the text-mode video interface the system could be enhanced with an option board providing a high-resolution graphics capability and an additional TTL RGB interface for a colour monitor. The board was fitted with a dedicated bank of 16 KB of video memory and supported three graphics modes: Extra high resolution: 640×192 pixels, 1 bit per pixel, 1 page. High resolution: 320×192 pixels, 2 bits per pixel, 1 page. Medium resolution: 160×96 pixels, 4 bits per pixel, 2 pages. A programmable lookup table with an 8-bit output mapped the pixel value to one of 256 different intensities (composite video) or one of 8 colours (TTL RGB output).
Output from the graphics board was mixed with output from the text-only video interface, allowing text and graphics to be easily overlaid. The graphics output covered only the top 20 lines of the text display and therefore text output could be set to use only the bottom 4 lines if overlap was not desired.
Memory in both the text and graphics video interfaces was accessed by the processor using port-mapped I/O and therefore did not consume memory address space.
Storage
Mass storage was either via cassette tape, floppy disk, or an external file server. The cassette interface operated at either 300 bit/s or 1200 bit/s.
ROS 1.2 (see below) and later systems could be connected via a serial interface to an external single or dual 5¼-inch disk drive unit with a built-in double density Intelligent Disc Controller (IDC). Disk capacity was 180 KB (48 TPI drives) or 360 KB (96 TPI drives) per side.
Research Machines also offered plug-in ROM Packs, containing up to 64 KB of ROM. These connected via the parallel interface allowing applications to be quickly loaded into RAM.