Alstom Trains Use New PCB Design for ATC System

Alstom Signaling Inc. in Rochester, N.Y., is continually striving to improve the safety of rail transport systems. Recently the company began an upgrade of its onboard Micro Cabmatic automatic train control system typically used on transit and commuter rail systems. This project dovetailed with infrastructure updates and changes to optimize manufacturing operations at its Rochester facility.

New Backplane Design

Originally the equipment for this ATC system was located entirely on the wayside, but now much of the electronics are carborne installations. This modular microcomputer-based system consists of one or more subsystems that provide train operations, vital safety control functions and two-way communications between a train and wayside devices. The heart of each ATC subsystem is an onboard microcomputer. However, the interconnection of various electronic boards was using a lot of cables and connectors. Cables are prone to wear spots and various maintenance problems. So, a major objective of the Alstom upgrade project was to reduce the cabling by going to a backplane design that allows direct insertion of various functional cards, and takes less space than cable connections.

A backplane can reduce wiring errors, make troubleshooting much easier and simplifies maintenance. It also tends to make the system more tamperproof. In addition, a backplane provides greater flexibility and ease in making changes to the design of functional cards when upgrading the ATC system.

A crucial consideration in the adoption of a backplane design was the fact it required different technology and manufacturing equipment than Alstom currently had in-house. After careful consideration of the cost and timing implications, Alstom decided to investigate outside vendors.

Vendor Selection

The first step in vendor qualification is usually looking for one with expertise and a successful history in supplying the product you need. In the case of a computer backplane, the vendor needs experience in the design and manufacture of boards that accommodate high-speed digital signals without excessive signal latency and pulse distortion. Beyond board engineering, the vendor must have cost-effective manufacturing and test facilities that ensure reliability in the finished assemblies.

In the case of the Micro Cabmatic ATC backplane boards, Alstom wanted to retain control of the design process, just as it did with the PCBs produced in its own plant. It also wanted to create and retain ownership of the board artwork, which it could then send out for bids from qualified vendors. This requires a close working relationship between engineers at Alstom and its vendors.

Key characteristics of a viable backplane vendor include:

• Backplane system design, simulation and validation
• Design and manufacture of the connectors and application machines
• Overall system integration and backplane application expertise
• Careful management of its supply chain
• Component and connector assembly and inspection
• Full electronic and functional testing
• Vertical integration (racks, enclosures, power supplies, wiring, etc.)
• Value engineering (cost effectiveness, design improvements, production engineering)
• Full documentation of its entire engineering, manufacturing and quality control systems

Critical Design Details

Based on Alstom’s own experience in the production of PCBs, its engineers had some definite ideas about the characteristics of a good backplane board. In addition to a cost-effective design, it wanted to be sure it had a robust board that would stand up to the rigors of rail service. This of course meant having reliable card connectors and robust connections when they are installed on the backplane, which is true of all the other components. A critical detail was to add a conformal coating that would improve the backplane’s resistance to environmental conditions.