Page 702 - Mechanical Engineers' Handbook (Volume 2)

2 Programmable Controllers (PLCs) 693

FUNCTION_BLOCK CMD_MONITOR (* Begin definition of FB CMD_MONITOR *)

(* Definition of external interface *)
VAR_INPUT
AUTO_CMD : BOOL; (* Automatic Command *)

AUTO_MODE : BOOL; (* AUTO_CMD Enable *)
MAN_CMD : BOOL; (* Manual Command *)
MAN_CMD_CHK : BOOL; (* Negated MAN_CMD for debouncing *)
T_CMD_MAX : TIME; (* Maximum time from CMD to FDBK *)

FDBK : BOOL; (* Confirmation of CMD completion by operative
unit *)
ACK : BOOL; (* Acknowledgement/Cancel ALRM *)
END_VAR
VAR_OUTPUT

CMD : BOOL; (* Command to operative unit *)
ALRM : BOOL; (* T_CMD_MAX expired without FDBK *)
END_VAR

(* Definition of internal state variables *)
VAR
CMD_TMR : TON; (* CMD-to-FDBK timer *)
ALRM_FF : SR; (* Note over-riding “S1” input,
Command must be cancelled before ACK can cancel

alarm *)
END_VAR

(* Definition of Function Block Body per
Figure (a), (b) or (c) *)
END_FUNCTION_BLOCK (* End definition of FB CMD_MONITOR *)
(a)
Figure 11 Function block encapsulation: (a) textual declaration in ST language; (b) graphic represen-
tation.