Thursday, February 3, 2011

Configuring the PLC System

PLC configuration should be considered during logic sequencing and flowcharting. The PLC configuration defines which modules of I/O will be used with which types of I/O signal, as well as where the modules will be located in the local or remote rack enclosures. The modules locations determine the I/O addresses that will be used in the program control.

The user should consider the following: possible future expansions, special I/O modules, such as fast response or wire fault inputs, and the placement of interfaces within a rack during system configuration. These details consideration, along with system configuration documentation, will result in a better system design.

The Inputs And Outputs assignment is one of the most important procedures that occur during the programming organization and implementation stages. The I/O assignment table organizes and documents what has been done so far. It indicates which PLC inputs are connected to which input devices and which PLC outputs drive which output devices. The assignment of internals, including counters, timers, and MCRs, also takes place here. These assignments are the coil representations and actual contact that are used in the program ladder diagram. In applications where electromechanical relay diagrams are available, i.e. modernization of a process or machine, identification of real I/O can be done by circling the devices and then assigning them I/O addresses.

These assignments can be extracted from the logic gate diagrams or ladder symbols that were used to describe the logic sequences. They also can come from the circled elements of an electromechanical diagram. The numbers used for the I/O addresses depend on the PLC model used. These addresses can be represented in octal, decimal and hexadecimal.

The user should group associated inputs and outputs. This grouping will allow the manipulation and monitoring of a group of I/O simultaneously. For example, if 16 motors will be started sequentially, they should be grouped together, so that monitoring the I/O registers associated with the 16 grouped I/O systems.

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