| 摘要 |
1,140,291. Data storage. HONEYWELL Inc. 26 April, 1966 [17 May, 1965], No. 18248/66. Heading G4C. Apparatus for detecting drop-outs in data frames successively recorded at predetermined intervals along the length of a multi-channel medium (e.g. magnetic tape), on failing to detect a frame in one of said intervals, counts subsequent frames detected, an error signal being produced if the count exceeds a predetermined value. The tape has 7 channels, one being used for a transverse odd parity bit for each frame, each frame being across the tape. A multi-frame record is followed by a longitudinal even parity frame for the record, separated from the nearest data frame by 3 times the gap between successive data frames (the latter corresponding to a delay of 17Ás). The bits 1 and 0 are recorded as transistion and no-transition of the magnetic medium respectively. Since transverse parity is odd, there should be at least one transition per frame. Referring to Fig. 2, any frame with a transition, on being read, produces a 3Ás clock pulse from 10 which produces 25Ás and 0À5ms pulses from units FPD and RPD respectively, partially enables AND gates 34, 36, 38, and causes a transverse parity check to be performed on the frame. Assuming forward read, signal RF is present. In this case absence of transitions from a frame allows the 25Ás pulse produced by FPD in response to the previous frame to disappear before being replaced by another, so flip-flop 32 is set via AND gate 26, thereby enabling any subsequent clock pulses from 10 to pass through AND gate 24 to counter 42. If the count reaches 2 an error signal is given. Even in the absence of error, the count will reach 1 since the gap between the longitudinal parity frame and the nearest data frame is more than 25Ás. However it is not more than 0À5ms, so there will be no gap between successive pulses from RPD until the parity frame has been passed. When a gap occurs, the flip-flop 32 and counters 42, 48 are reset. For backward read, in which signal RB is present, the parity frame precedes the data frames. The 3Ás clock pulses are counted at 48. After a count of 2 has been reached, gate 30 is enabled so that a missing frame can set flip-flop 32 causing subsequent frames to be counted at 42, an error signal being produced as before on reaching a count of 2. Since the parity frame may be all 0s, the two data frames nearest to it should have a large number of is so that all will not drop out. Similarly (for backward read) the two data frames at the other end of the record should have a large number of 1s. Writing on the tape (forward movement only) is immediately checked by a read-after-write operation. |
