This article presents some examples of typical configurations of Fieldbus blocks and function blocks of the FCS.
Foundation Fieldbus Loop Configuration Examples
Input Replacement Type
In this configuration, the I/O signals of an analog control system that uses the conventional 4 to 20 mA signals are replaced with Fieldbus equipment. Control calculations are performed by the FCS.
Cooperative Type
The FCS provides setting values for the local loops of the Fieldbus network in a cascade control configuration. In this configuration, the Fieldbus equipment and the FCS cooperate and perform control. Conventionally, when a local device were to perform a control calculation, the setting values were provided to one loop controller, etc.
Backup Type
In this type of configuration, the control calculation performed in FCS is backed up by Fieldbus devices. This is similar to the conventional one-loop local controller in which the control calculation is performed. The fieldbus device in backup type configuration must be a LINKMASTER device.
Operating Mechanism
When FF-AO block is set to RCAS mode, FF-AO block sends output signal based on the signal received from FCS. While, the fieldbus block FF-PID is in IMAN(CAS) mode, its SV is tracking the PV. If the signal from FCS becomes abnormal, FF-AO block shifts its mode into CAS and the FF-AO block sends output signal based on the signal from FF-PID.
To Learn of Basic of Foundation Fieldbus: What Is Fieldbus Engineering? Technical Approach
Setting Items on Control Drawing Builder
If an abnormality occurs in FCS, bumplessly switching the control to FF-PID (fieldbus PID controller device) is necessary. Thus the following settings are required.
1. In terminal of PID should be connected to OUT_V of FF-AI block in the loop using data reference connection. For this purpose, assign a %Z to the OUT of FF-AI on the Function Block Detail Builder. Since the FF-AI and PID are connected via the %Z without using terminal connection, the data cannot be updated periodically. The connection between fieldbus function blocks must be terminal connection.
2. The setting item [Action at communication timeout (SHED_OPT)] of FF-AO block should be set to ant of the followings. This setting item can be found on [Input] tab of Function Block Detail builder.
Otherwise, the FF-AO does not shift to CAS mode when communication error occurs.
• When communication recovers, return to normal mode:
Setting number 1, [Normal shed, normal return].
• When communication recovers, remain CAS mode:
Setting number 2, [Normal shed, no return]. Normally the loop is in CAS mode.
3. The time required to detect “Communication time-out” is set to the parameter SHED_RCAS on a resource block. The parameter can be set on the parameter edit dialog box of the Fieldbus Builder.
The %Z Assignment window of Function Block Detail Builder is shown below. Enable the [%Z Assignment] checkbox of the OUT terminal.
Discrete Signals
The following illustration shows the structures for displaying and operating discrete I/O signals (DI, DO) of Fieldbus using a switch instrument block.
Description of Operation
To use open/closed answer back (input) or a 3-position type device with independent open/closed output, it is necessary to connect input/output with separate open/closed DI/DO blocks.
For a 2-position type device, connect input/output with one DI/DO block.
Note that any single DI/DO block that supports three values (open, close, stop, etc.) cannot be used.
To understand more clearly with simple examples please read this article about foundation fieldbus Foundation Fieldbus Overview.