FB structured text

[TheFox 0]

Okay, Basically my first time playing with Omron so this may be a simple question. I have written some structured text that will compare a temperature to a setpoint with deadband (all REALS). Basically when enabled and the temperature < SP+DB then turn on output 1 and turn off output 2. Keep output 1 on until the temperature > SP+DB then turn off output 1 and turn on output 2 and vice versa. Now my problem is if they disable the FB I want to set both outputs to 0. Here is my text: (*Compare two real values to see if A > B or A < B*) (*If A > B latch on until A < B, and if A < B latch on until A > B*) H_DB:= VALUE_B + DB; L_DB:= VALUE_B - DB; IF VALUE_A > H_DB OR (A_GT_B AND NOT (VALUE_A < L_DB)) THEN A_GT_B:=TRUE; ELSE A_GT_B:=FALSE; END_IF; IF VALUE_A < L_DB OR (A_LT_B AND NOT (VALUE_A > H_DB)) THEN A_LT_B:=TRUE; ELSE A_LT_B:=FALSE; END_IF; Now what I need to do is on the first run of this set the A_GT_B and A_LT_B to FALSE. Is there a first run bit? I tried P_First_Cycle but that didn't work. I also set the initial values to False for the outputs but it doesn't seem to reset with the enable/disable bit? It only resets with power on/off.

[TheFox 0]

So I figured this out by adding a reset input. It is not my favorite thing in the world so if anyone has a cleaner solution let me know. Basically before I had a selector switch as my FB EN bit. Now I have CF113 (always on) in that place and I use a N/C contact on my selector to a reset bit. Here is the structured text: (*Compare two real values to see if A > B or A < B with a H_DB*) (*If A > B latch on until A < B, and if A < B latch on until A > B*) H_DB:= VALUE_B + DB; L_DB:= VALUE_B - DB; IF RESET =TRUE THEN A_GT_B:=FALSE; A_LT_B:=FALSE; ELSE IF VALUE_A > H_DB OR (A_GT_B AND NOT (VALUE_A < L_DB)) THEN A_GT_B:=TRUE; ELSE A_GT_B:=FALSE; END_IF; IF VALUE_A < L_DB OR (A_LT_B AND NOT (VALUE_A > H_DB)) THEN A_LT_B:=TRUE; ELSE A_LT_B:=FALSE; END_IF; END_IF;

[BITS N BYTES 66]

If you declared your BOOLEAN variables A_GT_B and A_LT_B as Internals then changing the Initial value to FALSE should do the trick. You could also add these two lines at the beginning of the function block:- A_LT_B:=FALSE; A_GT_B:=FALSE; This will make the outputs false. Subsequent ST statements will be evaluated and turn ON [TRUE] the desired BOOLEAN flag. The function block must always be enabled by P_ALWAYS_ON for this method to work. Edited 20 Nov 2007 by BITS N BYTES

[beegee 7]

When I see the code, I would have done it in ladder.... (or nested FB ST and ladder)

[scottmurphy 1]

Not sure if this is what your after, and I may be bit late in posting, but I have done something similar. (* Calculate Low / High Deadband Values based on PV & Deadband Value *) DB_Low := SV - Deadband; DB_High := SV + Deadband; (*Enable Cooling Output*) Cool := ( PV >= DB_High ) or ( Cool and ( PV > DB_Low) ); (*Enable Heating Output*) Heat := ( PV <= DB_Low ) or ( Heat and ( PV < DB_High) ) ; This code basically swaps between heating & cooling modes, based on the setpoint & feedback values. This can easily be done in ladder also, by doing in ST makes doing the math calcs easier.

[Guest anonymous]

Hello TheFox, Just a bit of info... The "P_First_Cycle" flag will not work within a FB, as during the first cycle, the FBs are being initialised, therefore, any internal operation within a FB will not take place until the second scan! Also, as I'm sure that many of you already know ( but is probably worth re-iterating ), switching OFF the EN parameter, does not reset anything within a FB. I.e. the state of all data within the FB is retained. Hope this helps