PROFIBUS - Overview

Theory

The network access technique for PROFIBUS corresponds to the "Token bus" method specified by EN 50170, Volume 2 for active stations and the "Master/slave" method for passive stations. The access technique is independent of the transfer medium.
All active stations (masters) form, in a pre-specified sequence, the "logical token ring" whereby each active station is aware of the other active stations and their sequence in the logical ring (the sequence is independent of the topological arrangement of the active stations on the bus).
The right to access the medium (the "token") is passed from active station to active station in accordance with the sequence specified by the logical ring. When a station receives the token (addressed to it), it has permission to send telegrams. The time allowed is specified by the so-called token holding time. Once this has elapsed, the station is only permitted to send one more high-priority message. If the station is not waiting to send a message, it passes the token onto the next station in the logical ring immediately.
The corresponding token timers ("max. token holding time", etc.) are configured for all active stations.
If an active station is in possession of the token and connections to passive stations are configured for it (master/slave links), these passive stations are queried (e.g. variables are read) or data is sent to them (e.g. set-point values).
Passive stations never receive the token.
This access technique allows stations to be added and removed under operating conditions.

1. Applications for PROFIBUS

The PROFIBUS sub-net for the cell and field level supports the exchange of information between field devices and with systems at a higher system level. It is used to transfer small to medium quantities of data. In SIMATIC S7, a CP is always required for PROFIBUS.
The following services can be used simultaneously on PROFIBUS
- FDL, FMS and S7 functions or
- DP, FDL and S7 functions.
Defined bus circulation times are guaranteed by the token passing technique.
PROFIBUS-DP allows data to be exchanged between master and slave stations from different manufacturers without the need for special adaptation of the interface.
In SIMATIC S7/M7, PROFIBUS-DP interfaces have been integrated into the CPUs (second interface for S7 CPUs or interface sub-module for M7 CPUs). To the customer, this means:
- the single-master with integrated interface guarantees faster response times (1 to 2 ms for 12 Mbit/s).
- integrated interfaces are less expensive than separate interfaces and consume less space.
? Extremely fast response (1 to 5 ms) on querying DP slaves.
PROFIBUS allows data to be exchanged between stations via FMS or FDL without the need for special adaptation of the interface.

2. FMS Interface

The FMS interface (open communication on Layer 7 of the ISO reference model according to the PROFIBUS standard) is used principally to connect non-Siemens systems to PROFIBUS. Data volumes of up to 240 bytes can be transferred.
The specific advantage of the FMS service is that the data structures are transferred in a neutral format and then converted in the communication partner. In the user programs of the stations, you can still use the respective "programming language" regardless, e.g. STL for SIMATIC S7 and C for the PC applications.
The FMS services comprise variable services for structured data (variables) and administration services.

2.1. Features
For open communication, there are special blocks on the SIMATIC S7 which support FMS services.
The FMS interface supports simple data transfer between two communication partners without an acknowledgement at the user program level via a link configured using STEP 7:
- from a SIMATIC S7 with PROFIBUS-CP
- from a SIMATIC S5 with PROFIBUS-C
- from a PC/programming device with PROFIBUS-CP
- Non-Siemens systems that support FMS services.
All global S7 variables such as bit memories, inputs, outputs and structured DBs are mapped onto VMD/VFD-specific communication variables.
These variables within a VMD are usually identified by names.

2.2. Links
FMS links configured using STEP 7 are required for the communication FBs.
These FMS links are established when the stations start up and remain established permanently, even when the S7 CPU switches to the STOP state.
When an S7 CPU restarts (warm), the links are not established anew.

2.3. Blocks