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Andrew Blakeley-Smith BSc (Hons) MIEAust MIRSE

Andrew Blakeley-Smith & Associates

Railway signalling technology has evolved over the years from the primary discipline of the mechanical engineer to the electrical engineer to the IT/computer science engineer. The design philosophy and hardware underlying analogue communications links is very familiar to those with, or exposed to, a telephony background but does not seem to have been adequately covered in most electrical engineering courses.

There are still many analogue links forming essential, and even vital, components of signalling systems. Some of these links, in service for many years, from the author's experience, have never been tested and commissioned properly and fail when relatively minor configuration changes are made. System testing is often prolonged because new communications bearers are not tested systematically. Effort is then mistakenly put into reconfiguration of the equipment at the ends of the bearer, perhaps because the digital interface is better understood.

This paper covers some telephony history and basic principles, some elementary transmission line theory that may have been forgotten (or passed over as no longer relevant or a black art) and discusses, from experience, some practical testing strategies that do not need a truck full of expensive and difficult to drive test equipment.