Computer science and technology :
Robert E. Rosenthal · 1987
Fietd buses were introduced to replace the traditional wiring of sensors and actuators to programmable controllers.After a brief summary of the advantages presented by field buses , we examine the features of the interconnected devices and the requirements that these networks must fulfill.We then describe Phoebus, a prototype field bus developed for a machine tool.This implementation is compared to other proposals .In a second part, an architecture based on the OSI reference Model is presented with elements of services and protocols which match the requirements for a machine tool field bus.Finally, we examine a series of problems which are introduced when several automation equipments share the same field bus.Multi PLCs When several reflex automation devices run a set of sensors and actuators over a single field bus, their sample periods are generally different.If these periods are constant, the network can be used optimally by externally synchronizing the different automatons.Otherwise, the field bus will introduce another penalty due to simultaneous requests for acquisition or update.The excess delay will not be constant.Comments When a single automation device (PLC, control loop, CNC) is linked to its sensors and actuators via a field bus, a single master/multiple slave configuration is an obvious choice.When several reflex automatons are connected to the network, the configuration choice becomes more difficult.However, the problems mentionned above show that, due to the real time constraints, the field bus cannot be made transparent and all cycles have to be designed carefully before implementing the network.In that sense, design and management can be made much easier with a single master station. CONCLUSIONA conventional master slave protocol like SDLC can be used to design a field bus for medium speed applications, but entails to much unnecessary overhead for high performance process control.This remark also applies to all other proposals when connecting a single on/off sensor or actuator to the field bus.Care should be taken to minimize the delays due to higher level software that may exceed the transmission time.It has been shown that the "old" single master/multiple slave configuration exhibits good behaviour in case of hard real time constraints.Furthermore, this configuration is much easier to handle when dealing with complex control applications.In these applications and because of the real time constraints, a field bus cannot be made transparent unless used inefficiently.Finally, due to the complexity of handling mutliple scan cycles, it becomes necessary to provide CAD tools for building the network operations.