These are connected to the outside world. They physically exist and receive signals from switches, sensors, etc. Typically they are not relays but rather they are transistors.
Internal Utility Relays
These do not receive signals from the outside world nor do they physically exist. They are simulated relays and are what enables a PLC to eliminate external relays.
There are also some special relays that are dedicated to performing only one task.
These do not physically exist. They are simulated counters and they can be programmed to count pulses.
Typically these counters can count up, down or both up and down. Since they are simulated they are limited in their counting speed.
Some manufacturers also include highspeed counters that are hardware based.
These also do not physically exist. They come in many varieties and increments.
The most common type is an on-delay type.
Others include off-delay and both retentive and non-retentive types. Increments vary from 1ms through 1s.
These are connected to the outside world. They physically exist and send on/off signals to solenoids, lights, etc.
They can be transistors, relays, or triacs depending upon the model chosen.
Typically there are registers assigned to simply store data. Usually used as temporary storage for math or data manipulation.
They can also typically be used to store data when power is removed from the PLC.
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