Having said that, it is still possible to address them all directly from the ladder program, as the earlier PLC versions had to.
The central processing unit CPU. This type of system is called 'Closed Loop' control. This is achieved by using a multi-pole connector on top of the card which connects via a data cable to another card with all the screw terminals on it.
That's because the rack pictured is what's known as an expansion rack. The completed program which would be much longer than this one in practice is downloaded from the PC to the PLC using the special programming or connection cable. The power supply plugs into the rack as well one for each rack and supplies regulated DC power to all the modules that plug into that rack.
PLCs come in so many makes and different shapes and sizes. Here it is again with the PSU slotted in place.
Can you see the rack data cable connecting the expansion rack on top to the CPU rack below? However any more than 16 inputs on a card usually means having a breakout connector as it's just not possible to connect that many wires white mage best in slot heavensward the top of an input card on the PLC with built in screw terminals.
More basically, this translates into talk Tx and listen Rx. Reading the inputs and setting the outputs accordingly.
In Stop mode it's as it says, stopped. Now you might not think this is a mode as such but since some programming can be done while running, while some PLC programming functions require it to be stopped. Given the millions of people that use the London Underground Tube railway, gives an indication of just how PLCs work very reliably day in and day out.
This type of device is what's called a bit device. Putting a PLC into Stop mode also turns off all the outputs. As you can see from the name the process is reversed. In Programming mode it accepts the downloaded program logic from the laptop or PC you would use to write the controlling program.
The completed program pages look like a ladder, which is why it's called Ladder Logic. The memory in the CPU stores the slots best odds in non-volatile RAM, which means it won't lose the program if the power is lost.
This addressing is easily done within the PLC parameters area of the ladder program these days, this tell the PLC which cards are fitted in the rack and at what position. For our purposes of closed slot trunking here, we will be looking at the modular rack based systems as they have easily definable parts.