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LGP-21

Overview

The LGP-21 was a transistorized successor to the LGP-30, produced by General Precision (Librascope) in Glendale, California, and marketed by Royal McBee Corporation beginning in 1963. It was designed as a budget machine – one-third the price of the LGP-30, but also approximately one-third as fast. It was 100% software backward-compatible with the LGP-30, making it possibly the only case in computing history where a follow-on product was deliberately slower than its predecessor.

Technical Specifications

Parameter Value
Word size 32 bits (31 bits + 1 sign; or 32 bits including sign, depending on source)
Number representation Two’s complement, fixed-point (changed from LGP-30’s one’s complement)
Memory type Magnetic drum
Memory capacity 4096 words
Clock frequency 80 kHz (vs. 120 kHz on LGP-30)
Minimum drum access time 0.78 ms
Instruction execution time 0.39 ms (excluding access, multiply, divide)
Multiply time 24.96–25.74 ms
Divide time 26.13 ms
Effective speed ~2564 instructions/second (~1/3 the speed of LGP-30)
Instructions 23 (vs. 16 on LGP-30)
Transistors ~460
Diodes ~375 (some sources say ~300)
Architecture Bit-serial, transistorized, internally stored program
I/O capacity Up to 32 devices
Branch switches 4

Physical Specifications

Parameter Central Computer Basic System (with printer and stands)
Height 11.5 inches 36 inches
Width 31 inches 62.25 inches
Depth 20 inches 21 inches
Weight ~90 pounds (41 kg) ~155 pounds (70 kg)
Parameter Value
Power consumption ~300 watts maximum
Power input 110V AC, 60 Hz, single-phase

Cost

  • Price: $16200 (some sources say $16250) – equivalent to ~$170000 in 2025 dollars
  • This was one-third the cost of the LGP-30’s $47000

For context, $16200 in 1963 was approximately the cost of an average 2-bedroom suburban house.

Differences from the LGP-30

Feature LGP-30 (1956) LGP-21 (1963)
Technology 113 vacuum tubes + 1450 diodes ~460 transistors + ~375 diodes
Clock 120 kHz 80 kHz
Speed ~400+ additions/sec ~2564 instr/sec (~1/3 slower)
Number system One’s complement Two’s complement
Instructions 16 23
Weight (CPU) 800 lbs 90 lbs
Power 1500W operating 300W maximum
Price $47000 $16200
Software compatibility – 100% backward compatible with LGP-30

The LGP-21’s lower clock speed and slower drum resulted in reduced performance, but the dramatic reduction in weight (from 800 to 90 pounds), power (from 1500W to 300W), and price (from $47000 to $16200) made it attractive to budget-conscious buyers.

Design History

The LGP-21 was a straightforward transistorization of the LGP-30 design. Librascope (by then part of General Precision) chose to optimize for cost and simplicity rather than performance. The transistor count (~460) was modest, and the design used printed circuits throughout.

The machine lacked index registers and many other features that were becoming standard on larger machines, staying true to the LGP-30’s minimalist philosophy.

Key Innovation

The LGP-21’s main innovation was extreme affordability for a general-purpose computer in 1963. At $16200, it was accessible to small colleges, departments, and laboratories that could not justify a mainframe but needed more than a desk calculator.

Known Customers

  • Los Alamos Scientific Laboratory – purchased one in September 1963 for $16200 (one of the earliest documented sales)

Total production numbers are not well documented; the LGP-21 was less commercially successful than the LGP-30.

Notable Anecdotes

  • The LGP-21 may be the only follow-on computer product that was deliberately slower than its predecessor – the tradeoff was accepted because the dramatic cost, weight, and power reductions opened new markets.
  • At 90 pounds for the central computer, one or two people could physically move it – a remarkable contrast to the 800-pound LGP-30 or the multi-ton IBM mainframes.
  • The central computer was roughly the size of a large typewriter (11.5” x 31” x 20”).

Current Status

LGP-21 emulation is available through SIMH, the free, open-source multi-platform simulator.

Some surviving units exist in private collections and smaller museums, though they are rarer than the LGP-30.

Sources