<9. Advanced Engineering >
As described in the online help, when a terminal is connected to another terminal via
a link in FBD or LD, data can only be passed through the link in one direction. Bi-
directional data communication though a link is not allowed.
Read back of status information from downstream to upstream can, however, be
implemented by connecting the same variable to the RB_IN terminal of an upstream
NPAS POU and the RB_OUT terminal of a downstream NPAS POU.
In the cascade loop example, device label AO001 of analog output is at the most
downstream. The read back input of device label AO001, AO001_RB, returns the
state of output equipment such as a valve.
In the figure below, AO001_RB is connected to the RB_IN terminal of output
processing POU NPAS_AO_ANLG_1, and written to variable RB_AO001 of the
RB_OUT terminal.
By connecting variable RB_AO001 (enclosed by red box) to the RB_IN terminal of
NPAS_PID_2, status information of AO001_RB is passed to NPAS_PID_2.
Status information of NPAS_PID_2 is written to variable RB_PID2 (enclosed by
brown box), which is connected to its RB_OUT terminal. By connecting variable
RB_PID2 to the RB_IN terminal of NPAS_PID_1, status information of NPAS_PID_2
is passed to NPAS_PID_1.
If the read back connection between two POUs is implemented using a direct link
instead of a variable, the execution order of the two POUs will be decided solely by
their relative vertical positions.
To avoid this problem, implement the read back connection using a variable instead.
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