The invention is related to magnetic tape storage systems for use in data processing systems. More specifically, the invention is concerned with a magnetic tape formatting scheme for facilitating the positioning of read/write heads at a desired location along a tape that carries a magnetic storage medium and the associated logic circuitry for detecting the physical ends of the tape and the location of data records along the tape.
A typicl magnetic tape storage system includes at least four essential and basic components: namely, a magnetic tape, a tape transport, data transfer circuitry and control circuitry. The magnetic tape generally comprises a flexible tape-like plastic strip having a thin coating of ferromagnetic material along the surface thereof as a storage medium. The tape transport moves the tape between supporting reels, or spools, in a forward or reverse direction past one or more associated read/write heads in the data transfer circuitry. The data transfer circuitry receives signals from the reading heads and converts them into binary signals for transfer to the data processing system and converts binary signals received from the data processing system into signals for energizing the writing heads thereby to store information on the magnetic tape. The control circuitry responds to commands from the data processing system to control the operation of the other components.
This invention is particularly adapted to a class of magnetic tape storage devices in which the tape transport may be driven in a fast access, or "seek", mode for the purposes of positioning a desired record at the read/write heads and in a slower, "read/write", mode during data transfer operations that enable data to be read from or written onto the magnetic tape. In such magnetic tape storage systems, efforts are made to achieve optimum performance in both the seek and the read/write modes. Specifically, it is desired to achieve a maximum spatial signal, or bit, density along the tape for signals that represent the data to be stored and various control information in order to maximize the storage capacity of the tape. However, in practice the maximum density that can be achieved is established by several conflicting operating criteria. For example, while increasing signal density increases the data transfer rate for a given tape speed, the probability of errors during data transfers also increases. It also is desirable to minimize record searching times during the seek mode as no data is being transferred. The faster transport rate achieves this objective, but many times at a rate that exceeds the bandwidth of the data transfer circuits.
Several magnetic tape storage systems utilize prerecorded formats on the magnetic tape to facilitate the operation of systems which have both seeking and read/write modes.
One such formatting scheme is shown in U.S. Pat. No. 3,387,293. In accordance with the description in that patent, the magnetic tape has plural, parallel tracks. One track, a mark track, contains prerecorded formatting information; another track is a timing track that contains timing information. The mark track defines different areas along the tape including end zones at the physical ends of the tapes and a plurality of intermediate blocks. Each block comprises contiguous frames including plural frames in the middle portion of each block for storing data. In a block the frames on either side of the data frames contain positioning information and control information that facilitate the operation of the system during a seeking mode and during a read/write mode.
More specifically, control circuitry utilizes positioning information associated with each block to relatively position the tape medium with respect to the read/write heads. This control circuitry and data transfer circuitry may also include detectors for detecting the end zones corresponding to the physical ends of the magnetic tape and the boundaries of adjacent blocks. Additionally, some magnetic tape systems may further incorporate switching and buffering circuitry for improving the data transfer characteristics between the magnetic tape storage system and the data processing system to which it connects.
Another formatted arrangement is depicted in U.S. Pat. No. 3,879,752 that discloses a tape or disk medium in which incoming data from the storage medium contains binary data in discrete records, or blocks, and sector information defining the boundaries between adjacent records or blocks. The sector information is stored at a frequency which is greater than the maximum frequency of the signal produced by the binary data. A frequency discriminating circuit detects the occurrence of each burst of higher frequency signal, thereby to indicate that an area of sector has passed the read/write heads, and generates a sector pulse. Other circuitry uses the sector pulse for ascertaining the position of the medium. Apparently, however, this formatting is limited to a medium that travels at a constant speed as the frequency of the sector information signal is dependent upon the velocity of the medium. Doubling the velocity would double the frequency of the sector information signal. Moreover this invention is disclosed as being applicable to both tape and disk media, and disk media are constant speed devices.
One disadvantage of the first formatting scheme is its inefficient use of the overall data storage capacity of the recording medium and the additional read circuitry required to reach each of the timing and mark tracks simultaneously with the data track. A similar system incorporates an optical detector in conjunction with transparent or reflective markers disposed at the physical ends of the magnetic tape. The tape may incorporate a reflective metal element on its surface, have its oxide coating absent at a portion thereof, or possess an arrangement of holes that pass light therethrough. In such a system, the control circuitry knows, directly or indirectly, whether the tape transport has reached the beginning or ending of the magnetic tape and thereby causes the tape control circuitry to take appropriate control action. Expensive optical detectors and associated logic circuitry are obvious disadvantages of this method.
Another system incorporates a null signal area, or "gap", as a boundary between adjacent records to identify inter-record positions. When a predetermined threshold signal level is not exceeded by the signal from the read heads, the system assumes that an inter-record position is passing the read/write heads. This method is limited by noise factors concommitant with the transfer of low level electrical signals generally associated with transducers. Thus, the data transfer circuitry becomes more complicated because it must have the capability of discriminating noise signals from valid signals. When multiple speeds are used, either different transducers or different threshold signal levels must generally be employed to sense position data at the relatively higher tape speed. All of these factors increase the costs of the storage system.