Surface Inductive Modem
SIM

Print version     Configuration options & accessories

DESCRIPTION

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:

• SIM, housed in a buoy or on land. The SIM provides the link between the underwater IM instruments and the computer / buoy controller. Communication with the computer / buoy controller is via RS-232 (optional RS-485).
• Underwater IM instruments. The SIM can link to up to 100 inductively coupled instruments on a jacketed mooring wire. Compatible instruments include:

• SBE 37 MicroCAT C-T (optional pressure) Recorder -- 37-IM, 37-IMP (integral Pump), 37-IMP-IDO (integral Pump and Integrated Dissolved Oxygen), or 37-IMP-ODO (integral Pump and integrated Optical Dissolved Oxygen).
• SBE 39-IM Temperature (optional pressure) Recorder.
• SBE 16plus-IM or 16plus-IM V2 SeaCAT C-T (optional pressure) Recorder, which can acquire additional data from optional auxiliary sensors (oxygen, fluorescence, etc.)
• Underwater Inductive Modem Module (UIMM) or SBE 44 Underwater IM, which links to a current meter, Doppler profiler, etc. with a standard serial interface.
• Instruments by other manufacturers with built-in Sea-Bird underwater inductive modems.

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.

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

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.

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.

• Manual -- description and programming of SIM is described in manual for each IM instrument (16plus-IM V2, 37-IM, 37-IMP, 37-IMP-IDO, 37-IMP-ODO, 39-IM, 44)
• Comparison of IMM to SIM and UIM
• IM power requirements
• Typical IM Mooring Components
• Print version of this specification sheet / brochure
• Application Notes:
  - 68: Using USB Ports to Communicate with Sea-Bird Instruments
  - 92: Real-Time Oceanography with Inductive Moorings and the Inductive Modem Module (IMM) -- Technical paper provides background information on inductive modems, principles of inductive coupling, mooring configurations, use in fresh water, system components, mooring cable and cable fitting resources, and OEM components available from Sea-Bird

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

• Configuration options & accessories:
  - Details for SIM
  - Listing for all products
• Cables

Links to Other Instruments of Interest

• IM system:
  - Inductive Cable Coupler (ICC) -- for connection to SIM or IMM
  - Inductive Modem Module (IMM) -- alternate surface buoy link in IM system; more flexibility than SIM
• Underwater IM instruments:
  - SBE 16plus-IM V2 SeaCAT C-T (pressure optional) Recorder -- can acquire data from auxiliary sensors
  - SBE 37-IM MicroCAT C-T (pressure optional) Recorder
  - SBE 37-IMP MicroCAT C-T (pressure optional) Recorder -- integral pump
  - SBE 37-IMP-IDO MicroCAT C-T (pressure optional) Recorder -- integral pump & Integrated Dissolved Oxygen (membrane-type sensor)
  - SBE 37-IMP-ODO MicroCAT C-T (pressure optional) Recorder -- integral pump & integrated Optical Dissolved Oxygen
  - SBE 39-IM Temperature (pressure optional) Recorder
  - SBE 44 Underwater Inductive Modem -- integrates RS-232 instruments with IM system
  - Underwater Inductive Modem Module (UIMM) -- integrates RS-232 instruments with IM system, more flexibility than SBE 44, requires external power

Specifications are subject to change without notice. 

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