Choose IO Module for S7 300

http://en.wikipedia.org/wiki/Sinking_and_sourcing

• Sinking - a logic circuit output, when connected to the inputs of one or more logic gates, and the output is in logical Low (zero) state, has a capability of providing a current to the ground or zero, so the voltage at that point will indeed correspond to the Low state.

If the sinking capability of a circuit is exceeded (for example when too many inputs of other gates are connected to it), then the voltage at that point may be too high, higher than that corresponding to a Low state. The circuits then may malfunction.

• Sourcing - a digital circuit output, when connected to the inputs of one or more logic gates, and the output is in logical High (one) state, has a capability of providing a current to the power supply, so the voltage at that point will indeed correspond to the High state.

If the sourcing capability of a circuit is exceeded (for example when too many inputs of other gates are connected to it), then the voltage at that point may be too low, lower than that corresponding to a High state. The digital circuits then may malfunction.

"Sinking and sourcing" apply to 24 VDCtransistor input/aoutput modules, Relay outputs are used to control either different voltage levels at the output or high power ( the circuit rating in Amps is part of the description of the module), They are more rugged than transistor modules (within their specs), but generally have some hysteresis (they have a recovery time that you must take into account if you are switching at relatively high-speeds) and they have a shorter lifespan that their transistor equivalents (the electromechanical parts wear out after a certain number of operations, also described in the datasheets),


Hope this helps,
Daniel Chartier