Fail-safe speed command signal decoder

Abstract

In a typical transportation vehicle control system, coded control information is transmitted to the transportation vehicles on vehicle command signals comprised of multiple message frequencies. A decoder is disclosed for decoding, in a fail-safe manner, information from message words which are comprised of message units carried on the multiple message frequencies. Vehicle speed command signals which are received by an antenna are amplified and frequencies which are substantially the same as the message frequencies are filtered from these signals. The filtered message frequencies are again amplified before message units are separately detected from each message frequency by a related filter-discriminator. The message units detected from a first message frequency by the related filter discriminator are delayed for the number of message units which comprise a message word. The message units detected from a second message frequency by the related filter-discriminator are inverted. If a threshold detector has determined that the filtered signals should be valued, the delayed message units are compared with the inverted message units and, when they correspond, the equivalent message unit is stored in relation to a clock which is synchronized with the rate that message units are detected. A decoding tree decodes information from message words comprised of stored message units. When the delayed message unit does not correspond with the inverted message unit, that message unit and all the stored message units are reset so that there is no information word for the decoder tree to decode. Non-correspondence between the delayed message unit and the inverted message unit may be due to transient responses of the filter, noise received by the antenna, or failure of a component in the receiver. In such case, no control information will be delivered to the vehicle because all the stored message units are reset. Since a no-information condition is deemed to be safe for the transportation vehicle control system, the receiver will fail in a safe manner and may therefore be described as fail-safe.