The MRB Z 1013 () was an East German single-board computer produced by VEB Robotron Riesa, which was primarily intended for private use and educational institutions. It was powered by a U880 processor (a Z80 clone) and sold together with a membrane (flat foil) keyboard. Initially, the kit was equipped with 16 kilobytes of DRAM, which was later replaced by a 64 KB version.
The kits first became available for sale in 1985 and were distributed in a unique way at the time. To purchase it, buyers had to send a postcard to the Robotron shop in Erfurt and wait six to twelve months and then to pick the kits up in person. The package contained the assembled and tested motherboard, a membrane keyboard, various small parts and detailed technical documentation. This basic kit was shipped without power supply or casing for the PCB. Most users tended to program the kit using the BASIC interpreter, which was loadable from compact cassette or by using a ROM cartridge. The BASIC interpreter contained a common core binary, which was identical across home computer models. In consequence, programs were widely compatible among different models of GDR-manufactured computers despite differences in capabilities.
Robotron was also the manufacturer of another line of computers, the Z 9001, KC 85/1 and KC 87, which shared some of the same expansion modules – offering more options also for Z 1013.
The expansion connector was based on the K 1520 bus standard for 8-bit computers made in GDR. This conformity to one standard across computers, ranging from tank-sized minicomputers to small home computers, allowed for reuse of hardware from all computers with the same bus interface. This meant that most hardware and binary code from one platform could be used across very different platforms and allowed for the sharing of resources.
This groundbreaking standardization was due to the need for common standards and compatibility between computers in the Eastern Bloc.
Opinion is divided over the widespread use and popularity of the MRB Z 1013 in the GDR. With a total of 25.000 kits sold over its lifetime from 1985 to mid-1990, it fared well in comparison to other models. However, some analysts put this success down to the relative ease of access to the kits compared to other computer offerings.
After all, key point for success was the simplistic makeup down to bare minimum. It is still debatable if it repelled potential users or actually lead to higher production volumes. Either way, demand and production kept kind of balance. To this extent, it was the only computer freely available for private purchase.
History Despite the Cold War and the associated high-technology embargo CoCom, the government pushed for an ambitious program to keep up with international development in engineering and microelectronics. That generated huge interest among individuals who tried to develop electronics at work or at home aside from government's economic programs. In 1984, the first two lines of home computers, the Z 9001 and HC 900 were presented to the public. Due to the small scale of production those computers were difficult to obtain and very expensive.
With production yield of a few percent, a significant amount of integrated circuits failed to pass the acceptance criteria. The components were called "Anfalltyp" or "rejects". Most of them were functional but exceed allowed tolerances. Within limits, e.g. speed or access time, they may work fine. Consequently, manufacturing companies pushed for the development of simple fault-tolerant learning and hobby computers, which can make use of rejects. This approach could lower reported defect rates and partly close the gap in demand for home computers.
Following the concept of selling rejects for a fraction of the original price, the computer design would only consist of the cheapest and easiest available circuits. Therefore, three single board computers were selected for industrial production: the LC80 with calculator display and keyboard, the Polycomputer 880 with 8-digit seven-segment display and the more comfortable Z 1013 with TV output.
The initiators of the Z 1013 concept favored a caseless single-board computer with membrane keyboard. Through simple appearance and design, the price had to be kept under 1000 Marks for the targeted group of electronics amateurs. Development and production was transferred to well-established industry PCB manufacturer VEB Robotron Riesa in early 1984.
Development The state planning targets for the mostly young engineers and employees from the corresponding group of developers ("youth research collective") of VEB Robotron Riesa envisioned an expandable single board computer with a minimum of material and manufacturing costs. To reduce production costs, the device was designed as a kit without a housing whose prefabricated parts were to be fully assembled by the user. Also, the computer had to utilize existing home electronics such as television sets and tape recorders already present in East German households.
Production should be based exclusively on rejects made in GDR and Comecon states. Additionally, the new system would have to be largely compatible with regards to interfaces and software to Robotron microcomputers Z 9001 and KC 85/1. It was only feasible to meet such tight requirements by a system architecture utilizing the very affordable and field-proven U880 8-bit microprocessor.
True graphics modes and specific interfaces for peripheral devices fell victim to cost pressure. However, the concept as modular computer provided for the possibility of connecting additional peripherals, such as memory expansions, via modules for the K 1520 bus.
Development work began in mid-1985. The first prototype with 16 kilobytes (KB) of memory and a membrane keyboard was presented to responsible authorities in fall of 1985. After acceptance, planning and preparations for the series production began, which lasted until November 1985. Because of the rejects used, the system clock was reduced from 2 MHz as usually found in home computers to 1 MHz for higher reliability.
Z 1013.01 and Z 1013.12
The first batch of 150 kits with 16 KB of memory went into production in December 1985. They were available for pick up at end of the year exclusively, and only after pre-order, in the store for home electronics of VEB Robotron in Erfurt and in a store of state-owned Handelsorganisation (HO) in Riesa for 650M. In addition to display unit and tape recorder, the user also had to provide an appropriately sized power supply and to solder the connecting cable for the keyboard onto the mainboard before commissioning. The single-board computer was officially presented to a wider audience for the first time at Leipzig Spring Fair in 1986.
In addition to the supply of the home computer market, a few kits with designation Z 1013.12 were manufactured for the industrial sector by 1987. It used regular circuits, an increased system clock of 2 MHz, 1 KB of video memory as well as 1 KB of static RAM as main memory.
Robotron also developed additional components for its internal usage, like a pixel graphic extension or a battery-backed memory expansion of 4KB. Even though those were not available via official Robotron sales channels, goal was to provide that information as reference and to enable independent reuse.
Despite extensive tests and multi-stage, multi-day duration tests during production, the installed rejects often caused problems and thus complaints from users. Economic disadvantages due to elaborate test procedures and subsequent repairs could no longer be outweighed by the low price of rejects and led to a rethinking by persons in charge. From July 1987 onward, production was changed to use regular components, which have fallen in price by then, and the updated kit was sold as Z 1013.16.
Z 1013.16 and Z 1013.64 Since differences between rejects and regular chips was limited to their reliability only, the decision to use regular components could be made in 1987 without major changes to the mainboard and therefore cheaply. In addition to increased reliability, the Z 1013.16 variant also has a higher system clock of 2 MHz, which is equivalent to doubling of the computing power. Also, the system software has been updated to support a much more comfortable regular keyboard with 58 keys. Owners of older kits could after procurement of components and modified operating system also upgrade their systems with the aid of a soldering iron.
An important aspect of upgrades made to the memory subsystem is - apart from improved reliability - superior compatibility of the Z 1013.16 kit with microcomputers Z 9001, KC 85/1 and KC 87. In addition to availability of their software also their expansion modules were now usable. Due to the interim relaxation of the CoCom embargo, and with it associated falling prices in late 1988, a more modernized version of the Z 1013 series was added. This Z 1013.64 kit with 64 KB of memory was made until production ceased in mid-1990.
Modern replicas The simple and manageable system architecture, its extensive documentation from the manufacturer and not at least the free availability of the system software allows for miniaturized replica of the Z 1013 with today's technical means and with manageable effort. Such modern realization was done for the first time in 2013 - like with other home computer systems too - as implementation via programmable gate array (FPGA), inclusive its development environment. Replica using FPGAs were initially intended only as a technical feasibility study, but in retrospect proved also its practical utility: Due to miniaturization and possibility to run in battery mode, it is an easily stowable, reliably working and portable alternative to the preserve-worthy original technology.
Technical details The fully assembled printed circuit board by Robotron contains the CPU, RAM and ROMs, video generation circuits and multiple peripheral connectors. The mainboard measured 215 mm × 230 mm (8.4 in × 9.1 in). It was delivered along with a keyboard of dimension 80 mm × 160 mm (3.1 in × 6.3 in), connection cables, various small parts and documentation.
CPU
The system architecture is based on the U880 microprocessor, an unauthorized replica of the Z80 microprocessor from Zilog. Z 1013.01 models were clocked with only 1 MHz as they used chips which had passed only limited quality criteria ("rejects"). For other models, fully qualified, regular types like the 2 MHz U880 microprocessor were used. The CPU has an address range of 64KB, whereby 59 KB are available as RAM for programs. To overcome this limitation, external RAM disks and bank switching were used frequently.
Memory allocation, RAM, ROM and interfaces The memory that is addressable by the CPU is segmented into regions for use by the system software, freely usable memory, pluggable expansions and the graphics memory. The system software often simply referred to as "machine language monitor" is either flashed on a 2KB or 4KB ROM chip, depending on computer model. Memory addresses for the system software range from to or respectively. After power on, the system software also uses the memory between and to store various variables, so this space should not be used by the user either.