700 Series Technology

Electronics

The 700 is constructed entirely from small-scale integration ICs, mostly 7400 series TTL with some 9900 series DTL. Just over 350 individual ICs are used, spread over 15 circuit boards.

RAM memory is magnetic core, with 8 planes of 32x32 bits forming 8k bits that are arranged as 1k 8-bit bytes. Intestingly, the logic engine works with 4-bit nibbles but accesses RAM in 8-bit bytes.

Microcode is held in a wire-braid core ROM where each ROM address has a fine copper wire that is threaded through, or beside, a series of 43 current sense transformers. 1792 addresses are defined in this way and threaded with nearly 1.5km of wire to define 77056 bits or 9.6k bytes.

It’s interesting to note that 1k bytes of core RAM occupies 12x5x1 cm = 60cm3 per k, while 9k bytes of core ROM occupies 43x30x4 cm = 5610cm3 or 620cm3 per k. This 10x greater volume illustrates the limits to ROM technology in the 1960s. Microcoding offered great benefits to hardware design but these could not be realised until small, fast and relatively cheap ROM stores became available. HP encountered the same issues when designing the 9100 calculator which also required a large microcode ROM store.

Architecture & General Design

The 700 was Wang’s first IC-based machine and uses small-scale integration ICs mostly from the 7400 TTL family. A small number of parts from the 9900 DTL family are used, mostly in cases where DTL specifics are useful.

RAM is magnetic core, continuing Wang’s association with this technology from its invention.

The 700 represents another step into the future for Wang, being a microcoded architecture. The 300 series was entirely hard-wired logic, where all functions had corresponding circuits and wiring. Microcoded architectures are wired to perform only a basic set of logic functions, and these basic functions are then programmed to perform more complex functions as required. This requires a microcode ROM that generates control words corresponding to the microcode program that is required. The advantages of this architecture are firstly that the hardware logic engine is smaller and simpler, and secondly that the function of the machine can be upgraded or changed by changing the ROM, without the need to change the underlying hardware.

The limiting factor on microcoded architectures in the late 1960s was that the size of the microcode ROM pushed against the limits of the day.

Construction

The 700 series are desktop machines, the cases are a major step up from the 300 series in their design but still look somewhat utilitarian and flimsy. The keyboard is typical Wang, short-travel clicky microswitches with paper labels under generic clipon keytops. A large fan is tacked onto the back of the machine and was indeed a hack, being added after early machines suffered from overheating. The result is in stark contrast to the HP9100 and carries on Wang’s uninterested approach to industrial design.

The main electronics are carried on 12 high-quality double sided PCBs. Two additional boards hold the nixie drivers. While the PCBs are high-quality construction, Wang persists in declining to gold plate the edge connector fingers.

The microcode ROM is built on a large PCB that is mounted in the base of the machine with a rather flimsy plastic cover that does not seem entirely safe from a large coffee spill. See the Wire ROM section for more information about the theory and operation of wire braid ROMs.

The power supply is a basic transformer design with series pass regulation on all supplies.