If a system malfunctions, the machine operator will be unable to switch modes, or if the machine is in automatic loop mode, the operator cannot enter manual mode. The machine can only be placed into manual mode upon receiving a return-to-base request. If no malfunction occurs, the automatic loop instruction in the ladder logic diagram will initiate an automatic loop in automatic mode, and the loop will only stop if the automatic program has not been started.
Such a setting helps the machine maintain its current state when a malfunction occurs. Other possible settings include allowing the cycle to start only when the machine is in its current or initial position. It's also possible to require holding the button down for three seconds when starting a cycle (at which point an alarm may sound), or other special requirements defined by the customer.
In addition, some possible settings include an immediate stop button (which can stop the machine even in automatic loop mode), or a memory bit information for the cycle stop (so that the loop can be completed without pressing the button again).
If the system malfunctions, the machine operator will be unable to switch modes, or if the machine is in automatic cycle mode, the machine operator will be unable to enter manual mode.
One point to note regarding the last rung: the first two rungs use latching or "set" bits in both automatic and manual modes. The last automatic cycle rung is also latched via a "hold" contact; these parallel automatic cycle bits have the same address as the coil. This is a common technique in programmable logic controller (PLC) ladder diagram programming, but it's actually just a preferred choice.
Another important point is not to start the machine immediately. Instead, the user should warn those around the machine that it is about to be started. This requirement applies to most machines, except for simple machines such as test stations.
This logic can also display how the alarm uses pulses to alert people that the system is about to start. The person starting the machine is instructed to press and hold a button to generate a start-up request signal. The next step is to start the system.
This is by no means the only implementation method, but it clarifies the general idea. There are four key points that users should pay close attention to:
The operator must hold down the cycle start button for the system to boot. Releasing the button will restart the timer.
The system can only start in automatic mode and when there are no faults.
If the system does not stop in the middle of automatic program execution, this should be modified according to different situations. The program should be allowed to pause in a natural state; a specific program step or position can substitute for a valid program bit.
A failure will immediately stop the automatic loop. Again, this may not apply to all situations.
The auto-cycle bit or state itself is not a true mode; it's more like a state within the auto mode. Typically, it's used to allow the auto-program to start or proceed, not to disable the output signal.
