Data Organization and Timing

Electronics Technician Volume 06-Digital Data Systems
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head or group of read/write heads. The moving tape can then have data written upon it as is shown in figure 9-9. Data may be read from tapes having information stored on them. Each individual write head can store data one bit at a time along an invisible line on the tape called a track. The number of bits written per inch of track (bpi) is one of the factors used to determine the density of data on the tape. An industry standard 0.5-inch tape may have seven or nine tracks of data stored on it. Data Organization and Timing The read/write heads of magnetic tape devices are usually designed to write and read data concurrently across the width of the tape. This grouping of bits is known as a frame. On multitrack tapes, density is determined by the number of frames per inch (fpi) instead of bpi for a single track. Common densities for multitrack tapes range from 200 to 1,600 fpi. Most magnetic tape devices are capable of writing and reading several different fpi densities. A frame of data on a seven-track tape consists of six data bits, and a check (parity) bit. A nine-track frame has eight data bits, and a parity bit. Frames are determinedby the a shift of the magnetic field in any bit position within the frame. With the proper combination of parity checking (odd parity) and data, at least a single binary ONE is stored in each frame. Using the NRZ recording method, every frame contains at least a single binary ONE. The presence of a ONE, when detected by any read head, will indicate the presence of a frame. This recording method is known as non-return-to-zero indiscrete (NRZI). Magnetic Tape Markings Magnetic tapes have many common features and data recording formats. Each tape is marked, in some manner, at beginning of tape (BOT) and at end of tape Figure 9-9.—Writing data on magnetic tape. Figure 9-10.—A magnetic tape layout 9-8

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