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Ocean Logger

Many data acquisition systems work by having a digitiser card located inside the data acquisition computer - this is good for applications requiring high-speed digitisation, but requires that sensors are located very close by. This is because the signal cables are prone to pickup noise, which can obliterate the data-carrying signal.
Most BAS work environments usually suffer high densities of electro-magnetic noise - this is mainly due to the extensive use made of HF and VHF radio communications and this has been one of the main constraints on the design of the Ocean Logger and Shipboard Three Component Magnetometer (STCM) data acquisition systems by the Antarctic & Marine Engineering Department.
This and many other potential problems have been overcome by using a distributed digital architecture. In a system designed in this way, a digitising module is placed as close as possible to each of the sensors and connected back to the host computer by an RS485 serial data bus. Each module can usually connect to 8 sensors, and 128 modules may be placed on the RS485 bus.
In this configuration, cables to each sensor are kept at a minimum length, thus reducing susceptibility to electromagnetic interference. The modules are low cost (approx £100 for an 8 channel module) and the ability to add or remove modules as required gives the system enormous flexibility and capacity for expansion.
The serial data stream uses the multi-drop RS485 system. RS485 was designed as a robust serial data transmission medium. It only requires 2 wires and can connect up to 128 devices over several kilometres at fast baud rates.
The transmission system is "balanced" and therefore highly resistant to noise contamination - even when using an unscreened twisted pair for the connection. In the Ocean logger, which has been running almost continuously since the James Clark Ross came into service, foremast sensors are connected to modules located locally in junction boxes.
These modules are interrogated every 2 seconds from a PC located in the UIC room. Calibrated data are then displayed on the PC using software written within the LabWindows environment. Connection between the host PC and the modules is via a 4-way cable - 2 wires for the RS485 bus and 2 wires for the sensor and modules power (15Vdc). The Ocean logger PC also interrogates locally installed instruments, such as the Vaisala digital barometer, via its RS232 ports.
The software and hardware has been designed to allow re-configuration of the system from cruise to cruise. Visiting scientists have brought along additional sensors linked to modules purchased by themselves which are simply plugged into an expansion socket on the foremast junction boxes. The software is easily reconfigured to interrogate these additional modules.