Event function block

How it applies

• Why they exist: In IEC 61499, nothing executes without an event. Event function blocks provide the common event logic as standard types, so that applications don't need a custom block for every split, merge or timer. Their names start with E_ and their behavior is defined in Annex A of IEC 61499-1; some are basic function blocks, others are composite function blocks built from the simpler ones (E_TRAIN, for example, combines E_CYCLE and E_CTU), and E_DELAY and E_RESTART depend on timer and start-up services of the resource.
• The standard set: Annex A defines the following types.

Block	Purpose
E_SPLIT	Splits one input event into two output events, issued in order (EO1, then EO2)
E_MERGE	Merges two input events into one output event
E_REND	Rendezvous: issues an output event once both inputs EI1 and EI2 have occurred; R resets
E_PERMIT	Passes an event only while the Boolean input PERMIT is TRUE
E_SELECT	Passes EI0 or EI1 to the output, selected by the Boolean input G
E_SWITCH	Routes one input event to EO0 or EO1, depending on the Boolean input G
E_DELAY	Issues an output event a time DT after START; STOP cancels it
E_CYCLE	Issues output events periodically with period DT between START and STOP
E_TRAIN	Issues a finite train of N events spaced by DT, with the count CV
E_TABLE	Issues a finite train of events with individual delays from the array DT
E_N_TABLE	Like E_TABLE, but each event on its own output EO0, EO1 …
E_RESTART	Issues COLD, WARM or STOP events when the resource starts or stops
E_SR, E_RS	Event-driven bistables: set and reset events switch the Boolean output Q
E_D_FF	Event-driven D flip-flop: latches D on the clock event, issues an event only when Q changes
E_R_TRIG, E_F_TRIG	Pass the input event when the Boolean input has risen or fallen since the last event
E_CTU	Event-driven up counter: counts CU events, Q becomes TRUE when CV reaches PV; R resets

• Engineering: E_RESTART typically starts the initialization chain of an application (its COLD and WARM events trigger the INIT events of other blocks). E_CYCLE turns the event-driven model into periodic execution where a control loop needs a fixed sampling time. Tools such as Eclipse 4diac ship these types in their standard library, often together with additional, tool-specific event blocks that are not part of the standard.
• Portability: Only the Annex A types can be expected in every compliant tool. Blocks with similar names from a vendor library (for example demultiplexers or extra counters) are extensions and need to be ported or replaced when an application moves to another tool.
• Documentation: In application descriptions, name the event block type and its parameters (for example E_CYCLE with DT = T#50ms) instead of describing the event flow in prose. Mark any event block that doesn't come from Annex A as a library extension.

Event function block vs. IEC 61131-3 timers and triggers

IEC 61131-3 blocks such as TON, R_TRIG or CTU are evaluated in every scan and work on Boolean signals. Their IEC 61499 counterparts E_DELAY, E_R_TRIG and E_CTU react only when an event arrives and produce events. An E_R_TRIG therefore detects the edge between two events, not between two scans: if no event arrives, no edge is seen.

External references

• IEC 61499-1:2012, Function blocks — Part 1: Architecture (IEC Webstore)
• Holobloc FBDK library: E_SPLIT and the other Annex A event function blocks