I . Channels and Gating Circuitry 



Signa I -cond itioning circuits are included in the system and may 

 be connected to receive signals from (I) instruments with a modulated- 

 frequency output, (2) sensors with low-level outputs, such as strain gages, 

 (3) variable resistance sensors and those with high-level outputs, and (4) 

 instruments with outputs in pulse form, as from the digital wave staff. 

 A block diagram of the data acquisition system is shown in Figure 3. Output 

 signals from all sensors are converted to pulse form prior to being scanned 

 by gating circuitry in the acquisition system. Therefore, the error voltage 

 and cross-talk of an analog multiplexer are not introduced, since scanning 

 of the input channels takes place in digital networks where voltage fluc- 

 tuations do not present a problem. Using digital methods in the remainder 

 of the processing system allows the input variable to be compared to a time 

 reference, which can be held to greater accuracy than a voltage reference. 

 The time base for this data acquisition system is introduced by a crystal- 

 controlled clock pulse generator accurate to two parts in one hundred 

 thousand. The signal from a modu lated-f requency device, such as a Vibroton 

 pressure sensor (United Control Corporation, Redmond, Washington) must be 

 treated individually to obtain sufficient resolution. The data acquisition 

 system has a period counter to measure frequency changes too sma I I to detect 

 with a simple cycle (line-crossing) counter. 



The acquisition system in use at the Scripps Institution of Oceanography 

 has a total of five counter/memory circuits to sample simultaneously one 

 modu I ated-f requency input and three pulsing (digital) or three variable- 

 level (d.c.) inputs. The fifth counter is used to maintain an accurate 

 record of elapsed time. The total number of available input channels is 

 doubled by time-sharing each counter with two input signals. The period 

 counter may be used as a cycle counter; in which case, eight digital, or 

 d.c, inputs may be recorded. 



It should be noted that a single data counter could have been used if 

 all inputs had been time-shared. However, there were several reasons for 

 simultaneous sampl-ing: (I) Phase comparison between data channels can be 

 made without the inconvenience of repeatedly referring to the time channel 

 for the relative time of sampling. (2) Each channel has its own counter, 

 which is used to store information for a sufficient time to al low a simple 

 integrating circuit to provide an analog read-out. 



2. Power Supply 



Power in the field for the acquisition system is usually obtained 

 from automobile storage batteries. Current consumption for the logic 

 circuitry is about 4 amperes. An additional 12 amperes is required for 

 the d.c. to a.c. converter that powers the recorder. About eight 15-minute 

 data runs may be made from a set of fully charged batteries without re- 

 charge. However, it is often convenient to maintain the charge on the 

 batteries with a 20-ampere d.c. generator driven by a gasoline engine. 

 This arrangement has been found to be more practical than to use a 60- 

 cycle alternator which has the difficulty of keeping voltage and frequency 

 constant. 



