1756-A7 A-B  Memory Board, Dynamic

When the E–Stop is pulled up, power flows through CR3(a) and CR1(b) energizing the coil CR2. When this coil is energized, the NO contacts CR2(a), CR2(b), and CR2(c) close and energize coil CR3. The NO contact CR3(b) closes and holds the coil CR3 energized. The device is now “armed and ready”.

When the operator pushes the start button, coil CR1 energizes. CR1(a) closes to hold in CR1. Then the NC contact CR1(b) opens and disconnects power from coil CR2. The final state is: coil CR1 on, coil CR2 off, and coil CR3 on. This condition allows the contacts of the output line [CR1(d), CR2(d), CR3(c)] to be closed and the load energized. The output line is also called the “monitored output” or the “safety output”. The terminology is the same for safety relay outputs.

Fault Examples • Start Button Welded (Fault): If the start button is welded (i.e. held energized) when the E–Stop is reset, then coil CR2 will not energize because the circuit is never complete in the rung with CR3(a) and CR1(b). Since coil CR2 can not energize, it is not possible to pull in coil CR3 or CR1 because CR2(c) will not close. • Output CR2(d) Welded (Fault): After resetting the E–Stop, coil CR2 will energize, but since contact CR2(d) is welded and CR2 is positive guided, the NO contact of CR2(c) will not close, thus coil CR3 can not energize and the system can not be started again. Similarly, if CR1(d) welds, CR1(b) will be held open. If CR3(c) welds, CR3(a) will be held open. Both situations prevent coil CR2 from energizing, so the system can not be rearmed. • E–Stop Failure: If there is a short across the channel 2 contact block of the E–Stop, then CR3 will not de–energize, but CR1 will open the load. Restart is not possible since CR3(a) did not re–close preventing CR2 from re–energizing. Similar fault detection is provided for the channel 1 E–Stop circuit via CR1(b).

The structure and details of the product：

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