Using the PROFIBUS-DP Compact Mode

Compact Mode is one of two modes that can be used with the RMC PROFIBUS-DP module. The other mode is called Message Mode, which is described Using the PROFIBUS-DP Message Mode. Be sure to read both topics and consider each carefully before choosing the mode you will use.

Compact Mode uses two 16-bit input registers and two 16-bit output registers per axis. These registers are described in detail in Input registers and Output registers. The role of PROFIBUS-DP is to carry the commands given from the master through the Output registers to the RMC100, and to carry requested data from the RMC100 through the Input registers back to the master. PROFIBUS-DP achieves this by continually sending these registers back and forth.

Compact Mode gives two choices for synchronizing this data within the PROFIBUS master, called Compact Mode with Sync and Compact Mode without Sync.

Compact Mode with Sync

This sub-mode provides one additional input and one additional output register called the Synchronization Input and Synchronization Output registers respectively. In this mode, commands are not processed by the RMC until the Synchronization Output register changes. When this register changes, all commands that change on all axes are processed. As a result the PROFIBUS master’s data-update cycle can be asynchronous with its programming cycle. For example, some PLCs do not guarantee that the PROFIBUS I/O will be done at the top of the ladder logic scan. Therefore the input register could change at any time during the PROFIBUS scan.

The following general steps should be taken in a control program:

Write all new commands and command values to the Output registers.
Increment the Output Synchronization Register.
Wait for the Input Synchronization Register to match the Output Synchronization Register.
Process data in the Input registers.
Repeat the process.

Note: When using Wonderware’s InControl with sequential function charts, it is necessary to increment the Output Synchronization Register in a step by itself, rather than in a step that also updates the command or command values. The reason for this is that InControl updates all the output registers at once, and apparently can have the data sent to the RMC100 module in the middle of updating the block. Therefore, some commands can be separated from their command values.

Compact Mode without Sync

CAUTION: In this mode, the synchronization is left to be the PROFIBUS master’s responsibility. This is very important to realize, because attempting to control the RMC over PROFIBUS in this sub-mode from a PLC that asynchronously updates the PROFIBUS data will not work.

No additional synchronization registers are added to the two registers per axis. Instead, the RMC processes a new command any time either output register for an axis changes. Two ways of controlling the synchronization are to have the PLC either (1) enforce updating the PROFIBUS data at the top of the scan (the SIMATIC TI505 can operate in this mode, but does not by default), or (2) provide a function block for explicitly updating the PROFIBUS data (the SIMATIC S7-300 series can use this option using the DPRD_DAT and DPWR_DAT system function codes).

Once all commands have been processed and the data in the Input Registers has been updated, the RMC100 copies the value from the Output Synchronization Register to the Input Synchronization Register. This should be used as a cue for the Master control program to process the data returned and/or issue the next commands.