LINK 480Z

Hardware

The LINK 480Z combined its keyboard and main system unit into one piece. The earliest versions came in a black sheet-metal case, but this was soon swapped for a cream-coloured plastic one. An external 5¼-inch floppy disk drive was available as an add-on. All models used a single processor: a Z80A running at 4 MHz.

A few early machines had only 32 KB of RAM, but most shipped with at least 64 KB. Through bank switching, memory on the main board could reach 128 KB, and with the option board—which also brought high-resolution graphics—it could go up to 256 KB. Since the Z80 could natively address only 64 KB without software tricks, the extra memory was usually set up as a RAM disk called the Silicon Disk System. This silicon disk could be pre-loaded with software and data when the 480Z booted to the network, saving a lot of time in classrooms where programs could be ready before students arrived.

It also kept its contents if the system was soft-reset. Up to 32 KB of firmware was included, and it could be bank-switched out of the normal address space when not needed. That left 58 KB of RAM directly usable by the user, with 2 KB reserved for system tasks and 4 KB hidden because the firmware ROM occupied that space. Nearly all 480Zs, except for some very early ones, had software-switchable text-only monochrome video that could show 40 by 24 or 80 by 24 characters.

A composite video output worked with an external monitor, and an internal RF modulator let you use a television. The text display had its own dedicated memory. An option board added high-resolution graphics and a TTL RGB interface for a colour monitor. This board came with 16 KB of dedicated video memory and supported three modes: extra high resolution (640 by 192 pixels, 1 bit per pixel, 1 page), high resolution (320 by 192 pixels, 2 bits per pixel, 1 page), and medium resolution (160 by 96 pixels, 4 bits per pixel, 2 pages).

A programmable lookup table with an 8-bit output mapped each pixel value to one of 256 intensities on composite video or one of 8 colours on the TTL RGB output.

Firmware

The 480Z system included up to 32 KB of firmware, split into three parts: an 8 KB ROM monitor called ROS, 4 KB of Z-Net network firmware, and 20 KB of BASIC in ROM. ROS, short for Resident Operating System, acted as both a monitor and a provider of basic system services. Through the monitor, users could launch BASIC from ROM, load programs from cassette, or start the operating system. It also featured a software front panel that let users view registers and memory, set breakpoints, and step through machine code one instruction at a time.

ROS handled fundamental hardware tasks like reading the keyboard, writing text to video memory, and managing disk input and output. These functions were accessed using the Emulator Trap (EMT) pseudo-opcode, which relied on the Z80 RST 30H instruction; the EMT handler checked the byte immediately after that instruction to identify the requested function, with all parameters passed in registers. Another pseudo-opcode for call-relative operations used RST 28H.

ROS services were largely compatible with the COS services found on the earlier Research Machines 380Z. The Z-Net firmware enabled network booting from a file server and was removed from the main address space after booting. Research Machines supplied a full BASIC interpreter in the standard firmware, mainly for cassette-based systems; it was swapped out if the machine booted from disk or network. ROS versions included 1.0, 1.1, 1.2 (which added floppy disk support), and 2.2.

Software

The 480Z could run CP/M 2.2 from a local disk or CP/NOS from a file server. With CP/M, network services were accessible via CP/NET. A multi-user MP/M variant allowed several computers to share a single disk unit, though the serial connection made simultaneous saves slow. This setup was an inexpensive step toward a 380Z server and basic networking. Standard CP/M software like WordStar was available, alongside Research Machines’ own tools: the ZASM assembler, TXED text editor, and a BASIC interpreter. Software written for the earlier 380Z in high-level languages such as BASIC, or using only standard CP/M and firmware calls, could run directly on the 480Z. However, programs that accessed hardware directly, like those using high-resolution graphics cards, were generally not compatible.