Universal Fail-Safe Block Interface

Proven Digital Excellence in Railway Block Signalling

The Universal Fail-Safe Block Interface (UFSBI), developed in accordance with RDSO Specification RDSO/SPN/147/2005 later revised and upgraded to IRS:S-104/2012 Ver. 0, is a fail-safe, Triple Modular Redundant (TMR) digital multiplexer designed to transmit 32 relay states (16 transmit + 16 receive) between physically separated railway locations via secure voice or data channels over optical fibre, microwave links, or quad cable.

Conceptualised under the initiative of the Ministry of Information Technology, the UFSBI was developed with expert guidance from IIT Delhi and the Centre for Knowledge-Based Systems at Jadavpur University, with invaluable field support from South Eastern Railway.

Entirely indigenously designed and manufactured at our facility in Kolkata, the system benefits from full in-house engineering capability, ensuring timely after-sales support and a dependable supply of spares throughout its operational life.

Initially approved by RDSO for interfacing traditional Single and Double Line Block Instruments over OFC voice/data channels, the UFSBI has evolved into a versatile fail-safe multiplexing solution. It now plays a key role in Intermediate Block Signalling (IBS) and Automatic Block Signalling (ABS) applications, efficiently replacing hundreds of copper conductors with a single optical fibre pair, significantly streamlining installation and maintenance.

With a proven track record of over 25 years and successful deployment at more than 10,000 locations, the UFSBI stands today as one of the most trusted, cost-effective, and widely adopted solutions for block signalling interfaces in Indian Railways.

Its outstanding performance has been consistently validated across numerous operational sections across all 17 zones of Indian Railways over the past 25 years. Encouraged by this success, newer applications and deployments are underway, further cementing UFSBI’s position as the benchmark in digital railway signalling solutions.

Dual System Cabinet of UFSBI

System Design Features

Galvanic and/or Optical Isolation for Safety & Flexibility: All UFSBI inputs, outputs, and block interface circuits are extended through optically isolated transformer coupled circuits, independent Q-series relays (as per BRS: 930) or dedicated potential-free contacts, offering galvanic isolation from other circuits and electrical potentials within the installation. This ensures safe and reliable performance across both RE (electrified) and non-RE railway sections, even in environments with high electrical noise.
Universal Interlocking Compatibility: The system is engineered to be fully compatible with all types of station interlocking systems, enabling seamless integration into diverse signalling architectures.
Triple Modular Redundancy (TMR): A fail-safe multiplexer architecture operating on 2-out-of-3 logic provides enhanced safety and system availability, even in fault conditions.
Noise-Resistant Software Architecture: The system incorporates fail-safe data structures, message redundancy, and a minimum Hamming distance of 5, ensuring resilient data transmission even in noisy or degraded telecom environments.

Fail-Safety & Regulatory Approvals

Approved by RDSO, as per RDSO/SPN/147/2005 , later revised and upgraded to IRS:S-104/2012 Ver.0, for deployment across Indian Railways.
Software validated to CENELEC SIL-4 standards by the Centre for Knowledge-Based Systems, Jadavpur University.
Hardware independently evaluated and validated by the Department of Electrical Engineering, IIT Delhi.

Functional Role of UFSBI

The UFSBI acts as a fail-safe digital multiplexer, replacing conventional signalling cables for vital relay state repetition across block section lengths, maintaining CENELEC SIL-4 safety in Intermediate Block Huts (IBH) or Auto Signal Huts and end stations. It enables the exchange of 32 relay states (16 each direction) over a single channel—via OFC Voice Channel, dark fibre, 1-Quad copper, or microwave radio links—optimising both cost and complexity of implementations.

Triple System Cabinet of UFSBI

Quad System Cabinet of UFSBI

Key Advantages

Exceptional Reliability and Fail-Safety: Equipped with Automatic Media Changeover, the UFSBI enables seamless transition between OFC and microwave links in the event of a failure, ensuring uninterrupted signalling communication.
Cost and Time Efficiency over Copper Cable based Conventional Signalling: In a typical ABS or IBS section, the UFSBI eliminates the need for a dedicated 6-Quad / 24-core signalling cable, reducing implementation costs by over 50% and installation time from two months to just fifteen days, as the work is limited to indoor setup.
Simplified Deployment: Largely mitigates or completely avoids the need for new copper cable laying, except tail cables, significantly reducing on-site labour, maintenance challenges, and project timelines.
Extended Operational Range: Can operate over distances of up to 30 km using 1-Quad cable, and substantially longer using dark OFC backbone or E1 / Ethernet channels, making it suitable for varied terrain and extended block sections.
Integrated Backup for Maximum Uptime: The same UFSBI can operate on multiple media simultaneously using Automatic Media Changeover units. It can work on diverse optic fibre cable or inter-station E1 channel and/or Quad Cable, providing a backup path for UFSBI communication—ensuring zero interruption in the event of a media damage.
High Noise and EMI Immunity: The UFSBI is engineered for excellent electromagnetic compatibility, making it highly suitable for deployment in railway electrified environments with significant electrical interference.

System Composition of an UFSBI

Input Card: There are 6 Input Cards in an UFSBI unit. The Input Cards are designed to accurately sense relay contact status by providing opto-isolation and converting 24V signals to 5V logic levels. It monitors both pick-up and drop contacts of each relay, ensuring precise input detection. These conditioned signals are forwarded to the corresponding CPU cards, where they are validated using a 2-out-of-3 majority voting logic. This architecture ensures continued system reliability, even in the event of a CPU or Input card failure—supporting robust and dependable operation in safety-critical environments.

Central Processing Unit (CPU) Card: There are 3 CPU Cards in an UFSBI unit. The CPU Card serves as the central processing unit of the UFSBI system, handling both Input signal validation and Output decision-making with high integrity. It receives opto-isolated, noise and surge-suppressed signals from the Input Cards and performs de-bouncing to filter out spurious fluctuations. These stabilised inputs are then validated through inter-processor communication among the three CPUs (A, B, and C), ensuring data consistency via a 2-out-of-3 software voting mechanism. CRC codes are appended to validated data for error checking before transmission to the Control & Communication Card. On the output side, the CPU processes incoming data from the remote UFSBI unit (via the modem) and collaborates with the other processors to securely activate output relays. Relay state decisions are executed through a hardware 2/3 voter and verified independently by each CPU to ensure correct operation. This triple modular redundancy (TMR) architecture provides fault tolerance, making the system robust and fail-safe for critical railway signalling environments.

Output Card: There are 2 Output Cards in an UFSBI unit. The UFSBI Output Card plays a pivotal role in ensuring the reliable and secure operation of the system, meticulously designed to deliver precision and redundancy at every stage. Upon receiving validated data from the Control & Communication Card, the Output Card employs a robust 2-out-of-3 Hardware Voting Unit to ascertain consensus among the three CPUs (A, B, and C). This stringent voting mechanism guarantees that only verified and agreed-upon commands are executed, enhancing system integrity. The card then processes the output through a DC-to-DC converter, ensuring optimal power delivery for downstream components. A dedicated decision read-back feature allows each CPU to independently verify the output state, further reinforcing operational accuracy. While the first card governs relays 1 to 8, the second card manages relays 9 to 16.

Control & Communication (CC) Card: There is only 1 CC Cards in an UFSBI unit. The Control and Communication Card (CCC) serves as the critical link between the local and remote UFSBI systems as well as manages the Inter-CPU communication necessary for ensuring TMR architecture. It performs RS-232 to TTL-level signal conversion, enabling seamless data exchange between the modem and the triplicated CPU modules (CPU_A/B/C) on TDM basis. The card ensures synchronized communication by handling protocol-compliant serial transmission while validating data integrity through embedded checks. Additionally, it drives the health-monitoring relays (BIPR1 & BIPR2), providing real-time system status feedback. By bridging processor outputs with modem interfaces, the CCC guarantees fail-safe data routing in alignment with the 2-out-of-3 voting architecture's decisions.

Input Card of UFSBI

CPU Card of UFSBI

Output Card of UFSBI

CC Card of UFSBI

DC-DC Power Supply Unit: This unit ensures delivering robust and isolated power distribution with unwavering reliability. This sophisticated unit accepts a 24V DC input (with ±20% tolerance) and efficiently converts it into four precisely regulated outputs: 24V DC (isolated), 5V DC, and ±12V DC, each with dedicated current ratings to meet system demands. Its advanced design incorporates comprehensive protection mechanisms, including input over/under voltage safeguards and output short-circuit resilience, while maintaining exceptional efficiency exceeding 75% under all weather with remarkably low ripple (<50mV). Operating in "Hot Stand-By" mode, it ensures uninterrupted performance, complemented by an integrated Power Supply Monitoring Unit (PSMON) that triggers immediate audio-visual alerts at the Station Master's panel during anomalies.

DC-DC Power Supply Unit of UFSBI

IBH-Model with UFSBI & HA-SSDAC on OFC_e

Working Arrangement of IB Signalling using Redundant UFSBI & Dual Axle Counter on Dual OFC

Application of UFSBI as a 16-Input & 16-Output Digital Multiplexer

In high-density traffic corridors of Indian Railways, the Signalling & Telecommunications (S&T) Department plays a pivotal role in enhancing line capacity—without the need for physical track expansion. To achieve this, advanced block signalling methodologies such as Intermediate Block Signalling (IBS) and Automatic Block Signalling (ABS) have been strategically implemented.

These modern signalling schemes typically rely on Single or Multi-Section Digital Axle Counters for accurate track occupancy detection. Traditionally, the transmission of vital relay states between block huts and adjacent stations has been managed through extensive outdoor cabling networks, supported by cutting relays installed approximately every 2 km along the section.

However, this legacy infrastructure demands a complex network of cables and relay huts throughout the block section, resulting in high capital expenditure, costly and time-consuming maintenance, and susceptibility to cable theft and vandalism. Troubleshooting faults in this distributed relay state transmission system is also increasingly challenging.

While Electronic Interlocking (EI) solutions have been introduced for similar applications, they often lead to substantially higher implementation and maintenance costs. Furthermore, EI systems typically require specialist manufacturer involvement for installation, maintenance, or modifications—incurring significant charges and operational delays. This contrasts sharply with the UFSBI system, whose simple and robust design is well understood by Indian Railway S&T staff, enabling them to install, maintain, and modify the equipment independently, without dependence on external vendors.

The UFSBI-based solution thus offers a clear advantage over both traditional copper cabling and Electronic Interlocking alternatives. By replacing bulky cable networks with a digital, fail-safe, and galvanically isolated multiplexer, UFSBI not only reduces infrastructure complexity and cost by over 50% but also delivers superior operational reliability, enhanced noise immunity, and ease of maintenance. This makes it the preferred, highly cost-effective, and user-friendly choice for digital block signalling applications across Indian Railways.

Working Arrangement of Auto Signalling using Redundant UFSBI & Dual MSDAC on Dual OFC

Downloads

Product & Application Presentation for UFSBI (as per IRS:S-104/2012 Ver.0)

RDSO Approval Certificate as per IRS:S-104/2012 Ver.0

User's Manual for UFSBI (as per IRS:S-104/2012 Ver.0)

Deltron Equipment & Systems Pvt. Ltd.