Small computer KC 85 / 2-4

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The small computers of the series KC 85 / 2-4 were from 1984 in the GDR by the state- owned company VEB Mikroelektronik "Wilhelm Pieck" Mühlhausen from the Kombinat Mikroelektronik Erfurt in the models HC 900, KC 85/2, KC 85/3 and KC 85 / 4 built. The manufacturer initially thought of the hobby and private sector, but most of the computers for popular education were complained about . Until shortly before the end of the GDR (around 1988), these computers were difficult to obtain for private individuals. The high price (3,900 M for the KC 85/3 - later reduced to 1,750 M, 4,600 M for the KC 85/4 - later reduced to 2,150 M) ensured that the "small computers" hardly found their way into private households.


In 1984, two home computers based on the 8-bit U880 (Z80) microprocessor were presented in the GDR :

  • Z 9001 was developed in the VEB Robotron-Meßelektronik "Otto Schön" Dresden and
  • HC 900 was developed in the VEB Mikroelektronik Mühlhausen. This development was promoted by Werner Domschke and was not a development covered by the GDR government. Despite some weaknesses, the development led to some upsets at Robotron, as this computer was fully graphics capable in its basic configuration.

The 1985 renaming from Z 9001 to KC 85/1 and from HC 900 to KC 85/2 was due to the change in the application, because the KCs should not be used as home computers, but as small computers in schools and other training facilities.


Basic device

KC 85/4

The system architecture of the HC 900 was anything but perfect: The screen control implemented with counter circuits was laborious to program. Essential system functions were implemented with PIOs (e.g. bank switching) and CTCs (cassette interface, flashing, sound output). The sound was generated by two CTC channels with a downstream flip-flop . Extras such as blitters or sprites were completely missing. The operating system CAOS (Cassette Aided Operating System) and HC-BASIC were quite comfortable, but slowly. With the original Mühlhausen operating system, scrolling the screen took 0.6 seconds and clearing the screen took 1.75 seconds. This improved significantly with the KC 85/4. The KC 85 / 2-4 implemented some standard interfaces such as Centronics for parallel printers, and serial interfaces (such as RS232C or V.24 ) via additional modules . From 1989 there were floppy disc drives as an additional device for the series, which represented its own CP / M computer and used the basic device as a terminal .

HC 900 and KC 85/2

It was based on the 8-bit CPU U880 (a Zilog Z80 - Clone ) 1.75 MHz (HC 900 = KC 85/2, 85/3 KC) or 1.7734475 MHz (KC 85/4) processor clock. The typical application of the Mühlhausen computers was a KC 85/3 with 32 KByte RAM (expandable with additional modules ), 16 KByte ROM - BASIC , connected cassette recorder for data storage and connection to a television used as a monitor (via UHF modulator, FBAS output or RGB output). The KC 85/2, it only had 4 KByte ROM, lacked the ROM-BASIC and the lower case letters. The KC 85/4 came with 128 KB of RAM and improved graphics options, but these were hardly used due to the collapse of the GDR . All KC 85s from Mühlhausen were graphics capable; the screen resolution was 320 × 256 pixels. However, the “color resolution” was significantly lower: In a pixel rectangle of 4 × 8 pixels, there could only be one foreground color (out of 16 possible) and one background color (out of 8 possible). With the KC 85/4, this restriction was reduced to a line of 1 × 8 pixels, and a “real” color mode with 4 colors and no limitation could also be switched on. It was not until the KC 85/4 that the disruptive characteristic of the Mühlhäuser KC series that memory accesses by the CPU to the screen memory caused picture interference.

At the University of Leipzig , visiting students gain their first experience at KC 85/3 (1989)

KC 85/3

The extensions of the KC 85/3 were:

  • Instead of 4 KByte ROM it now had 16 KByte ROM
  • integrated BASIC interpreter (approx. 10.5 KByte)
  • extended operating system CAOS 3.1 with commands such as DISPLAY, KEY and KEYLIST (approx. 4.5 KByte)
  • Upper and lower case letters (ASCII 00 ... 7F) available in the ROM
  • Piezo buzzer

KC 85/4

CAOS 4.2 for the KC 85/4

The main extensions of the KC 85/4 were:

  • Expansion of the ROM from 16 KByte to 20 KByte
  • Expansion of the internal RAM from 32 KByte to 64 KByte
  • Different organization of the screen memory (thus incompatible with KC 85/2 and KC 85/3 at hardware level)
  • Modification of the processor clock from 1.75 MHz to 1.734475 MHz in order to generate the system clock and chrominance signal with the same crystal
  • Access to screen memory without picture interference
  • Proper debouncing of the RESET button
  • Conversion of the keyboard socket from 3.5 mm jack to DIN
  • Larger text characters (capital letters 7 lines instead of 6 lines high)

The biggest change was the expansion (with an organization incompatible with the KC 85/2 and KC 85/3) of the screen memory:

  • Instead of 1 bank with 16 KByte, 4 banks of 16 KByte each were available.
  • The 4 banks resulted in 2 screen pages between which it was possible to switch.
  • 2 banks were used for each screen page.
  • There was a 16-color mode in which the pixels in one bank and the foreground and background colors for 8 horizontal pixels in the other bank.
  • There was a 4-color mode in which there was 1 bit each in both banks, which together made it possible to select one of four colors.
  • The organization of the screen memory was much easier: 256 bytes in the screen memory described a screen column of 8 × 256 pixels.
KC-85 series data
property KC 85/2 or HC-900 KC 85/3 KC 85/4
Dimensions 385 mm × 270 mm × 77 mm
Dimensions approx. 4.1 kg
Power consumption approx. 25 W (without extensions)
CPU UB 880 D (Zilog Z80 Clone)
Clock frequency 1.75 MHz (112 × PAL line frequency) 1.75 MHz (112 × PAL line frequency) 1.7734475 MHz (0.4 × PAL color subcarrier frequency)
RAM circuits 16 × KR 565 RU 3 G
ROM circuits 2 × U 2716 C 2 × U 2364 D
R.A.M. 32 KByte 64 KByte
ROME 4 KByte 16 KByte 20 KByte
free RAM for users approx. 17 KByte approx. 64 KByte
Refresh memory 10 Kbytes of pixels + 2.5 Kbytes of color (additional 1.25 Kbytes for storing the characters shown) = 13.75 Kbytes 2 × (10 KByte pixel + 10 KByte color (additional 1.25 KByte for storing the characters shown)) = 42.5 KByte
operating system HC-CAOS V2.2 (3.5 KByte + 0.5 KByte character table) HC-CAOS V3.1 (4.5 KByte + 1 KByte character table + 10.5 KByte BASIC interpreter) HC-CAOS V4.1
Image composition fully graphic, colored
Image size 320 × 256 pixels
Character representation 8 × 8 pixels each, 32 lines (typically 30 used), 40 columns
Foreground colors 16 (including black / white), additional flashing possible, frequency programmable
Background colors 8 (including black / white)
Character image tables 1 (capital letters) 2 (upper and lower case letters)
characters contained in the ROM 64 128 128
definable characters as many
Character generator through software
Screen layout through windows
simultaneously definable windows 1 10 10
Programming language in ROM no BASIC (10.5 KByte) BASIC
available programming languages BASIC, FORTH, assembler (EDAS + 2 debugger), standard Pascal compiler (status 1989)
Number of BASIC keywords 94 (as of 1989) 107 (as of 1989)
of which mathematical functions 10 (as of 1989) 10 (as of 1989)
of which are string functions 10 (as of 1989) 10 (as of 1989)
mathematical operators 14 (as of 1989) 14 (as of 1989)
available CAOS sub-programs 54 70 72
Sound output channels 2
  • 5 octaves (max.sample rate = (CPU clock in Hz) / (16 or 256) / (1 ... 256))
  • programmable via U857 (Z80-CTC)
Sound output
  • Two-channel diode socket, constant level
  • Single-channel RGB output, volume control
Sound output in the device no Piezo buzzer
Image output Color or b / w television or color or b / w monitor
Connections for image output HF modulator (VHF channel 2) without sound, FBAS, RGB HF modulator (UHF channel ~ 36) without sound, FBAS, RGB
Television standard PAL
Button click - adjustable
Display 2nd keyboard level - by cursor
Module slots 2
external connections
  • Expansion interface (computer bus)
  • Diode socket for recorder
  • RGB and FBAS
  • Keyboard jack
Data recording on tape Diphase process, compatible with KC 85/1 and KC 87
Recording speed
  • about 1200 baud gross
  • net about 1,000 baud
Block number 1 byte block number (Z80 programs: starting from 00h, last block 0FFh, BASIC programs: starting from 01h, last block identified by line number 65535).
Block length always 128 bytes of data, last block filled with padding bytes
data backup Check sum 1 byte per block (sum of all data bytes excluding the block number)
Data synchronization by separators
Transmission frequencies
  • logic 0: full oscillation 2400 Hz
  • logical 1: full oscillation 1200 Hz
  • Separator: full oscillation 600 Hz
Byte structure: 1 separator (600 Hz), 8 useful bits (1200 or 2400 Hz)
Block construction: 160 separators (600 Hz), block byte, 128 useful bytes, check byte

Expansion modules

There were a number of expansion modules for the KC 85 / 2-4.

The modules could be recognized by reading from the I / O port xx80, where xx stands for the number of the module slot. The module slots 08 and 0C were available in the basic device D001. Four additional slots were available in the D002 extension units (10, 14, 18 and 1C, can also be switched to other addresses).

By reading from port xx80, the module identifier called "structure byte" was obtained:

  • E7 : M010 ADU (1 8-bit A / D converter C570C with 4 switchable inputs) - circuit diagram
  • EE : M003 V24 (2 serial interfaces)
  • EF : M001 Digital In / Out (16 parallel inputs / outputs and CTC connections)
  • F4 : M022 16 KByte RAM, is automatically switched to address 4000 on cold start
  • F6 : M011 64 KByte RAM
  • F7 : M025 8 KByte user PROM
  • F8 : 16 KByte user PROM
  • FB : M027 Development (Editor + Assembler + Debugger)
  • FC : M006 BASIC interpreter for the KC 85/2
  • FF : no module or module without identification
  • And many others. Like this list .

This made it possible to achieve a certain degree of plug and play .

Modules could be activated and configured by writing to this address:

  • Bit 0 usually activated a module, i. H. the actual components (RAM, ROM or peripherals) can be addressed
  • Bit 1 activated writing to a module (only affects RAM modules),
  • the more significant bits usually define the base address or the rotation of the addresses (16 KByte, 64 KByte).


  • Busdriver D002 housing attachment with its own power supply unit , could accommodate 4 additional modules per device.
  • Floppy disk base D004 with its own power supply, contained a UA880 CPU with 4 MHz system clock and 64 KByte RAM and could operate up to four 800 KByte floppy drives ( floppy disk drive , in a further article). The D004 is actually a complete CP / M computer that only processed input / output via the basic device D001, which served as a terminal (there was, however, an operating mode in which the CAOS in the KC only used the D004 as a fast mass storage device used).


The Junost-402B black and white television set was often used as a monitor. The device only had one antenna input. The image quality was very mediocre, letters were provided with disturbing ghost images. The HF output provided no sound and no sound carrier; the volume control on the television had to be turned all the way to the left in order to suppress the loud noise.

Additional signals were available via a connector on the rear:

  • FBAS
  • RGB (see above composite sync signal as composite sync)
  • Mono tone, mixed from the two square tone generators, with a common 16- or 32-level volume control

They allowed the connection of televisions with composite input (which enabled medium quality) or with RGB input. Since most televisions did not have such inputs, you had to

  • content themselves with the HF input
  • or with some devices an extension was possible (built-in module) e.g. B. in the RFT Colormat 4001 or robotron RC 6073
  • From the beginning of 1989 the first devices with RGB input appeared: RFT Color 40, robotron RC 9140
  • In principle, an analog RGB monitor (Commodore 1084S) can be connected if it supports composite sync or if HSYNC / VSYNC is tapped from the motherboard.

Mass storage

KC85 / 3 with documentation, software cassettes and peripheral devices, etc. a. the GC6020 as mass storage

Cassette recorders, in particular the Geracord GC 6000, GC 6010 or GC 6020 cassette recorders manufactured by VEB Elektronik Gera, were used as mass storage devices. The easy accessibility of the azimuth adjustment screw was particularly important.

Although the recording frequencies of 600 Hz to 2,400 Hz were quite low (the signal can be transmitted by phone), the azimuth adjustment of different devices was often quite different. Another problem was the drop-out rate of cartridges from the VEB ORWO Wolfen .

There have been a few approaches to speed this up:

  • various turbochargers, partly as a pure software solution, partly as a hardware solution
  • Communication via V.24 with computers that have other mass storage devices (up to 57600 bps)
  • Connection of a floppy disk drive


The first software library was opened in Dresden in 1989

Programming languages

The KC 85 / 2-4 could be programmed in machine language and (before the KC 85/3 only with an additional module or RAM-BASIC from cassette) also with a BASIC dialect, which was significantly more extensive than the BASIC in the C64 . In contrast to most home computers , the system always started with the CAOS operating system (rather a better monitor ); BASIC had to be called explicitly from this monitor, if it was in the ROM at all. The monitor commands could easily be extended by assembler programmers.

Other, but not widely used, programming languages ​​for small computers were Pascal and Forth . In the last years of the GDR a floppy disk attachment was built for these computers. This enabled CP / M (Mühlhausen's name for it: “MicroDOS”) and software to be used.

The BASICODE programming language was also available for the KC 85 / 2-4 . It is a BASIC dialect that is compatible with a number of computers, with programs for BASICODE also being broadcast on the radio.

Word processing

There were essentially two word processing programs available

  • TEXOR: Worked with a 40 × 32 character screen and 8 × 8 pixel characters
  • Wordpro: Worked with an 80 × 32 character screen and 4 × 8 pixel characters


There were essentially two categories of games:

  • Games written in BASIC
    • The character table was often redefined in order to enable the necessary graphic output at a useful speed.
  • Games written in machine language

Hobby projects around the KC 85 / 2-4

KC 85/4 tower with hard disk

Popular self-construction projects were / are:

  • Connection of another keyboard
  • RAM disk
  • Interface modules
  • Text systems
  • Connection of electronic typewriters as keyboard and printer (for example Erika type wheel typewriter S3004, 3005, 3006, 3015 and 3016 )
  • Connection of a 3.5 "floppy drive
  • Use of a hard disk via GIDE interface
  • Scanner module M051
  • Network / USB module M052
  • Project: KC85 Recorder (cassette recorder replacement)
  • KC85 / 4 VGA connector

The CAOS operating system is subject to regular further development. There are also newly developed versions for the CP / M supplied with the floppy disk extension. A graphical user interface is being worked on. There were also model tests to implement industrial control tasks ( programmable logic controllers , PLC) for teaching purposes with the small computers. At the Breakpoint 2009 the KC 85/4 was used for a demo .

KC 85/5

At the time of manufacture, the circuit board of the KC 85/4 was already prepared for the use of more powerful memory types. The use of these memories was no longer implemented by the manufacturer until the series was discontinued. The built-in 64Kbit DRAMs can easily be exchanged for 256Kbit types. The CAOS and BASIC ROMs can also be replaced by larger types with further developed program versions without the need for further changes to the original hardware. A KC 85/4 upgraded in this way is commonly referred to as a KC 85/5.


Main board of the KC85 / 2
Main board of the KC85 / 3
  • Circuit diagrams: The circuit diagrams were disclosed, which led to numerous additional circuits and programs. The magazine Funkamateur especially took care of thesmall computers. She published circuits, listings and information about computer clubs. Further circuits and information appeared in the magazines Practic and radio fernsehen elektronik .
  • Thermal problems: The KC 85 / 2-4 had no fan. Summer room temperatures or contacted modules with increased power requirements could temporarily overload the computer's power supply. The system then worked unstably.
  • First reset : A feature in the ROM of the KC 85/3 made it possible to delete the main memory on RESET. Since this feature was based on a single bit (bit 7 of (IX + 7), where IX was not initialized before the query), the main memory was often unintentionally cleared in the event of a crash.
  • Second reset : Since you forgot to initialize the stack pointer in ROM before calling the first sub-function, you could intercept the reset with the stack pointer prepared. However, you could then no longer use the subroutine stack of the CPU for the commands POP / PUSH / CALL / RET and for interrupts.
  • Reset for the third time: The reset was not properly debounced on the KC 85/2 and KC 85/3, so that there could be errors when refreshing the DRAM during the reset.
  • Printer: Dot matrix printers capable of graphics were produced in the GDR, but were hardly available through free trade. Most of the specimens wereexportedto the Federal Republic of Germany and distributed there under the Quelle trademark Privileg . The electronic typewheel typewriters Erika S3004, 3005, 3006, 3015 and 3016,also built in the GDR, could also be used as printers. Through persistent programming ('.' And a lot of micro-steps were involved) graphics could also be printed with it.
  • RF emissions The reception of VHF transmitters was severely disturbed in the vicinity of the computers. Many functions generated characteristic disturbances (clear screen, display menu, scroll screen).
  • Rem : There was a popular computer program called "Rem" on the radio in the GDR. In addition to programming courses (“basic for advanced users”) and tips and tricks, what was special was that software was broadcast over the airwaves. The bits and bytes were converted into audible noiseswith the aid of frequency modulation and pulse modulation , which could be recorded using a cassette recorder and later loaded into the KC.

More small computers from the GDR

  • Z 9001 or KC 85/1 and Z 9002 or KC 87 : The KC 85/1 (identical to Z 9001) wasdeveloped and builtby Robotron with the experience of the K 1520 system that hasprovenitself in the A 5120 . Apart from the processor clocked at 2.5 MHz, the BASIC dialect and the format for data storage on cassettes, it had little in common with the Mühlhausen KCs. The KC 87 was an improved KC 85/1 with BASIC in the ROM. These computers were also available with a color option with an RGB output, but they did not offer pixel graphics, only text mode pseudographics. The keyboard was built into the base unit and consisted of tiny keys with a typewriter-like spacing that was very difficult to use.
  • Z1013 : Kit (RAM-Basic, assembler, flat membrane keyboard, connection for black-and-white television, additional modules)
  • LC80 : (LC von L ern C omputer) single-board computer, only machine language , only with pocket calculatorkeyboardand 7-segment LED
  • POLY880 : Learning computer for use as a learning machine in general schools
  • KC compact : The present for the 40th birthday of the Republic, very low numbers, CPC - Clone ; came with a neat, integrated into the housing keyboard, case and keyboard ( not but the electronics) are classified by BIC A 5105 adopted since the end of 1989 sold at Robotron existing systems for 1 million marks to Muhlhausen this. Mühlhausen later offered a D005 comfort keyboard for the KC 85/4 based on it.


  • Karl-Heinz Schubert (Ed.): Electronic Yearbook 1990 . Military publishing house of the GDR, Berlin 1989, ISBN 3-327-00770-5 .

Individual evidence

  1. Z80-CTC (in the GDR U857)
  2. GIDE
  3. KC85 Recorder (cassette recorder replacement)
  4. KCVGA ( Memento of the original from March 4, 2016 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. @1@ 2Template: Webachiv / IABot /
  5. Article on MLDOS
  6. Waypoint X by Moods Plateau
  7. System manual KC 85/5 with CAOS 4.4

Web links

Commons : Kombinat Mikroelektronik computers  - collection of images, videos and audio files