Sea-Bird Electronics            Products            Support            Software            Sales            Service            Search/Sitemap

Surface Inductive Modem
SIM

[HRule Image]

Print version     Configuration options & accessories

SIMDESCRIPTION

The Surface Inductive Modem (SIM) is a vital link in Sea-Bird’s inductive modem (IM) telemetry systems, which provide data communications without the need for underwater electrical connections. Each system requires:

The SIM (a modem is a modulator/demodulator) impresses (modulates) the mooring cable with a DPSK signal that is encoded with the commands received from the user’s computer/controller. The encoded signal is demodulated by underwater inductive modem instruments coupled to the mooring cable. The underwater inductive modem instrument interprets the commands and transmits replies via the mooring cable, which are demodulated by the SIM. The DPSK communication link between the SIM and underwater modem is half-duplex, so talking and listening is sequential only. Although the data link between the SIM and the user’s computer/controller is established at 1200, 2400, 4800, or 9600 baud, the DPSK modem communication between SIM and underwater modem operates at 1200 baud.

SIM VERSIONS

The SIM is available in two models, SIM-Direct and SIM-Coupled:

SIM-Direct: In a direct connection (typical cable to shore applications), the bottom end of the wire is grounded to seawater, and the top end remains insulated all the way to the connection to the SIM. A second wire from the SIM connects to seawater ground, completing the circuit.

SIM-Direct: Direct Connection

  

SIM-Coupled: In typical surface buoys it is often preferable to connect the jacketed mooring wire to the buoy with a length of chain, grounding the jacketed wire to seawater at each end. An Inductive Cable Coupler (ICC) connects the SIM to the jacketed wire above the uppermost underwater inductive instrument and below the point where the wire is grounded.

SIM-Coupled: ICC Connection

DPSK (Differential Phase Shift Keyed) DATA TRANSMISSION

Sea-Bird's Inductive Modem telemetry system uses a DPSK data transmission method that overcomes most of the disadvantages of Frequency Shift Keyed (FSK) transmission, resulting in superior transmission efficiency and much lower error rates. The Sea-Bird system uses a carrier frequency of 4800 Hz, permitting four cycles of carrier frequency during the time allotted to each data bit (i.e., 1200 baud).

The encoding scheme is straightforward: if the next bit is a one, the phase of the carrier is inverted (shifted 180 degrees); if the next bit is a zero, the carrier phase does not change. With DPSK, both the modulation and demodulation hardware are extremely simple. Modulation requires only an OR gate and flip-flop, and demodulation is inherently coherent (bit energy is averaged rather than spot-sampled) using minimal hard logic, a shift register implementing a one-bit delay being the principle component. Further advantages are that the transmission of all zeros creates a single coherent frequency (4800 Hz) that is readily detected in inductive modem instruments as the ‘wake up’ signal, and that - unlike FSK - the connection polarity of the transformers used for coupling does not matter.

SIM SPECIFICATIONS

Sensor Interface to computer or buoy controller:
Standard -- RS-232C; 1200, 2400, 4800, or 9600 baud; 8 data bits; no parity
Optional -- RS-485

Current:
30 mA when communicating,
27 mA in quiescent state (PwrOff).
With control line, < 10 microamps when turned off.

Power: 7-25 VDC

 

 

ADDITIONAL INFORMATION / LINKS:

Documentation -- manual, photos, technical papers, application notes, etc.

Sales Information -- options, accessories, cables, mount kits, spares, etc.

Links to Other Instruments of Interest

[HRule Image]

Specifications are subject to change without notice. 

Last modified:09-Dec-2010

Sea-Bird Home     Phone: (+1) 425-643-9866     Fax: (+1) 425-643-9954     E-mail: seabird@seabird.com