KONGSBERG NN791 Modules: NN791.10 CPU 5MHZ I/O Processor Card —

Technical Dossier

Product Details And Specifications

KONGSBERG NN791 Series: Comprehensive Module Range and Technical Overview

The KONGSBERG NN791 series is a distributed control system (DCS) module family developed by Kongsberg Maritime / Kongsberg Simrad, deployed extensively across global heavy industry sectors including offshore oil & gas platforms, petrochemical refineries, LNG processing facilities, and nuclear auxiliary control systems. The NN791 backplane architecture supports modular I/O expansion, CPU processing, and inter-module communication via a proprietary parallel bus. Its ruggedized design meets the environmental demands of continuous-duty industrial installations, with documented installations spanning facilities in Norway, the UK North Sea, the Middle East, and Southeast Asia. The series entered service in the late 1980s and remained in active production through the mid-2000s, placing the majority of installed units now in the mature-to-end-of-life phase of their operational lifecycle.

The Evolution of NN791 Architecture

The NN791 platform was designed as a successor to earlier KONGSBERG modular control architectures, introducing a 5 MHz CPU bus structure that allowed tighter synchronization between I/O processor cards and the central controller. Early revisions (Rev. A/B) used through-hole PCB construction with socketed EPROMs for firmware storage, requiring physical chip replacement for firmware updates. Mid-generation revisions (Rev. C/D) transitioned to surface-mount components and introduced onboard flash memory, reducing field maintenance overhead. Late-generation modules incorporated enhanced EMI shielding and extended temperature tolerance to address installation environments with poor climate control.

Compatibility across revisions is constrained by backplane slot addressing and firmware version matching. Mixing early-revision CPU cards with late-revision I/O modules requires firmware alignment verification. The NN791 series communicates with supervisory SCADA layers via RS-422 and proprietary KONGSBERG serial protocols; direct integration with modern OPC-UA or Modbus TCP infrastructure requires protocol gateway hardware. Facilities operating legacy NN791 installations must account for these integration constraints when planning control system upgrades or partial migrations.

NN791 Full Catalog & Functionalities (SKU List)

CPU & Processor Modules

• NN791.10: CPU 5MHz I/O processor card; central processing unit for NN791 backplane control loops
• NN791.11: CPU 5MHz enhanced variant; extended onboard RAM for larger scan tables
• NN791.12: CPU co-processor card; handles floating-point arithmetic offload for PID loops
• NN791.13: Redundant CPU module; hot-standby failover for critical process control nodes

Digital Input (DI) Modules

• NN791.20: 16-channel 24VDC digital input module; field device status acquisition
• NN791.21: 32-channel 24VDC digital input module; high-density discrete signal collection
• NN791.22: 16-channel 110VAC digital input module; legacy field wiring compatibility
• NN791.23: 16-channel isolated digital input; galvanic isolation for hazardous area signals

Digital Output (DO) Modules

• NN791.30: 16-channel relay output module; dry-contact switching for actuators and alarms
• NN791.31: 16-channel 24VDC transistor output; solid-state switching for high-cycle applications
• NN791.32: 8-channel high-current relay output; motor starter and valve actuator control

Analog Input (AI) Modules

• NN791.40: 8-channel 4–20mA analog input; process variable acquisition (pressure, flow, temperature)
• NN791.41: 16-channel 4–20mA analog input; high-density transmitter loop termination
• NN791.42: 8-channel thermocouple input; direct TC signal conditioning with CJC
• NN791.43: 8-channel RTD input; Pt100/Pt1000 resistance temperature measurement

Analog Output (AO) Modules

• NN791.50: 4-channel 4–20mA analog output; control signal generation for control valves and VFDs
• NN791.51: 8-channel 4–20mA analog output; expanded loop control for multi-valve skids

Communication & Network Modules

• NN791.60: RS-422 serial communication adapter; KONGSBERG proprietary supervisory link
• NN791.61: Dual-port RS-485 adapter; multi-drop field device communication
• NN791.62: PROFIBUS DP slave adapter; integration with third-party field device networks

Power Supply Modules

• NN791.70: 24VDC backplane power supply; primary rail for I/O module bus
• NN791.71: Redundant 24VDC power supply; N+1 power architecture for critical installations

Sourcing Hard-to-Find & Obsolete NN791 Parts

The NN791 series has been formally discontinued by KONGSBERG, with OEM spare parts support withdrawn. The installed base across offshore platforms and onshore process plants continues to operate under extended lifecycle maintenance programs, creating sustained demand for tested replacement modules. DriveKNMS maintains a dedicated inventory of NN791 series modules sourced through controlled decommissioning projects, authorized surplus channels, and direct acquisition from facilities undergoing DCS migration.

All NN791 modules held by DriveKNMS are catalogued by part number, hardware revision, and firmware version where readable. For facilities requiring exact revision matching to maintain backplane compatibility, revision-specific sourcing requests are accepted. DriveKNMS also supports cross-reference identification for modules where original labeling has degraded, using PCB layout and component signature analysis to confirm part identity before dispatch.

Quality Control for the NN791 Range

The NN791 backplane uses a parallel bus architecture with address-decoded slot selection. Functional testing of NN791 modules requires a live backplane environment or a dedicated NN791-compatible test rack. DriveKNMS operates test fixtures that replicate the NN791 backplane electrical environment, allowing full functional verification of CPU cards, I/O modules, and communication adapters prior to shipment.

Test procedures for CPU modules (NN791.10 series) include: boot sequence verification, RAM read/write integrity check, bus arbitration response timing, and EPROM/flash firmware readback. I/O modules are tested under simulated field load conditions: analog input modules are verified against calibrated signal sources across the full 4–20mA range; digital I/O modules are cycled through all channel states under rated voltage and current. Communication modules are tested for protocol handshake integrity and data throughput at rated baud rates. All modules are inspected for capacitor condition, solder joint integrity, and connector pin serviceability before being released to inventory.