Thursday, February 3, 2011

Control Programming and PLC Description

Modernization applications involve the transfer of a process or machine’s control from conventional relay logic to a Programmable Controller. Conventional hardwired relay panels, which house the control logic, generally present maintenance problems, such as contact welding, contact chatter, and other electromechanical problems. A PLC switching can improve the performance of the machine as well as optimize its control. The new programmable controller program machine is actually based on the instructions and control requirements of the original hardwired system.

We will use the example of a midsized PLC capable of handling up to 512 I/O points (000 to 777 octal) to explain how to configure and implement a PLC program. The I/O controller structure has 4 I/O points per module. The PLC has eight racks, 0 through 7, each one with eight slot, or groups, where modules can be inserted.

The PLC can accept four channel analogue input modules, which can be placed in any slot location. Discrete I/O can not be used in the same slot when analogue I/O modules are used. These multiplexed modules provide the enable lines for the I/O devices and require two slot positions.

Addresses 000 through 777 octal represent device connections of input and output mapped to the table of I/O. The address of first digit represents the rack number, the second digit represents the slot, and the third digit specifies the terminal connection in the slot. The PLC detects whether the slot holds an input or an output.
Point addresses 10008 to 27778 may be used for internal outputs, and register storage starts at register 30008 and end at register 47778. Two types of timer and counter formats can be used block format and ladder format, but all timers require an internal output to specify the ON delay output. Ladder format timers place a ”T” in front of the internal output address, while block format timers specify the internal output address in the block’s output coil.

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