pressing the External Data Button on the front panel. 



Data on the Primary Data Tape is organized into Blocks of 

 252 Measurements plus a Block Number at the beginning of each 

 Block. Blocks are simply numbered sequentially from the be- 

 ginning of the tape. The block number register is not reset until 

 the tape is changed. Blocking of the data is necessary to allow a 

 computer to read the tape. The Primary Data Tape is written one 

 character at a time (asynchronously); the computer, however, 

 reads it back at fixed speed, one block at a time. Each block on 

 the tape must be transferred at one time, as it is read, into the 

 computer's primary memory (core storage). The Block length 

 used in the Primary Data Tape amounts to 1766 characters 

 (252 7-character Measurements plus a 2-character Block Number). 

 The input buffer in the computer's core storage must therefore be 

 1766 6-bit characters in length (or 1766 8-bit bytes in the IBM 

 System/ 360). This choice of block length requires a buffer which 

 could be accommodated in the core storage of most any computer 

 which might be used to process data from the Oceanic Telescope. 



Before final data reduction and analysis, the Primary Data Tape 

 is examined by a set of preprocessing programs. These programs 

 check for consistency of identification numbers, location of delimiter 

 characters, and the like. From a knowledge of the starting time of 

 the Primary Data Tape and of the Measurement Interval, the pre- 

 processing programs construct an array containing real time versus 

 measured resistance for each of the seven inputs from the Sensor 

 Cable. Any inconsistencies in the Primary Data Tape are flagged 

 in the array. This array forms the input to the Data Analysis and 

 Reduction Programs. 



3.3.2 The Measurement Bridge 



Wherever it is necessary to make very accurate measurements, 

 it is desirable to have the accuracy of the measurement depend on 

 the accuracy of as few "standards" as possible. In most commercial 

 digital ohmmeters, a constant current is forced through the resistor 

 being measured, and the resulting voltage is measured by a digital 

 voltmeter. This scheme has the disadvantage that the accuracy of 

 the measurement relies on the accuracy of both the current source 

 and the digital voltmeter. 



A bridge configuration was therefore selected for making resistance 



215 



