Manchester decoding algorithm for multifunction vehicle bus

The multifunction vehicle bus (MVB) is a local area network for telematic applications of trains. It uses Manchester encoding in order to interchange data among field devices on board. The physical level is accurately defined since RS-485 standard is followed. So electrical signal are bipolar and the point-to-multipoint structure is the most common topology. The railway communication systems must be hardly reliable, so any error in a received frame supposes discarding it. The decoding algorithm in this paper detects wrong values of voltage along a defined symbol and compares the placement of edges with the theoretical right position. It is based on an over-sampling process, by taking into account the admitted tolerance in the medium. AC model of an MVB circuit has been created in order to validate the decoding algorithm and establish the start point in the design flow. Exhaustive simulations have proved that the decoder can detect any error in the symbol level with a resolution of 41.67 ns, 12.5 % of the symbol time and that all misplaced edges are rejected and correct ones might be discarded in a range of 25 ns, 7.5 % of the symbol time. In addition, the decoder has been described in VHDL and synthesized in a EPM3032ALC44 of Altera. It has been arranged in the following modules: the start bit detector, a clock signals generator, a shift register and the edges detector.