P ERSONAL C OMPUTERS

Common usage of the personal computer (PC) in our daily lives has led to the practical elimination of dedicated PLC programming devices. Due to the personal computer’s general-purpose architecture and standard operating system, most PLC manufacturers and other independent suppliers provide the necessary PC software to implement ladder program entry, editing, documentation, and real-time monitoring of the PLC’s control program. The large screens of PCs can show one or more ladder rungs of the control program during programming or monitoring operation (see Figure 4-25).

Personal computers are the programming devices of choice not so much because of their PLC programming capabilities, but because PCs are usually already present at the location where the user is performing the programming. The different types of desktop, laptop, and portable PCs give the programmer flexibility—they can be used as programming devices, but they can also be used in applications other than PLC programming. For instance, a personal computer can be used to program a PLC, but it may also be connected to the PLC’s local area network (see Figure 4-26) to gather and store, on a hard disk, process information that could be vital for future product enhancements. A PC can also communicate with a programmable controller through the RS- 232C serial port, thus serving either as the data handler and supervisor of the PLC control or as the bridge between the PLC network and a larger computer system (see Figure 4-27).

Figure 4-25.

A PLC ladder diagram displayed on a personal computer.

C o u rt e s y o f B &

n I d u s tr ia l A u to m a ti o n , R o s w e ll, G

S ECTION Components Processors, the Power Supply, C HAPTER 2 and Systems

and Programming Devices 4

-programming -editing -monitoring -data gathering -complex calculations -report generation

printer

personal computer

Figure 4-26.

A PC connected to a PLC’s local area network.

Mainframe computer system

PC as bridge

PLC network

Figure 4-27.

A PC acting as a bridge between a PLC network and a mainframe computer system.

Industrial Text & Video Company 1-800-752-8398

www.industrialtext.com

S ECTION Components Processors, the Power Supply, C HAPTER 2 and Systems

and Programming Devices 4

In addition to programming and data collection activities, PC software that provides ladder programming capability often includes PLC documentation options. This documentation capability allows the programmer to define the purpose and function of each I/O address that is used in a PLC program. Also, general software programs, such as spreadsheets and databases, can communicate process data from the PLC to a PC via a software bridge or translator program. These software options make the PC almost invaluable when using it as a man/machine interface, providing a window to the inner workings of the PLC-controlled machine or process and generating reports that can be directly translated into management forms.

K EY block check character (BCC) T ERMS checksum

constant voltage transformer cyclic exclusive-OR checksum (CX-ORC) cyclic redundancy check (CRC) diagnostics exclusive-OR (XOR) Hamming code I/O update scan isolation transformer longitudinal redundancy check (LRC) microprocessor miniprogrammer multiprocessing parity parity bit program scan scan time vertical redundancy check (VRC)

Industrial Text & Video Company 1-800-752-8398

P ERSONAL C OMPUTERS