Browse Prior Art Database

Edge Bit Detection and Recovery for Magnetic Card Recording

IP.com Disclosure Number: IPCOM000077883D
Original Publication Date: 1972-Oct-01
Included in the Prior Art Database: 2005-Feb-25
Document File: 3 page(s) / 34K

Publishing Venue

IBM

Related People

Cooper, DW: AUTHOR [+2]

Abstract

Under worst-case recording conditions, a magnetic card may come out from underneath the read gap after recording the last character on the track on a high-speed magnetic card deck. When the card motion is then reversed and the logic begins to error-check read in the reverse direction, an invalid error could be detected due to the flux transition caused by the edge of the card coming back under the read gap.

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Edge Bit Detection and Recovery for Magnetic Card Recording

Under worst-case recording conditions, a magnetic card may come out from underneath the read gap after recording the last character on the track on a high- speed magnetic card deck. When the card motion is then reversed and the logic begins to error-check read in the reverse direction, an invalid error could be detected due to the flux transition caused by the edge of the card coming back under the read gap.

The velocity of the high-speed read/record deck is 35 inches per second. Recording is accomplished by moving the card in a continuous forward motion while causing the write-head currents to change, causing flux changes on the card which correspond to character bits when later read. The bits within a character, as well as the characters, are written in a continuous stream while the card moves forward until the last character has been written. At this time, card travel is reversed and a checkread for record errors is done while the card is moved to its original starting position. Thus, the last character written during the forward motion of the card is the first character to be read and checked for errors during the reverse motion of the card. The control of the writing of bits is accomplished with strict windows of time clocked by the logic. The checkread for errors consists of detecting the flux changes on the card with the read gap of the read/record head, and ascertaining whether the flux changes fall within the acceptable windows of time; if not an error is indicated.

The magnetic card is long enough and the card velocity is such that it is guaranteed that the logic can record a maximum of 103 characters and be able to read them back later on any machine. However, due to the coasting tolerances of the card after the end of the forward drive of the card during recording, it cannot be guaranteed that the card will not coast out from under the read gap on the read/record head, as shown in Fig. 1, when more than 100 characters are recorded on a single track under worst-case conditions. When this does happen, the head is in a no-flux condition. When the card then begins moving in the reverse direction and the logic is gated to look for a transition to begin checkreading the last character written, a flux change is detected when the card moves back under the read gap, and the read gap sees a change from "no flux" to "erase flux". Under certain conditions this flux transition can look like a valid start sequence. If it does, an error would then be detected because no data bits follow the start bit within the acceptable time window. Without a solution to this problem, there is an exposure that an err...