Inductive Modem Module (IMM)

Inductive Modem Module (IMM)

Link between the underwater IM instruments and the computer / buoy controller. Communication with the computer / buoy controller is via RS-232.


Inductive modem telemetry is a proven technology for real-time communication between underwater devices and moored buoys. Early inductive modem systems were instrument-specific and highly constrained. The Inductive Modem Module (IMM) sets a new standard for inductive modem telemetry and opens the door to a new generation of compact, low-power devices for real-time applications. The IMM is designed to interface with existing devices or to be placed as a component on printed circuit boards in instruments such as the SBE 37-IMP, 37-IMP-ODO, 39-IM, and a growing selection of moored instruments made by other manufacturers.


  • Highly integrated, low-cost OEM component.
  • Very low power: 15 mW active, 300 µW idle.
  • Compatible with all Sea-Bird Inductive Modem (IM) products.
  • User-enabled wake-up tone detect.
  • RS-232 or logic level serial interface.
  • Optional hardware handshake lines (1 open collector output, 1 input).
  • Binary data support.
  • 8 Kbyte FIFO reply buffer.
  • Supports direct cable connection to IM line with additional transformer.
  • User-adjustable timeouts and delays.
  • 120 bytes/second IM transmission for characters > 0x20, TAB, CR, and LF.
  • Multiple transmit power levels.
  • IM line status monitor detects transmitting devices.
  • FSea-Bird's field-proven IM telemetry, with more than 3000 Sea-Bird IM instruments deployed since 1997.
  • Five-year limited warranty.


An IM system provides reliable, low-cost, real-time data transmission for up to 100
IM-enabled instruments using plastic-coated wire rope (typically 3x19 galvanized steel) as both transmission line and mooring tension member. IM instruments clamp anywhere along the mooring, which is easily reconfigured by sliding and re-clamping instruments on the cable. In a typical mooring, an IMM in the buoy communicates with IM instruments and interfaces to a computer/data logger (not supplied by Sea-Bird) via RS-232. The data logger is programmed to poll each IM instrument for data, and sends the data to a satellite link, cell phone, etc.


IMM advanced features assist existing techniques and support development of the next generation of mooring systems:

  • Output Tags — The IMM uses markup tags to pass data to the Host in a structured format. Markup tags are key words that identify the associated data. The similarity to XML is a convenience for data output only; the IMM does not require XML input. The markup tags allow a more structured approach to data processing without compromising traditional methods. IMM Output Tags and Data Tags should not be treated as XML unless the user-settable data follows the rules of XML. The IMM does not force XML compliance on users.
  • Host ID String — The Host ID is a user-settable 64-character string stored in the IMM's persistent memory. It provides a means of identifying the instrument connected to the IMM without interacting with the instrument itself.
  • Host Data File — The Host Data File is a general-purpose, 4-Kbyte persistent memory space in the IMM for application-specific information. It can store data describing the Host instrument, binary code for Host data processing, XML data, the Host command set, or network-specific interface data. The Host Data File can be uploaded through the IM interface.
  • Serial Number Addressing — Previous IM systems used user-programmed device IDs to address instruments on the network. If two devices were accidentally deployed with the same ID, neither instrument would communicate properly. The IMM solves this problem with optional Serial Number Addressing. Commands may be addressed to a specific IMM's 8-digit decimal serial number. IMM serial numbers are programmed at the factory and cannot be modified.
  • Group Command Addressing — Group commands allow addressing of pre-defined instrument groups. For example, it may be useful to put all SBE 39-IM temperature recorders in Group 1, all SBE 37-IMP MicroCATs in Group 2, etc. allowing you to set / reset parameters in similar instruments at the same time. A maximum of 9 groups can be defined per mooring.
  • Discovery — Discovery is the automated process of identifying devices connected on an IM network. This allows recovery from deployment or configuration errors and has considerable potential in more complex self-configuring systems. The Host starts the discovery process with the DISC command. The IMM responds in seconds with a list of serial numbers of IMMs detected on the network.
  • Sample Data Memory — Sample Data is a 16-Kbyte persistent memory space reserved for passing data from IMM-equipped instruments to data collectors or buoy controllers. Sample Data Memory is limited to 40 stored samples. Sample Data provides a level of independence between devices collecting data and devices recording data. Each new instrument can be programmed to write data to the IMM's Sample Data Memory. The buoy controller periodically checks IMMs for new samples, uploads data, and clears successfully uploaded samples. The buoy controller can tag the data with the Host ID stored in the IMM, allowing it to use the same code to upload data from any IMM-equipped instrument.



3.1 x 7 x 1.6 cm (1.2 x 2.8 x 0.6 inches)
Note: Photos above show electrical pins at bottom, to plug into electrical connection below it. See drawing 41453 for a side connection.

Power Requirements

Communicating with 1 or more IM instruments requires use of Surface Inductive Modem (SIM) or Inductive Modem Module (IMM), which provides standard serial interface (RS-232) between user’s computer & up to 100 IM-compatible sensors coupled to single cable. An Inductive Cable Coupler (ICC) is always required if specifying SIM-COUPLED, & is often required if specifying an IMM.

ICC Version 4a is a 20-turn coupler, while Version 4b is a 100-turn coupler. Other couplers are available for OEM applications (contact Sea-Bird for ordering information). Table below summarizes power requirements for each:

  20-turn ICC
(version 4a)
100-turn ICC
(version 4b)
8-turn ICC
(OEM version)
Direct Cable Connection
(no ICC)
SIM 35 mA 20 mA Not allowed 20 mA
Low power transmit setting 1
Not allowed 1.2 mA Not allowed 1.3 mA
Full power transmit setting 1
12 mA 4 mA Not allowed 40 mA
Underwater Inductive Modem (UIM) 2 Not allowed Not allowed 15 mA Not allowed

1 IMM power setting is set with SetEnableFullPwrTx= command.
2 Underwater Inductive Modem is a component used by OEMs to integrate Sea-Bird's inductive modem technology with a third-party sensor.


  • Logic level serial interface — 33062
  • Mechanical — 41453
  • PC serial cable with 9V power lead — 33049
  • Test coupler — 41440
  • Test coupler with 9V power lead — 41452
The list below includes (as applicable) the current product brochure, manual, and quick guide; software manual(s); and application notes.

For older Inductive Modem Module product manuals, organized by instrument firmware version, click here.


Inductive Modem Module

Communicating with 1 or more IM instruments (SBE 16plus-IM, 16plus-IM V2, 37-IM, 37-IMP, 37-IMP-IDO, 37-IMP-ODO, 39-IM, 44, or UIMM) requires use of Inductive Modem Module (IMM), which provides standard serial interface (RS-232) between user’s computer & up to 100 IM-compatible instruments coupled to single cable. IMM is housed in buoy or on land. Order IMM with ICC, or order IMM with 41456.
Note: SIM can be used in place of IMM.
IMM Inductive Modem Module (IMM) Developers kit, includes PN IMM.100 RS-232 or logic level modem module, straight connectors. Includes inductive test coupler (801588), IMM-to-computer serial cable (801583), 9V battery (other connector orientations also available, consult factory)  
    For typical cable-to-buoy applications, it is often preferable to connect jacketed mooring wire to buoy with length of chain, grounding jacketed wire to seawater at each end. Inductive Cable Coupler connects IMM to jacketed wire above uppermost underwater inductive instrument & below point where wire is grounded. Order IMM & Inductive Cable Coupler (ICC); see ICC listing.
41456A.1 IMM adapter PCB for direct connection method between surface buoy and top of mooring cable (DN 41546). Full size board.

Provides external transformer for typical cable-to-shore applications where bottom end of wire is grounded to seawater, & top end remains insulated all way to connection to IMM. Second wire from IMM connects to seawater ground, completing circuit. Order IMM & 41456A.1 or 41456A.2.

41456A.2 IMM adapter PCB for direct connection method between surface buoy and top of mooring cable (DN 41546). Half size board.


Many cables, mount kits, and spare parts can be ordered online.


  • 801583 To computer COM port, 0.25 m, DN 33049

Compare Inductive Modem Instruments

SBE Measures
(C, T, P)
Auxiliary Sensors Memory Power Comments
Internal External
SBE 16plus-IM V2 SeaCAT C-T (P) Recorder C, T, P* 6 A/D; 1 RS-232 64 Mb   Optional pump
SBE 16plus-IM SeaCAT C-T (P) Recorder
C, T, P* 4 A/D; optional RS-232 or PAR 8 Mb   Replaced by SBE 16plus-IM V2 in 2008
SBE 37-IM MicroCAT C-T (P) Recorder C, T, P*   8 Mb    
SBE 37-IMP MicroCAT C-T (P) Recorder C, T, P*   8 Mb   Integral pump
SBE 37-IMP-IDO MicroCAT C-T-ODO (P) Recorder C, T, P* Integrated DO 8 Mb   Integral pump; Replaced by SBE 37-IMP-ODO in 2014
SBE 37-IMP-ODO MicroCAT C-T-ODO (P) Recorder C, T, P* Integrated Optical DO 8 Mb   Integral pump
SBE 39plus-IM Temperature (P) Recorder T, P*   64 Mb   Includes internal USB connector for fast data upload
SBE 39-IM Temperature (P) Recorder T, P*   32 Mb   Replaced by SBE 39plus-IM in 2016
SBE 44 Underwater Inductive Modem (UIM)   1 RS-232 30 Kb buffer Integrates sensor with standard serial interface (current meter, Doppler profiler, etc.) to inductive modem telemetry system
Underwater Inductive Modem Module (UIMM)   1 RS-232 16 Kb Sample Data Memory   Integrates sensor with standard serial interface (current meter, Doppler profiler, etc.) to inductive modem telemetry system; more flexible and economical than SBE 44

C = conductivity, T = temperature, P = pressure, DO = dissolved oxygen
* = optional

1. See Inductive Modem Module (IMM) or Surface Inductive Modem (SIM), and Inductive Cable Coupler (ICC) to complete your inductive modem system.
2. See Application Note 92: Real-Time Oceanography with Inductive Moorings and the Inductive Modem Module (IMM) for background information.
3. products are no longer in production. Follow the links above to the product page to retrieve manuals and application notes for these older products.