am I misreading something here? ...
Maybe...I'll try to be a little more clear.
There are no safety issues with using latch/unlatch. Any motor or object that is a safety concern (can cause harm), are not allowed to be controlled by a PLC alone. So if the power goes off and then on, it doesn't matter if the outputs on the PLC are on or off.
The same thing happens when you press the emergency stop. It doesn't matter what the PLC is doing, if PLC outputs are on or off or in between. There has to be something else that is cutting the power to the motor, air supply or whatever it is.
This is machine safety 101.
In it's simplest form you can use a safety relay and you shutoff 24V to all outputs with it. A safety relay also needs a manual reset button hooked up to it. If you press the E-stop you have no 24V anymore and everything stops. To restart you need to pull out the E-stop and push the reset button.
If you cycle the power on the machine you have the same situation. The safety relay cuts all 24V and you have to push the reset button to get the power back. Then you are allowed to start machine again.
The safety relay is also connected to an input on the PLC so you know when it has been activated. You use that input to stop everything in the program, just like an additional stop button. So every rung that sets the output or latches it off is activated when the safety relay is activated.
For example
Code:
start_no stop_nc [COLOR=Red] safety_nc [/COLOR] output
+---| |---+---| |-------[COLOR=Red]| |[/COLOR]-------------( )---
| |
| output |
+---| |---+
or
Code:
start_no output
+---| |---+------------------------------(L)---
stop_nc output
+---|/|---+------------------------------(U)---
| |
|[COLOR=Red]safety_nc[/COLOR]|
+---[COLOR=Red]|/|[/COLOR]---+
This is just the simplest of examples. What safety realys you need and how you are allowed to use them depends on what level of safety the machine needs for that particular function.
On larger machines you often have a bunch of stuff going on with guards, light curtains, different safety zones and you use a safety PLC which is a PLC that can do both regular PLC stuff as well as doing the job of a whole case of safety relays. The principles are the same though.
.