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Published by the American Society of Agricultural and Biological Engineers, St. Joseph, Michigan

Citation:  Transactions of the ASAE.  VOL. 43(6): 1939-1946 . (doi: 10.13031/2013.3099) @2000
Authors:   S. R. Evett
Keywords:   SCADA, TDR, Time domain reflectometry, Bulk electrical conductivity, Soil water content, Waveform interpretation, Soil temperature

Despite the increased use of time domain reflectometry (TDR) for measurement of soil water content and bulk electrical conductivity (BEC), there are few public releases of software for TDR system control. The TACQ program, under development since the early 1990s on a wide variety of soils, allows control of multiplexed systems supporting up to 256 TDR probes. The program is DOS-based to ease creation of low-power, embedded computer systems; and to eliminate resource conflicts and timing difficulties inherent to multi-tasking, Windows-based operating systems. So that it can be easily used in embedded systems, the program was written to run with as little as 640 kbytes of RAM and 1 Mbyte of expanded memory, and with a variety of graphics standards and CPUs ranging from 80186 to Pentium. Embedded computer systems based on the PC-104 specification have been implemented using TACQ; and the program has been integrated into a supervisory control and data acquisition system (SCADA) for irrigation scheduling and control. Using a parallel port, the program controls multiplexers from both Campbell Scientific (Logan, Utah), and Dynamax (Houston, Texas); and it allows reading of probes in any user-defined order if using the latter. The user has complete control over multiplexer address assignments, interconnection of multiplexers, and probe locations on each multiplexer; including individual settings for probe length, window width, averaging, distance to each probe, gain, and type of data acquired (waveform, travel time, apparent permittivity, water content, relative voltages for bulk electrical conductivity, or a combination of these). Interfaces to TDR instruments including Tektronix 1502 (modified), 1502B, and 1502C cable testers are implemented through an RS-232 port. Concurrent temperature data may also be acquired (ComputerBoards models CIO-DASx). System power control is implemented through the computers own power management capabilities, and through direct control of power to the TDR instrument and video subsystem where applicable, thus allowing creation of very low-power systems. The program is stable and suitable for use in environmental measurement systems that are unattended for long periods of time. Waveform interpretation methods are discussed in the second paper in this series.

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