A typical 9-track unit consisted of a tape transport—essentially all the mechanics that moved tape from reel to reel past the read/write and erase heads—and supporting control and data read/write electronics. The transport typically consisted of supply motor, take-up motor, hubs for locking the tape reels in place, a capstan motor (though not necessarily a pinch roller, see below), tape head assembly, miscellaneous rollers which kept the tape in a precise path during operation, and vacuum columns. Problems with wow and flutter, stretched tape, etc could result in corrupted data. The transport had to guide the tape with low tension on the tape without damaging the tape edges, but with enough tension to keep the tape in constant contact with the read/write head.
To load a tape, an operator would remove the protective ring on the outside of the tape reel and install the tape on the supply hub, then thread the tape leader through the various roller assemblies and onto the take-up reel, installing three or four winds of tape to provide enough friction for the take-up motor to be able to pull the tape. The operator then initiated an automatic sequence, often by a single press of a button, that would start the vacuum system, then move the tape forward until the beginning-of-tape (BOT) foil strip was detected by an optical sensor in the tape path. The control electronics would then indicate to the controlling computer that the unit was ready for operation.
Like its audio counterpart, moving tape past the read/write heads on 9-track digital required precise control, accomplished by a capstan motor. The capstan motor was designed for very smooth operation. Feedback to the control electronics was accomplished by a tachometer, usually an optical "tone wheel", to control tape velocity. Starting and stopping the capstan was controlled by ramp generators to ensure a properly sized inter-record gap, the gap between blocks of information.
The vacuum system provided a physical buffer between the precision movements of the capstan and the large movements of the reels by storing a short length of tape in the vacuum column under relatively low tension. The vacuum columns were chambers open at one end, the openings being in line with the tape path before and after the capstan and roller assemblies. The amount of tape in the column was controlled by four optical or vacuum sensors on the sides of the columns. The control electronics kept the curve of the tape loop between the two inner sensors, cueing the supply reel to feed more or the take-up reel to take more as necessary. The outer two sensors, at the very top and bottom of the columns, served to sense malfunctions in the feed mechanism during operation, prompting the control electronics to shut off all operation of the tape transport and vacuum system to prevent damaging the tape. Because of the tension provided by the vacuum columns and the design of the tape path, tape was usually kept in sufficient contact with the a relatively high-friction coating on the capstan that a pinch roller was not used.
Tape motion on many systems was bidirectional, i.e., tape could be read either forward or backward at the request of the controlling computer. Because the supply vacuum column kept a small, constant tension in the reverse direction, the capstan could feed backwards without the tape bunching up or jumping out of path. Unlike most audio tape systems, the capstan and head assemblies were always in contact with the tape, even during fast forward and rewind operations. On some units, manufacturers installed a "fast search" capability which could move the tape quickly a certain number of blocks, then bring the tape to a halt and go back to read the requested data at normal speed.
Tapes included an end-of-tape (EOT) foil strip. When EOT was encountered, the unit would either halt or rewind the tape onto the supply reel, depending on the unit's design. The sensing of BOT and EOT was achieved by shining a small lamp at the tape's surface at an oblique angle. When the foil strip (glued to the tape) moved past the lamp a photo-receptor would see the reflected flash of light and trigger the system to halt tape motion. This is the main reason that photographic flash cameras were not allowed in data centers since they could (and did) trick the tape drives into falsely sensing BOT and EOT.
The above describes a typical transport system; however, manufacturers engineered many alternative designs. For example, some designs used a horizontal transport deck where the operator simply set the tape reel in the supply reel bay, closed the door and pressed the load button, then a vacuum system would draw the tape along the path and onto a take-up hub within the mechanism. Some designs eliminated the vacuum columns in favor of a microprocessor-controlled direct drive design.
|IBM Model||2400 Series||3400 Series|
|Model numbers||2401, 2415, 2420, 2440||3410, 3420, 3422, 3440|
|Density (bits/in/track)||800, 1600||1600, 6250|
|Tape speed (in/s)||18.75 - 200||120 - 200|
|Transfer rate (B/s)||15,000 - 320,000||1,250,000|
|Interblock gap (in)||0.6||0.3|
|Rewind speed (in/s, avg.)|
|Start time (ms)|
|Stop time (ms)|
|Length of reel (ft)||2400 max||2400 max|
The maximum data capacity of a 2400 ft reel, blocked at 32,767 bytes and recorded at 6250 BPI was 170 megabytes. Typically, much smaller block sizes, such as 4K (4,096 bytes) were used, in which case the storage capacity of the tape was reduced by 33%, to 113 megabytes.