文档介绍：Programmable Logic Controllers (PLCs)
1 。About Programmable Logic Controllers (PLCs)
PLCs (programmable logic controllers) are the control hubs for a wide variety of automated systems and processes. They contain multiple inputs and outputs that use transistors and other circuitry to simulate switches and relays to control equipment. They are programmable via software interfaced via puter interfaces and proprietary languages work options.
Programmable logic controllers I/O channel specifications include total number of points, number of inputs and outputs, ability to expand, and maximum number of channels.  Number of points is the sum of the inputs and the outputs. PLCs may be specified by any bination of these values.  Expandable units may be stacked or linked together to increase total control capacity.  Maximum number of channels refers to the maximum total number of input and output channels in an expanded system.  PLC system specifications to consider include scan time, number of instructions, data memory, and program memory.  Scan time is the time required by the PLC to check the states of its inputs and outputs.  Instructions are standard operations (such as math functions) available to PLC software.  Data memory is the capacity for data storage.  Program memory is the capacity for control software.
Available inputs for programmable logic controllers include DC, AC, analog, thermocouple, RTD, frequency or pulse, transistor, and interrupt inputs.  Outputs for PLCs include DC, AC, relay, analog, frequency or pulse, transistor, and triac.  Programming options for PLCs include front panel, hand held, puter.
Programmable logic controllers use a variety of software programming languages for control.  These include IEC 61131-3, sequential function chart (SFC), function block diagram (FBD), ladder diagram (LD), structured text (ST), instruction list (IL), relay ladder logic (RLL), flow chart, C, and Basic.  The IEC 61131-3 programming environment provides supp