Many instruments that operate in the marine environment use microcomputers for control and data-logging. RALPH, designed to study seabed sediment transport processes, is such an instrument. Although the design was started in 1987 and useful data have been collected since 1980 (DeIure, 1983; Boyd et al., 1983) RALPH, like many research tools, has been in a constant state of development. This development has mainly involved the additional of sensors and the provision for more data-logging. These two thrusts have led to RALPHNET.
To reduce the number of cables and connectors, the initial criteria of RALPH's design called for housing all electronics and most sensors in one large pressure vessel. This also facilitates interfacing the various components to the computer which controls all sensors and the data-logging. However, some of the sensors (e.g. flow meters) are required to be outside the pressure vessel on contact with the environment. As RALPH grew, more cables and more computer interfaces were required. In 1984 the concept of RALPH as an underwater local area network of special-purpose computers was adopted. This concept solved the problems cited, gave other unexpected advantages and has also found application in unrelated instruments.
This paper first gives a brief description of RALPH and the components which have been added to it. The details of RALPHNET are then described, its advantages listed and some other applications cited. An Appendix contains a more detailed description of RALPH and several of its components.
RALPH (Heffler, 1984) may be used from the outer limits of the surf zone (5 m depth) to the edge of the continental shelf (200 m depth). It is operated for periods of up to a month, and may be independent of cables to the surface or to the shore. Power is supplied by internal batteries and data are logged internally. This enables RALPH to continue sampling during a storm, the time when sediment transport is maximum and when most instruments cannot be used.
RALPH is an instrument system which includes a main pressure vessel housing a computer, battery power and some sensors. Additionally, various external devices are connected to this main pressure vessel, all components are mounted on a tripod structure. The external devices contain microprocessors which are interfaced to the computer in the main pressure vessel. RALPHNET is a network of the RALPH computer, the external devices and an operator's terminal. Figure 1 shows a block diagram of RALPH.
Fig. 1. A block diagram of RALPH components showing RALPHNET
A Local Area Network (LAN) is method of connecting a variety of computers so they can transfer information. Often one device is considered to be the master, but other devices-slaves-are able to transfer data without intervention of the master. Three simple topologies are obvious when connecting several slaves to a master. Figure 2 shows a star, a ring and a bidirectional line. The star uses many connections and interfaces to the master. The ring is sensitive to the failure of one device.