In an era before semiconductor memory became widely available, computer manufacturers had to devise creative solutions for program storage. One fascinating example that has recently captured attention is the wire-wrapped ROM (Fädel-ROM) used in the Wang 720C desktop calculator from the early 1970s.
A Memory Engineering Marvel
The Wang 720C, produced between 1970 and 1973, featured a remarkable piece of engineering: a wire-wrapped ROM that stored an impressive 77,056 bits (1,792 words x 43 bits). This memory system was physically large, measuring 43 x 31 cm - slightly larger than an A3 sheet of paper.
Technical Implementation
The ROM's architecture consisted of:
- 11 quadratic plastic structures, each containing four U-shaped ferrite cores
- 1,804 diodes
- 74 transistors
- 15 integrated circuits
- A massive decoding matrix of 1,792 diodes for microcode address selection
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| An intricate example of early computing technology, this motherboard reflects the complexity of components similar to those found in the wire-wrapped ROM of the Wang 720C calculator |
Manufacturing Process
The manufacturing process was particularly noteworthy:
- The prototype required over 1,500 meters of wire
- Initial programming took 6.5 weeks to complete
- Production units were largely hand-manufactured with assistance from specialized workshops
Cost and Market Impact
The Wang 720C was a premium device for its time, with prices ranging between DM 23,000 and DM 36,000 (approximately equivalent to $11,500 to $18,000 in 1970). The calculator featured:
- DTL/TTL logic
- 2-digit Nixie tube display
- Core memory
- Cassette storage capability for programs and data
Technical Operation
The wire-wrapped ROM functioned similarly to modern EPROMs:
- Address input followed by a 200ns pulse on CPB for data retrieval
- Required additional latching for extended data retention
- Maximum capacity of 2,048 words, though not fully populated in production units
This historical piece of computing technology demonstrates the ingenuity of early computer engineers in solving memory storage challenges before the advent of semiconductor memory chips, representing a crucial stepping stone in the evolution of computer memory systems.