3. Data Storage 



The acquisition system counts in binary numbers, as do most digital 

 counters, using the two characters, "zero" and "one". These are represented 

 electronically by the two states of a bistable component; on the magnetic 

 tape they are represented by changes in the state of the magnetic coating. 

 Seven locations, called "levels", are assigned across the width of the tape 

 for the digits of the binary system. At any one location a binary one is 

 represented by the presence of a magnetic pole; its absence indicates a 

 zero (Hoagland and Bacon, I960). In any level (sometimes called a "track") 

 the locations are evenly spaced five thousandths of an inch between centers. 

 Since each location contains information to define one "bi nary digit_", it is 

 said to contain one "bit". Note that a bit is the smallest particle of in- 

 formation handled by the electronic circuitry, yet ten bits can represent ^ -Z4 

 any data count from zero to over one thousand. This system handles 1200 

 bits per second, a rate limited primarily by the tape recorder. 



Both magnetic tape and paper tape were considered for the storage of 

 data. I^agnetic tape was selected because the high recording rate required 

 to sample many information channels eight times each second is not practical 

 with paper tape. The arrangement of the information bits on the tape was 

 designed to get the maximum density of information on the tape and to keep 

 computer programming at a minimum (Figure 4). Information is transcribed 

 into the seven levels (tracks) of each frame simultaneously. Zeros and ones 

 are recorded in the seventh level as required by the computer for parity 

 check. This format requires an odd number of ones to be in each frame. 

 The seventh level is reserved by the computer for this parity procedure. 



Because a tape record completely free of errors is virtually impossible 

 to obtain in the field, the format permits the computer to search for and 

 correct deviations from a specific arrangement of information on the tape. 

 One method the computer uses to locate errors is to check that the number 

 of ones in each frame is, in fact, odd. This "odd parity" check is used 

 instead of "even" to eliminate the chance that all seros may appear in one 

 frame. A series of frames with all zeros is used by the computer to signify 

 the end of a data record. 



Each set of data frames is preceded by a frame which contains a binary- 

 coded-decimal (BCD) alphabetic character to identify the data frames which 

 follow it, indicating which signal channel is being recorded. The letters 

 A, B, C, D, E, and J, K, L, M, N are used as identifiers for the ten 

 channels because each of these characters is represented by a BCD code 

 which includes a one in the sixth level. Figure 5 shows the coding for 

 the ten letters. This data acquisition system uses the sixth level as a 

 "flag" to enable the computer to sort the identification frames from the 

 data frames. In Figure 4, the rectangular "boxes" indicate locations on 

 the magnetic tape for individual binary bits of information. The solid 

 black boxes indicate ones, and those with enclosed circles are zeros. 

 Note that a one in the sixth level indicates to the computer the presence 

 of an identification frame. A technique similar to this was introduced by 

 Frank Snodgrass of the University of California, San Diego, in a punched 

 paper tape system. 



