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IBM 3420 magnetic tape drive

• Models 3, 5 and 7 announced November 5, 1970 and withdrawn October 20, 1987
• Models 4, 6 and 8 announced March 7, 1973 and withdrawn October 20, 1987

Introduced in 1970, the IBM 3420 incorporated a number of technological innovations. To improve the operation of tape and move some of its control function out of the central processing unit (CPU), some instructions were moved to the tape drive. Approximately 1,000 hardware-assist instructions were embedded in the circuitry of the 3420. That was the beginning of tape microcode, or embedded instructions, for controlling the tape. By implementing for the first time a microprogram in a read-only memory in the control unit, IBM engineers eliminated the need for a separate switching unit (previously, the IBM 2816) to permit more than one CPU to use the same drive. In addition, the 3420 made possible diagnostic testing of the unit from the controller without taking the tape unit offline.

Improved function, density and cost were achieved by using monolithic circuits (MST) (associated with the System/370) rather than the older SLT (associated with the System/360 of the 1960s) along with other design changes that achieved a three-fold reduction -- from 80 to 24 -- in the number of signal and control wires connecting the tape drives with their controllers.

The 3420 Models 3, 5 and 7 were first shipped in 1971 with linear densities of 800 and 1600 bits per inch. Two years later, three new models were rolled out, offering 6,250 bits per inch.

The following is the text of an IBM Information Systems Group marketing brochure published in March 1985.

The IBM 3803/3420 Magnetic Tape Subsystem combines the capabilities of the IBM 3803 Control Unit and the IBM 3420 Magnetic Tape Unit to provide a wide range of data rate capabilities for those systems requiring 10½-inch reel compatibility

At 6,250 bytes per inch (BPI) recording density, the 3420 Models 4, 6 and 8 have a nominal data rate of 470, 780 and 1,250KB/second, respectively. The Models 4, 6 and 8 can optionally read and write data at 1,600 BPI. The standard recording density for the 3420 Models 3, 5 and 7 is 1,600 BPI with optional 800, 556 and 200 BPI capability.

One 3803 Control Unit can provide access to as many as eight tape units. Sixteen 3420 tape units can be accessed through two, three or four control units.

Highlights

• Up to eight 3420s can be attached to each 3803.
  
  
• Attachment combinations: 3803 Model 1 to 3420 Models 3, 5 and 7; 3803 Model 2 to 3420 Models 3, 4, 5, 6, 7 and 8.
  
  
• 6,250 BPI encoding/checking logic allows error correction on the fly for any single track, or combination of two tracks simultaneously. Errors may be corrected in all nine tracks of a single data block, providing they occur in combinations of no more than two tracks at a time. Automatic in-flight single-track error correction is provided for 1,600 BPI recording.
  
  
• An optical tachometer can precisely control the speed of the capstan to help minimize tape stretching and breaking.
  
  
• Optical tachometers also control reel motors' speeds, providing fast rewind times and helping to minimize the possibility of erratic stacking of tape on reels.
  
  
• Automatic loading with the IBM wraparound cartridge for 10½" reels helps minimize tape contamination.
  
  
• Automatic tape threading takes about ten seconds. Automatic reel latch permits fast loading of tapes.

Optional features

• Dual Density - Models 3, 5 and 7 operate at either 800 BPI or 1,600 BPI. Models 4, 6 and 8 can operate at either 1,600 BPI or 6,250 BPI.
  
  
• Seven-Track Capability - Models 3, 5 and 7 can operate in seven-track mode at either 556 BPI or 800 BPI. 200 BPI is also available if these models are attached to an IBM 3803 Model 2.
  
  
• Tape Drive Switching - This increases the systems operating flexibility and availability of tape drives.
  
  
• Two Channel Switch - This allow the 3803 to be switched between two different channels under program control.