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MS1/MS2: acyclic data

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In the case of PROFIBUS, class 1 and 2 masters are differentiated from slave devices.  These devices communicate using different communications models.

Model of the PROFIBUS communication

Model of the PROFIBUS communication

A controller communicates with its periphery using the MS0 protocol. This protocol has been described in a previous chapter and will not be explained further here.


In this chapter we consider how a controller (class 1 master) or engineering station (class 2 master) exchanges acyclic parameters with the slave.


Smart slave devices (e.g. in process industries) require expanded communications capabilities. This includes the acyclic exchange of data, an expanded alarm model and the introduction of complex data structures. In this chapter the necessary extensions to the standard DP-V0 protocol are explained. We assume that the reader is familiar with the fundamentals of the PROFIBUS DP-V0 protocol.


These extensions are designated as DP-V1. They have been structured so that a DP slave can always communicate with a standard master with DP-V0 basic functions.


In the standard PROFIBUS-DP a class 1 master can exchange data with a DP slave. A class 2 master can simultaneously communicate with the class 1 master and check the configuration. However, when accessing the DP slave, the class 2 master is severely limited in its functions: cyclic process data can only be read, but not written.


In the extended protocol DP-V1 additional parameter data is specified alongside cyclic process data. This parameter data can be read and written by the class 1 and 2 master via special communications relationships. In addition, alarm handling is introduced for extended diagnostics.


We will therefore have to address the following chapters:

-How is parameter data modelled in these devices?

-How is acyclic communication between the class 1 master and slave established and managed?

-How is acyclic communication between the class 2 master and slave established and managed?

-How are the alarms handled?


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