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. Networking The LINK 480Z supported a proprietary 800 kbit/s CHAIN local area network that ran over a coaxial cable in a similar manner to 10BASE2 Ethernet. Each station on the network required a unique, 8-bit network address that was set by means of a DIP switch on the rear of the unit. Using the built-in Z-Net firmware a diskless 480Z could be directly booted from a network file server (typically a Research Machines 380Z). Interfaces and options The 480Z motherboard