18.1 SWITCHED INPUTS AND OUTPUTS
The Obvious:
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A PLC is just a computer, we must get information in so that it may make decisions, and have outputs so that it can make things happen.
Inputs:
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Switches - contact, deadman, etc. all allow a voltage to be applied or removed from an input.
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Relays - Used to isolate high voltages from the PLC inputs, these act as switches
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Encoder - Can keep track of positions
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etc.
· Outputs
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Motors - motors often have their own controllers, or relays because of the high current they require.
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Lights - can often be powered directly from PLC output boards.
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etc.
· PLCs, and other industrial controls often use methods called sourcing or sinking current for outputs of devices. In this method the output of a device does not supply any power. Instead, the device only switches current on or off.
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Sinking - When active the output allows current to flow to a common ground. This is best selected when different voltages are supplied.
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Sourcing - When active, current flows from a supply, through the output though the output device and to ground. This method is best used when all devices use a single supply voltage.
· When selecting sensors
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- NPN is sinking
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- PNP is sourcing
