【Mitsubishi Electric】 MELCOM 1530

The MELCOM 1530 was a mid-sized general business computer announced by Mitsubishi Electric in 1963 and developed through a technical collaboration with the American firm TRW. The logic operation circuitry on the MELCOM 1530 made use of a groundbreaking architecture called stored logic that was a forerunner of microprogram control. Stored-logic architecture was a logic system that could freely associate basic elements as needed to execute any kind of instruction, instead of having fixed computer logic functions. This architecture allowed for more flexible system configurations and contributed to a large increase in reliability because there were fewer circuit elements and because logic circuits could be consolidated and simplified.

The processor’s chassis had a 60-centimeter-square footprint and housed the main memory, operation circuitry, interfaces, power supply, and associated control circuits.

At the time, there was no high-speed read-only memory available suitable for storing microprograms, so microprograms were stored in main memory. A magnetic core was used for the main memory, and a microstack with a 50-mil-diameter core was developed for the MELCOM 1530. The microstack’s access time was three microseconds and the cycle time was six microseconds, which was fast by the standards of the day.

As the diagram below illustrates, the processor consisted of core memory, six 18-bit registers (L, E, M, P, A, and D), the AD full adder, the AC half adder, the carry indicator C, and the overflow indicator F. In general, data transfers were made as 18-bit parallel transfers. There were 64 types of micro operations that transferred data between these blocks each clock cycle.

Block diagram of the MELCOM 1530 processor

Block diagram of the MELCOM 1530 processor

Micro commands called LOGANDS (which stood for “logical building block of commands”) could be made by combining several micro operations. There were more than 8,500 varieties of LOGANDS. LOGANDS had the following 18-bit structure.

Primary field:
The primary command, such as load, store, add. These basic commands were executed in four or six clock cycles.
Address option field:
Register containing the address for memory reads and writes. Bits 10 and 11 indicated the M, L, P, or D register, and Bit 12 indicated either direct or indirect addressing.
Count option field:
This field specified the memory access or address change control (Count / Hold) or the word length for shift processing (Single / Double). The interpretation of this field was determined by the Address option field or the Primary field.
Secondary field:
Used as a secondary command, test conditions, parameters, or scratchpad memory addresses. The interpretation of this field was determined by the Address option field or the Primary field. Secondary commands were executed in the final clock cycle of the primary command.

A program created by combining LOGANDS was called a LOGRAM (which stood for “logical program”). Assembly language commands were implemented by LOGRAMs.
The main memory could hold 8K words, but this was expandable to 16K words or 32K words. One word consisted of 18 bits plus a parity bit. One word could contain three alphanumeric characters. The main memory was accessed from the operation circuitry, the I/O control unit (buffer controller), and the communications control unit (tele-control unit).
The operation circuitry and control circuitry were synchronous circuits timed to a 333 KHz clock pulse that was in sync with the memory cycle. The multiplication and division circuitry was also hardwired. The operations of up to three MELCOM 1530 processors could synced for direct data transfers by attaching a computer communications link (CCL) unit.

A buffer controller was included to perform I/O operations with peripheral devices. The buffer controller housed buffer memory, input channels, output channels, interrupt control circuitry, peripheral device control circuitry, and a power supply. With this controller, inputs, outputs, and calculations could be performed simultaneously. The buffer memory was a core memory with the same word length and access time as the main memory and could hold 384 characters. It had an 18-bit output register and a 132-bit output shift register, which were used for buffering outputs to slow devices such as card punches, line printers, paper-tape punches, and typewriters. Buffer memory was not used for input devices or high-speed output devices; an 80-bit input shift register buffered input data. These devices sent data to the main memory via interrupts or cycle-stealing, thereby increasing the system’s overall synchronous operation ratio.

A magnetic tape unit was used for large external storage. The magnetic tape units had seven tracks, which were IBM compatible, and a transfer speed of 41,700 characters per second. Up to eight units could be connected to the computer. Other devices that could be connected included a high-speed optical card reader with a scan speed of 1,650 cards per minute, a high-speed card reader/puncher with a punch speed of 300 cards per minute, a high-speed print-drum line printer with a print speed of 750 lines per minute with 132 characters per line, a 10-character-per-second typewriter, a paper-tape reader, and a paper-tape puncher. Also, up to four magnetic disks could be connected to the computer, each with a capacity of 228.3 x 106 bits (?32.5 million characters), an average access time of 190 milliseconds, and a maximum transfer speed of 697 x 103 bits per second.

The tele-control unit could handle up to 64 terminal stations, and each terminal station could be connected to up to 32 terminal devices.

MELCOM 1530 data processing system