Navitron Systems NT925 HDI & NT920 DHR MKII True Heading Interface

Navitron Systems NT925 HDI Heading Data Interface & NT920 DHR MKII True Heading Repeater

The Navitron Systems NT925 HDI (Heading Data Interface) paired with the NT920 DHR MKII (Digital Heading Repeater) forms a multi-protocol marine heading conversion and wheelhouse display package. Engineered by Navitron Systems Ltd. (UK), the NT925 HDI bridges legacy electro-mechanical gyrocompasses (Synchro and Step-by-Step) and pulse-output speed logs with modern digital bridge architectures.

The interface simultaneously converts raw analog or step heading signals into four distinct output protocols across six isolated channels—eliminating ground loops, common-mode interference, and impedance loading on primary shipboard gyro transmission lines. When coupled with the NT920 DHR MKII repeater head, bridge officers gain a high-contrast, night-vision-optimized digital true heading readout directly at secondary steering stations, bridge wings, or engine control rooms.

Model Details

• Primary Interface Unit Model: NT925 HDI (Heading Data Interface — Firmware/Software V3.0 onwards)
• Primary Interface MPN: NT925HDI / NT925 HDI
• Companion Display Unit Model: NT920 DHR MKII (Digital Heading Repeater Mark II)
• Companion Display MPN: NT920DHR/2 / NT920 DHR MKII
• Assembly / Kit Reference: Navitron Systems True Heading Conversion & Repeater Suite (11–40 VDC)

Compatibility

• Compatible Gyrocompasses (Input Side)
  • Anschütz: Standard 4, Standard 6, Standard 14, Standard 20, Standard 22 (Step-by-Step 24V/35V/70V & Synchro)
  • Sperry Marine: Mk 37, Mk 27, SR-120, SR-180, Navigat X MK1 / MK2 (Synchro 1:1, 36:1, 90:1, 360:1 & Step)
  • Tokimec / Tokyo Keiki: ES-11, ES-110, TG-5000, TG-6000, TG-8000 (Step & Synchro)
  • Yokogawa: CMZ-200, CMZ-300, CMZ-500, CMZ-700, CMZ-900 series
  • Plath / C. Plath: Navigat II, Navigat III, Navigat X
• Compatible Receiving Bridge Equipment (Output Side)
  • Navitron Autopilots & Repeaters: NT921G, NT951G, NT990G, NT991G MK2, NT777 MK2, NT888G, NT920 DHR / NT920 DHR MKII (up to 4 heads), NT920 AHR MKII
  • Radar / ARPA & ECDIS Systems: Furuno FAR-21×7 / FAR-28×7 / FR-15xx / FR-21xx (via Furuno AD-10 5V/12V ports), JRC JMA-5300 / JMA-9100, Kelvin Hughes MantaDigital, Transas Navi-Sailor, Kongsberg ECDIS
  • VDR / S-VDR & AIS: Consilium, Danelec, Rutter, Jotron, Saab R5/R6 AIS (via IEC 61162-1 / NMEA 0183 RS422 ports)

Features

1. Simultaneous Quad-Protocol Signal Distribution: Converts a single Synchro or Step-by-Step gyro input into NMEA 0183 (RS422), Furuno AD-10, Step-by-Step, and Navitron Serial formats concurrently without external splitter boxes.
2. 1 kV Galvanic Opto-Isolation: All gyro inputs, speed log pulse inputs, and dual NMEA 0183 output channels feature optical isolation to protect primary gyrocompass transmitter boards from downstream short circuits and ground faults.
3. Dual-Channel Independent Furuno AD-10 Ports: Each Furuno output port allows independent PCB jumper selection for update rate (5 Hz or 40 Hz), angular resolution (0.1° or 0.166°), and signal logic level (5 VDC or 12 VDC).
4. Combined Heading & Speed Log Multiplexing: Accepts 200 ppNm or 400 ppNm speed log pulses (10–70 VDC) and outputs synthesized $VDVHW (Water Speed and Heading) alongside $HEHDT and calculated Rate of Turn ($HEROT) sentences.
5. Fail-Safe Bridge Monitoring & Night Ergonomics: Incorporates a volt-free SPDT power-failure alarm relay, automated serial data watchdog alarms (no dAtA / dAtA Err), and variable red backlighting on both the NT925 HDI and 25 mm NT920 DHR MKII LCD.

Applications

• Commercial Vessel Gyro Retrofits: Upgrading legacy analog/step gyrocompasses to feed modern ECDIS, ARPA radars, and Voyage Data Recorders (VDR/S-VDR).
• Autopilot Heading Reference Conditioning: Supplying stabilized, high-update-rate (10 Hz or 24 Hz) true heading data to Navitron NT921G, NT951G, NT990G, and NT991G MK2 autopilot systems.
• Secondary Steering & Wing Repeater Stations: Driving up to four NT920 DHR MKII digital heading repeaters across wheelhouse consoles, bridge wings, and aft docking stations.
• DP & Survey Vessel Sensor Integration: Providing isolated $HEHDT and $HEROT (Rate of Turn at 1°/min resolution) data streams to hydrographic survey computers and dynamic positioning interfaces.

Part Number Cross-References:

• Navitron NT925HDI / NT925-HDI / NT925 HDI
• Navitron NT920DHR/2 / NT920DHR MKII / NT920 DHR MK2 / NT920DHR MK2
• Supersedes / interfaces alongside earlier NT920HDI and NT920DHR/1 series units

Frequently Asked Questions

What is the primary function of the Navitron NT925 HDI when paired with the NT920 DHR MKII?

The Navitron NT925 HDI acts as a central heading data converter that translates Synchro (1:1 to 360:1) or Step-by-Step (3, 6, 12, or 24 steps/degree) gyrocompass signals into simultaneous NMEA 0183 (RS422), Furuno AD-10, Step-by-Step, and Navitron serial outputs. The NT920 DHR MKII connects directly to the NT925 HDI serial output (or any standard NMEA 0183 heading source) to provide a 25 mm digital true heading display at remote bridge or steering locations.

How many NT920 DHR MKII digital heading repeaters can be driven by a single NT925 HDI unit?

A single NT925 HDI interface can drive up to four (4) Navitron NT920 DHR MKII repeater heads simultaneously via its dedicated Navitron Custom Serial Heading Data port (Terminals 29–32), which also supplies 12 VDC auxiliary power to the repeaters.

Does the Navitron NT925 HDI generate Rate of Turn (ROT) and Speed over Water NMEA sentences?

Yes. The NT925 HDI internally calculates Rate of Turn from the incoming gyrocompass heading signal and transmits $HEROT sentences (at 1 Hz or 5 Hz with 1°/min resolution) alongside $HEHDT true heading. When a 200 or 400 ppNm speed log pulse input is wired to the unit, it also transmits combined speed and heading via the $VDVHW sentence.

Can the NT920 DHR MKII operate independently without the NT925 HDI?

Yes. Because the NT920 DHR MKII features an opto-isolated input port compatible with standard NMEA 0183 heading sentences ($HEHDT, $HCHDG, $HCHDM), it can be wired directly to a GPS compass, fiber-optic gyrocompass, or autopilot NMEA output when Synchro or Step-by-Step signal conversion is not required.

What voltage supply is required for the NT925 HDI and NT920 DHR MKII?

Both units operate across a wide marine DC input range of 11 VDC to 40 VDC, making them directly compatible with standard 12 VDC and 24 VDC vessel battery banks without external DC-DC voltage converters.