A check for continuity and proper location of the identification 

 frames is another method that the computer uses to locate errors. The 

 first five levels of the frames between identification frames are used for 

 actual data storage in straight binary form. There are five binary bits 

 per frame with the five most significant bits recorded first and the five 

 least significant bits recorded in the last frame of the data group. Those 

 between, if any, are recorded in descending order of significance so that 

 in each frame, the most significant bit is recorded in the fifth level 

 and the least significant in the first level. This permits more numeric 

 information per frame than does the BCD system which can record only one 

 decimal digit in each frame. 



it was decided to use two frames, or ten bits, for each data channel 

 to give a measurement resolution of 0.1 percent per sample. Channels E 

 and N, which are used to record elapsed time, have three frames each, 

 enabling the binary time count to reach the equivalent decimal number 

 of 2^^ minus I, or 32,767. 



Section III. SPECIAL SENSORS 



The data acquisition system is designed to accept a wide variety of 

 sensor outputs. Several special sensors that have been designed or modified 

 for use with DAS are described in the following paragraphs. 



I . Digital Wave Staff and (Mini-Digit Staff 



The digital wave staff was designed to give accurate wave height 

 and water level measurements in the surf zone (Figure 6). In principle, the 

 staff operates by scanning in sequence a set of metallic contacts equally 

 spaced on an electrical insulator. As each contact is scanned, an electrical 

 pulse is transmitted over the signal line if the contact is under water. No 

 pulse is sent if the contact is above water level. The result Is a train of" 

 of pulses, the number of pulses representing the height of the water on the 

 staff (Figure 7). Attenuation or noise on the transmission line, within 

 limits, will not affect the read-out, since the information is contained In 

 the number of pulses present, not In their amplitude. 



The operation of the scanning circuitry for the digital wave staff Is 

 illustrated in the logic diagram of Figure 8. The interrogator drives the 

 input lines of the 16 x 8 matrix, applying a negative potential to each of 

 the 16 lines In turn and keeping the remaining lines at a high impedance 

 with respect to ground. Biasing circuitry maintains the high impedance 

 lines at a slightly positive voltage, and the sequential scanning of the 

 lines effects an alternating potential at each of the 128 external staff 

 contacts. The alternating current minimizes electrolysis at the contacts. 



Each of the output I ines of the matrix is selected, in turn, by its 

 line selector gate. The timing is arranged so that all eight output lines 

 are scanned during the time interval in which one Input line is interrogated 





