1. edge triggered means the event when a logic signal goes from false to true (positive edge) or from true to false (negative edge).
2. no, but they are essential for retentive timers, and very important for counters.
3. Timers on PLC-5s and Micrologix are 16 bit, so they are limited to a range of -32768 to +32767. ControlLogix timers are 32 bit and have a range of -2,147,483,648 to 2,147,483,647.
4. the un underflow bit. This may result in a fault in some PLCs.
21. The normal output ‘Y’ is repeated twice. In this example the value of ‘Y’ would always match ‘B’, and the earlier rung with ‘A’ would have no effect on ‘Y’.