Case Study: Custom modem for railway communications
The humble modem, operating at all of 2400 baud, is not exactly cutting edge technology, but to this day it is the communications engine that drives London Underground’s passenger information displays to be found on each platform.
Each line has a central control room from where train information is continually broadcast over hard-wired links that runs alongside the tracks between each station. Receivers at each station pick up the relevant data from the system, pass it to a message routing system and ultimately on to the display drivers for the platform signs.
BVM has supplied much of the message routing system, so, when the original modems were first declared obsolete by the original manufacturer, who then subsequently went out of business, it seemed as though it should be a relatively simple matter to design and manufacture an up-to-date replacement for the obsolete item. BVM's knowledge and experience of the system made the company the obvious choice to produce an up to date replacement for a 25 year old design. A wholesale replacement and update of the network was not an option because of the problems of limited physical access, safety requirements and the time required to obtain type and systems approvals meant that the availability of system spares was already causing difficulties. In addition to ongoing service requirements, a new project to implement platform signs on all stations throughout the London Underground network meant that another source of modems was imperative.
When the project was examined in detail, the challenges became clearer. The main issues primarily arose from the fact that the interconnects follow, quite logically, the Underground tunnels, with the cables hanging on brackets on the wall. There is minimal screening on the cables, so running low amplitude signals, albeit at very low data rates, through cables that are frequently within a metre or so of electric trains running on DC, in tunnels that are heavily contaminated with conductive dust, is not the ideal environment when high signal integrity is a key requirement.
The modem itself is non-standard, operating on a custom specification independent of the well-known CCITT Specifications, with an operating frequency range of 350Hz to 50kHz and a maximum data rate of 2400 baud. Actual operating frequencies are 2.5kHz and 4.0kHz.
The manufacturers of the original modem had rightly assumed that noise rejection would be a major issue, so, back at some point in the 1980s, a custom modem had been designed with bulky heavy duty wound components to filter out the noise. The high degree of noise on the line, picked up as both conducted and radiated interference, required a 45dB rejection input filter, implemented in the original design as a purely passive 5 stage Chebychev filter using physically large inductors, transformers and capacitors, with transformers to isolate the input from the rest of the device.
When it came to designing an up to date replacement, in addition to replicating the electrical characteristics and noise rejection of the original units, a solid state implementation was required so that the modems could be physically downsized in order to be easily mounted within the message routing equipment already supplied by BVM. The new design would also provide two modems per unit so that hardware requirements in stations with multiple lines could be reduced and also provide a degree of built-in redundancy. Environmentally, the finished units are classified as Signalling and Telecommunications Apparatus and therefore need to meet the requirements of EN50121-4:2000 for EMC compatibility.
The new design consists of an input isolation transformer, an input amplifier, the filter network, a demodulator with an active RS232 output and ESD-protected RS232 transceivers. The key element is the active 5th order band pass Chebychev filter, with a Lower Cutoff of 1.25kHz, an Upper Cutoff of 4.8kHz and ripple of 0.1dB. The pass-band gain of the filter is -12dB, allowing maximum dynamic range without clipping.