Communication and coupling subsystems

The link between the SAPHYDATA subsystems is the transmission of data from the observation points to the processing centres. The technical requirements are determined to a large degree by the observational subsystem, the volume of data, and the operational needs for the data.

Water data are utilized for two basic purposes: (1) to define the hydrologic rPgime of an area for instance for design and planning purposes and to develop information on existing conditions of water supplies and (2) the forecasting of short-term andlong-term trends in water supply and the forecasting of floods.

O n the basis of these two major divisions, hydrologic information systems can be subdivided into operational and resource development systems. At the present time these two informational systems may utilize the same sensors and the information transmitted via different routes to the processing centres.

Data collected primarily for water resource planning and devdopment are usually processed on a monthly, semi-annual, or annual basis. The final results are tabulated either in a printed yearbook or stored on punched cards or magnetic tape for future use. Data required for flood forecasting or the operation of water supply facilities are required on a real time basis. Data in the latter category must be transmitted from the observation point to the user immediately. A technology has been developed to fill this requirement on a local, regional basis, but as yet no continental system has been organized. However, with emerging requirements for the full utilization of the water resources on a national or continental scale, systems will have to be designed to meet this need.

Telemetering systems fall into two categories, analogue and digital, with varying degrees of development in each. Digital systems generally are the more reliable means of trans- mitting data and offer a higher degree of accuracy. The purpose of this section is to describe as an example a digital telemetry system designed specifically for transmission of hydro- logic data. In addition to having the advantages mentioned above, this system also provides for data to be received in computer compatible form. The main disadvantage of a digital telemetry system is its high initiaI cost. If, however, the data is recorded at the site in digital form (BCD; binary code decimal) this system can utilize the analogue-to- digital recorder as the encoding device; thus it is quite competitive in cost with a com- parable analogue system. Operating costs can be much lower because more economical transmission facilities can be used.

As data are recorded, they are stored in a memory bank in BCD form so that the infor- mation can be made immediately available for transmission. Recording and storage of data are initiated at a preselected time interval with a contact closure of the timer (Fig. 2.2).

The water stage recorder, ADR No. 1, is continually balanced, therefore its value is immediately recorded. The same contact closure within the timer that activates the water stage recorder also activates the programming device witbin the servo-programmer. The

information systems

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Description of different systems characteristics

programmer sequences Sensor No. 1 (specific conductance) into the system allows time for the servo-drive to balance ADR No. 2 and calls for the value indicated by the sensor to be recorded. It then sequences Sensor No. 2 into the system, balances the ADR, and records the value indicated. This same procedure is repeated until all active channels have been recorded, at which time the programmer returns to the off position and awaits a new initiating signal from the timer.

At the time water stage is recorded on ADR No. 1, a set of electrical contacts within the ADR, representing the recorded value in BCD code, is closed. This value is transferred into the water stage memory storage module through the input sequencing matrix. The sequencing matrix will then advance to the second storage module labeled ‘specific con- ductance’. The value for specific conductance is recorded on ADR No. 2 a few seconds after the recording of water stage on ADR No. 1 as a result of the lag in the servo- programmer. Its value as represented by the set of electrical contacts in ADR No. 2 is transferred to the specific conductance memory storage module through the input sequencing matrix and the matrix will then advance to the next storage module. The sequence is repeated until all parameters are entered into the memory storage modules.

Since the same contact closure of the timer which initiates the recording process resets the input sequencing matrix to its starting position, proper sequence into memory storage modules is maintained or recovered on the next recording in the event of a malfunction.

Upon completion of this cycle, digital data in BCD form representing the parameter measurements are stored in memory and are ready for telemetering at any time. These values will remain in the memory modules until a new value for a particular parameter is recorded, at which time the old value is erased and the new value inserted in its place.

A telemetry network may consist of several remote stations reporting to a central receiving station and each remote station may have several parameters. Therefore it is necessary to provide both station identification and parameter identification. This can be done by placing fixed identification numbers in each memory module within the memory bank. The first module always represents the station identification and is wired in such a manner as to maintain the identification number assigned to it. Each succeeding module represents a parameter and has space for eighteen bits of information. In the case of water stage the first two bits in its module are wired to read the digital equivalent of one and the following sixteen bits (four full digits) represent the value of water stage to the nearest one part in ten thousand. In the other individual parameter modules the first six bits are permanently fixed and represent the parameter identification number ranging from OD to 09. The next twelve bits (three full digits) represent the value of the parameter. This arrangement allows for an accuracy of one part in a thousand.

Parameter identification numbers can be assigned as follows:

Water stage 1

Specific conductance 00

Temperature 01

Dissolved oxygen 02

P H 03

Chloride 04 Turbidity 05

Numbers 06 through 09 are left uncommitted so that they may be assigned to less fre- quently used parameters.

Actual transmission of data is initiated by the programmer at the master central station.

The station identification code of the remote station from which data is desired is selected by the programmer and transmitted to all remote stations. Only the station that has this particular code stored in its station identification storage module responds. Its transmitter is activated, samples the value in the storage identification module, and transmits this value as a series of pulses. These pulses are received at the central station, causing the teletypewriter to type and punch out on paper tape the information transmitted.

The output sequencing matrix then advances to the water stage storage module and these data are transmitted and recorded. This procedure is repeated until all information

from the remote station has been transmitted and recorded. The programmer then calls for the next remote station to report and the entire process is repeated. Many remote stations can be interrogated by a single central station in this manner. Additional central receiving stations may be operated from the same system; however, they perform no control function and they are merely slave stations.

The communications linkage may be leased line (either telegraph or voice grade) radio, microwave, or a combination of these. Rate of transmission is limited by the type of communications linkage which is used. System speed is controlled by plug-in modules in the telemeter transmitter and receiver. These are easily exchangeable so that a system’s speed may be selected to best suit the communication linkage. Maximum rate over a telegraph grade line is 15 pulses per second; therefore approximately two seconds are required to transmit one parameter. Maximum rate over a voice grade line is approximately 2,300 pulses per second and transmission time is reduced proportinately. Rates over both radio and microwave can be considerably faster. However, when combinations of the different types of linkages are used in a single system, transmission rates cannot be faster than the capabilities of the slowest speed linkage.

Because this is a modular system, a high degree of flexibility is obtained. For example, it is not necessary that a teletypewriter be used for recording data; any other digital recording device can be used. Also, accessories such as digital clocks may be added.

Such a system can be adapted to the specific needs of the user.