Microprocessor interlockings: designing network equations

Railroad signal systems have evolved over the years to be one of the safest systems in the transportation industry. With the expanding use of microprocessor-based interlocking systems, the implementation of signalling logic using Boolean equations needs to be done with methods that retain the inherent safety in designs previously achieved using vital relays. Most circuits used in signalling can be represented quite well using Boolean logic. However, the author has found that networks, such as route check, home and distant networks used extensively in railroad signalling, pose a unique set of problems when they are represented in Boolean logic using conventional methods. These problems are especially severe when implementing the larger three or four track interlockings. This paper describes a new technique that can be used to program vital and nonvital microprocessor systems using Boolean logic. This technique implements the traditional relay networks by using what are called network node equations. A discussion of the problems using conventional methods is included. The network node equation method is introduced and the solutions to these problems are demonstrated. The method described allows the reader to program networks in a fashion that retains all the advantages of the relay style network. An example of an actual implementation of a full universal four track interlocking using this method is included as a model.