FOUNDATION Fieldbus Operational Modes


A common stumbling block for new users is the concept of operational modes for function blocks on the devices -- what they are and how to get a block into the desired mode. This document addresses these questions.


The Modes

The most common modes are Automatic (Auto), Cascade (Cas), Manual (Man), and Out Of Service (OOS or O/S).

  • In Automatic mode, the block is running under its normal automatic control, using a local setpoint value in the normal block algorithm to determine the output value.
  • In Cascade mode, the block is receiving its setpoint value from another function block for use in the normal block algorithm to determine the output value. A link object makes the connection between the two blocks. Note: If you are using the NI-FBUS Configurator, the link object is created implicitly when you wire the output of one function block to the input of another. The PID and AO function blocks often receive their setpoints from an upstream block.
  • In Out Of Service mode, the block is not running at all. Normally, this mode is used during block configuration. Also, some devices require that the function block be in Out Of Service mode when writing certain parameters.
  • In Manual mode, the block output is not being calculated. The operator writes the output of the block directly.

The MODE_BLK Parameter

The MODE_BLK parameter for a function block has four fields: TARGET, ACTUAL, PERMITTED, and NORMAL. PERMITTED and NORMAL are defined by the device manufacturer.

  • PERMITTED contains a list of all allowable modes for that block.
  • NORMAL is the mode the device manufacturer expects the block to be in during normal use.
  • ACTUAL is the current operating mode of the function block on the device.
  • TARGET is a field that is writable by the user. Writing this field tells the device to change to the specified mode. The device will attempt to change the mode. If it is successful, the ACTUAL mode changes to reflect the target mode. Note: If the block is not currently scheduled, it will always be in Out Of Service (OOS) mode. In the NI-FBUS Configurator, a block is scheduled by placing it in a Function Block Application Editor window and downloading the project to the device. Before this is done, the function block is not actually executing on the device -- it is Out Of Service. Any attempts to change its mode are ineffective until the block is scheduled to execute on the device.

Determining Why a Block Will Not Change Modes

If the device cannot change its ACTUAL mode to the TARGET mode, first verify that you have downloaded the configuration to the device. If the configuration has been downloaded, you can diagnose the problem by looking at the BLOCK_ERR parameter of the function block. This parameter often identifies the problem. For example, if the device's resource block is offline, the BLOCK_ERR parameter will read "Resource Block Offline", indicating that you must change the resource block to Auto mode before you can change the function block's mode. Also, if the device is an FP-3000, you can look at the additional parameters on the Diagnostics tab in the function block’s parameter window. Note that the LAST_BLOCK_EVENT is not cleared after an error condition clears. Check the time stamp to determine whether the event you are looking at is current.

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