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Dietz Mincal 523

Bild: Mincal 523 CPU

Manufacturer: Dietz Computersysteme Type: Mincal 523 Year: 1971 Technology: TTL, transistors 8 kwords core memory with 19 bits (+1 parity bit) 1280 words ROM for microcode and loader

The Mincal 523 is quite an exotic computer from today's point of view. Only the word length of 19 bits is unusual, but is has to be noted that this computer was designed for process control and processing of BCD values was normal. Of the 19 bits the top bit is the sign, so the remaining 18 bits can represent 4½ BCD digits (from 0 to +/-39999). The exceptional design of this computer was revealed only little by little during the reverse engineering (there was practically no documentation and all schematics had to be drawn and recreated by hand). The CPU itself is built simple. The machine consists of a lot of circuit boards with low integrated TTL ICs, three of the boards form the processor control, one contains the ALU and six are for the hardware registers A-F. The remaining boards control the interrupt and level logic as well as the I/O section. Nonetheless the computer's abilities are very powerful (but not particularly fast). This is achieved by micro programming that saves a whole lot of hardware effort at the cost of speed. The special about this, however is that the micro program is located in the normal address space of the CPU. It is realised in form of foil ROMs (see beneath) and is accessed automatically during an instruction fetch. The opcode of the machine instruction is used to form the jump address within the microprogram ROM. But it's also possible to write own microprogram routines and put them into core memory. There is an instruction to call a microprogram routine from core memory and continue execution with normal instructions afterwards.


The firm Dietz is known amongst computer firms mainly with the very successful system 621, intoduced 1971. Initially the company, created 1951 by Heinrich Dietz, developed and built electronic assemblies for the industry. They gradually moved from analog to digital electronics. So they were in best company with other firms like Digital Equipment or Hewlett Packard who also started with process control or measurement equipment and who only lated jumped on the computer train.

The ROM of the Mincal 523

ROM module of the Mincal 523 ROM module of the Mincal 523 Top view: One can see 8 words with 19 bits each. Such a board set contains up to 64 foils with 8 words at 19 bits each, i.e. 512 words in total. Closeup of nine bits Closeup of nine bits Each copper curl corresponds to a primary winding of a transformer whose core consists of the vertical ferrite rod. If a current pulse is sent through a copper trace a current is induced in the secondary winding (located below the yellow insulating foil) of the transformer. Accordingly no current is induced in those cores where the copper trace just goes past the core. The cores that read out the bits The cores that read out the bits The paper-thin circuit foils from the side The paper-thin circuit foils from the side 32 foils each are located above and below the top epoxy board. Thus the aforesaid transformer has 64 primary windings.


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