IT IS A COMMUNICATION SOFTWARE FOR PLC TO CONNECT TO PC
•Digital I/P (Pushbutton, Switches )
•Analog I/P (Temp, Pressure, Flow, Level)
•Digital O/P (Solenoids valves, Contactors)
•Analog O/P (Control Valves, Speed)
Sinking Source type modules gives out the current to the field digital devices while sink type modules draw current when the device is in high state
Sinking - When active the output allows the current to flow to a common ground.
Sourcing - When active, current flows from a supply, through the output device and to ground.
Forcing the I/Os means making the desired status of I/O in PLCs irrespective of its status coming from the field.
In certain cases when there is problem in receiving field input /output, we can force the i/Os so that the logic takes desired state.
PLC's scan cycle follows following path Scan cycle of PLC
•Input Image Updation
•Process Logic Execution
Output Updation
Scan time is the Time required to read the I/P, Process the logic and update the output in one cycle.
Typically it is less than 10 ms. It depends on the complexity of logic, PID algorithm etc.
Every PLC manufacturer have their own software for programming the PLC
Ladder Logic closely resembles electrical ladder diagrams
1.A Ladder logic program exists only in the PLC’s software.
2.In ladder Logic it is not the actual flow of current thru circuits.
3.In electrical diagram the devices are described as open or closed(ON or OFF), where as in Ladder Logic, 4.instructions are either TRUE or FALSE.
5.In Ladder Logic Program must contain at least one control instruction (output) and usually contains one or more conditions (inputs).
The capacity to switch from primary equipment to standby equipment automatically without affecting the process under control.
