Even though this example is a simple control sequence it provides the backbone for more complex additions.
Some examples of additional features include….
- Auto and manual modes so that individual components can be operated in case of failures and maintenance.
- Sequence status and indication lights and sirens.
- A stopping sequence and/or emergency stop function can also be added if required.
- Conditional logic that needs to be met before the sequence can be started.
- Sequence pause logic to remember the last step before the sequence was stopped, so that the sequence can resume from where it left off.
- Conditional start logic that decides the step that must be triggered to resume operation.
- Alarm and fault logic to capture failure of equipment.
It’s great using hold in logic to latch the sequence steps because all the transition and stopping conditional logic is located in the same area.
When we encounter more complex processes then using hold in logic for our sequence steps can become difficult because we are limited to using only one output. Using multiple output symbols with the same variable declaration is a recipe for disaster. We don’t want to do this because the output symbol may be overwritten inadvertently during the PLC scan causing haphazard like behavior.
If the ladder diagram complexity is high then we can latch and unlatch sequence steps using the Set (Latch) and Reset (Unlatch) symbols. These symbols provide a lot more flexibility with the sequence programming, but require extra caution and expertise.