The objective of this article is to realize an in-depth study of MMS semaphores. This article is not a tutorial on MMS semaphores although it does explain to some extent how MMS semaphores behave and can be utilized. What we are trying to achieve is a definitive clarification of the MMS semaphore concept in order to make its use easier to MMS programmers and to encourage its adoption in the industrial applications and products. We compare MMS semaphores to Dijkstra's classical semaphores and show that real differences exist. Other mechanisms are also discussed (monitors, CSP). Examples are provided to illustrate our analysis and references to the semaphore mechanism of some operating systems are given to ease the comparison. We come to the conclusion that MMS semaphores are flexible but cannot be utilized to solve all kinds of problems classical semaphores usually cope with. We also explain why the two kinds of semaphores provided by MMS are redundant.

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MMS is an application layer protocol developed for ensuring the interconnection and inter-working of heterogeneous industrial devices and more conventional computers. It is becoming widely accepted as the main protocol f o r open communications between heterogeneous machines in many areas that are not limited to manufacturing. In this paper, we first analyse the strengths and weaknesses of an MMS server implementation architecture proposed earlier. Then we refine the classification of MMS services and propose a generic architecture for the implementation of MMS servers.

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