NCR Century 100

The NCR Century 100 was NCR's first all integrated circuit computer. All logic gates were created by wire-wrapping NAND gates together to form flip-flops and other complex circuits. The console of the system had only 18 lights and switches and allowed entry of a boot routine, or changes to loaded programs or data in memory. A typewriter console was also available.

The 615-100 Series integrated a complete data processing system had 16KB or 32KB of short rod memory, 80-column card reader or paper tape reader, two 5MB removable disc drives, 600-line per minute printer. The system could be provided with a punched paper tape reader, or an external card reader/punch, and also allowed for the attachment of multiple 9 track 1/2 inch reel to reel magnetic tape drives. Two more disk drives could be attached to the system. The Century series used an instruction set with two instruction lengths: 4 bytes (32 bits) and 8 bytes (64 bits).

The memory of the Century Series computers used machine made, short (1/16 inch long and approximately the diameter of a human hair) iron-oxide coated, ceramic rods as their random access memories, instead of the hand-labor-intensive core memories that were used by other computers of the time. These rods were inserted into a plastic alignment sheet which was wound with read, write, and sense wire coils arranged in columns and rows. To get the rods to stand up straight on the sheet (so that they would drop into the coils for assembly) a large electro-magnet was turned on and made the rods stand up and "dance" into the individual holes. The economy of machine assembly was augmented by selling rod memory without paying patent royalties on core memory to (NCR's competitor) IBM.

The Model 655 removable disc drives were the first to employ floating or flying heads. Early marketing material made a big deal of this, but there were a number of problems that plagued all of the Century Series systems. Head crashes were common, because the head flew less than a human hair's width above the disc surface. (Unless a drive unit was repaired and carefully cleaned after a crash, the next disc pack loaded in that drive would also crash. And, if a crashed disc pack was loaded on an operational drive, it would destroy the head on that drive unit. Contributing to this was the fact that disc packs were hand-loaded: spun tight onto a spindle, then uncovered by hand, in open air, before the drive cover was closed. A head crash could be caused merely by repeatedly bumping the smoked polycarbonate pack cover against the edge of a disc platter while lifting it off. Although the typical head crash ruined heads and the pack, the worst kind of head crash was caused by an entire pack coming loose from its spindle and rising freely inside the machine while spinning at high speeds!)

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