IBM 1360 - Description

Description

From the Photostore Operator's Manual

Data was stored on small 25 by 70 mm cards of stiff film known as chips, each one holding 32 data "fields" in a 4 x 8 array. Each field contained 300 lines of data of 300 bits each, 0s written as a black-clear pattern, and 1s as a clear-black (using Manchester encoding). In total each chip held about 6.6 megabits. Chips were delivered in plastic boxes known as cells, each holding 32 chips. Cells were delivered in boxes of ten, wrapped in a lightproof wrapper. Boxes of cells were loaded into a hopper on the 1365 Photo-Digital Recorder unit, which would cut off the wrapper and drop the cells into a queue. When a cell reached the head of the queue it was removed and opened, chips being pulled out one at a time as needed.

Data was read off the card by moving it in front of a fixed photocell. Access time was improved by laying out the data in rows that were read in both directions. The head would read off a track of data as the card moved from right to left (say), and then reverse direction and read the other side of the same track from left to right. Once it returned to its original position it would move onto the next track in the field. The term for this method of data access is boustrophedonic, from a Greek root meaning "as the ox plows."

Data was written to the chips using an electron gun, similar to the operation of a television tube. Sensors and magnets on either side of the chip holder automatically focused the beam and corrected for focus as the filament wore down through use. The gun had eight filaments instead of one, automatically rotating a new one into position as needed to allow it to work for extended periods before replacement. After the chip had been written it was moved to an automated photo processing system similar to those found at most camera shops today; the chip was dipped into a series of liquid-filled stations to develop, and then pulled out to dry.

Flaws on the film, impossible to avoid, were addressed to some degree through the use of complex error correction codes, which used up about 30% of the overall storage capacity – thus each chip held just over 4 Mbit of user data of the 6.6 Mbit available. Error correction could correct for minor imperfections, but not for larger problems or bad developing, so after developing the chips were immediately passed to the 1364 Photo-Digital Reader to ensure they worked. A non-working chip was automatically discarded and another one made to replace it while the data was still in memory.

Once processed, the chips were re-inserted into the cell they had been removed from earlier. They were then moved out of the reader and into the 1351 Cell File & Control or additional storage-only 1362 Cell File units. Note the numbering; these units were intended to be shared with the 1350 system. Each file contained 75 trays (5x5 x 3 deep) holding 30 cells each, for a total of 2,250 cells, containing 1/2 a terabit. The system installed at LLNL used one 1361 and one 1362 for a total of one terabit, but other installations typically had two more 1362's for a total of 2 terabits. Cells could be manually moved about by loading them into the front-and-bottom-most set of trays, which could be removed.

Speed of the system was fairly good, writing at about 500 kbit/s, and reading at about 2.5 Mbit/s. Cells were moved between the Files and Readers using a pneumatic tube system similar to those used to move documents around in some stores and hospitals. The system could keep up to 13 cells "in flight" around the system in order to minimize delays.

Controlling the entire system was a small computer, programmed similarly to industrial control systems with a fixed number of tasks running all the time. The controller was also tasked with translating the data to and from the host format. IBM surprisingly offered the 1367 Data Controller for Control Data systems, realizing that most users with this sort of storage need had a number of CDC machines. Other Controllers were available for different host platforms.