IBM 3350 direct access storage

• Models A2, A2F, B2 & B2F announced July 15, 1975 and withdrawn September 5, 1994
  
  
• Models C2 and C2F announced November 4, 1975 and withdrawn September 5, 1994

Known during its development as "Madrid," the IBM 3350 was introduced in 1975 and first delivered the following year. It extended Winchester technology by increasing the number of disks per drive and the recording density to provide a 4.5 times increase in capacity per spindle. In addition, the developers eliminated the customer-removable disk pack and circled back to fixed disks, as in the IBM RAMAC, to achieve higher recording densities and lower cost per bit of online storage. In addition, the elimination of operator handling -- and exposure to external contamination -- provided high reliability.
The 3350 furnished a maximum storage capacity of approximately 317.5MB per drive (635MB per unit), allowing more than 2.5 billion bytes of online storage per 3350 string. The data rate was 1198K/second and average seek time was 25 milliseconds.
The 3350 Models A2F and B2F provided 1,144,140 bytes of zero seek time storage per spindle (2,288,280 per unit) when operating in 3350 native mode. Models A2 and A2F were two-drive units with associated controls, which attached to the IBM System/370 Models 135, 155-II and 165-II via the 3830 Model 2, and to the IBM System/370 Model 145, 158 and 168 via their ISCs and/or the 3830 Model 2. The 3350 Models B2 and B2F were two-drive units which attached to the 3350 Model A2 or A2F. Up to three B2 or B2F units could be attached to each A2 or A2F unit, for a maximum of eight drives per 3350 string.

Characteristics
Average seek time (ms):	25
Average rotational delay (ms):	8.4
Data Rate (KB/sec.):	1198
Bytes per track:	19,069
Tracks per logical cylinder:	30
Logical cylinders per drive:	555
Capacity per drive (MB) approx.	317.5

Features

• Rotational position sensing, which permitted improved block multiplexer channel utilization.
  
  
• Error correction of single data error bursts of up to four bits.
  
  
• Command retry, which enabled the storage control to recover from certain subsystem errors without recourse to system error recovery procedures.
  
  
• Read only switch, gave increased data security by providing for each drive the means to protect data from being overwritten or erased.