National Instruments NI-FBUS User Manual

Page 129

Appendix C

Cascade and Remote Cascade Setups

C-2

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Mode and Status Behavior During Cascade Initialization

Cascade initialization is arbitrated through the status of the backward
calculation path from the downstream block and the forward calculation
path on the upstream block. If the upstream block publishes a status of

Good, Non-Cascade

, it does not support cascade initialization, and the

lower block immediately transitions into a Cascade mode. This happens in
the case where an Analog Input (AI) block is acting as the upstream block
for an Analog Output (AO) block. Since the AI block does not have a back
calculation input, it does not support cascade initialization.

If the upstream block does support cascade initialization, it publishes a
status of

Good, Cascade

on its forward calculation output. This signals

to the downstream block to begin the cascade initialization process as soon
as it is able. If the downstream block is unable to begin cascaded control,
it publishes a status of

Good Cascade, Not Invited

on its backward

calculation output. This signals to the upstream block that the control path
from the downstream block to the process has been broken. As soon as the
ability to begin control is established, the downstream block publishes a
status of

Good Cascade, Initialization Request

on its backward

calculation output. This signals to the upstream block that it should
initialize itself for cascade control. While the initialization request status is
active, the downstream block is also publishing its current setpoint to the
upstream block. This enables the upstream block to prepare for a smooth
transfer to automatic control. While the upstream block is initializing itself
for automatic control, it enters an actual mode of Initialization Manual
(

IMan

). When it is ready to begin control, it publishes a status of

Good

Cascade, Initialization Acknowledge

to signal that it is beginning

cascade control. The lower block then enters Cascade mode.

To prevent windup, the control loop needs to be aware when it is unable to
control the process. If the downstream block can no longer control the

Downstream

OUT

CAS_IN

BKCAL_OUT

Upstream

BKCAL_IN