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 standard computer interfaces and proprietary languages and network 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 possible combination 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,
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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, and computer.
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 support for five languages specified by the global standard: Sequential Function Chart, Function Block Diagram, Ladder Diagram, Structured Text, and Instruction List. This allows for multi-vendor compatibility and multi-language programming. SFC is a graphical language that provides coordination of program sequences, supporting alternative sequence selections and parallel sequences. FBD uses a broad function library to build complex procedures in a graphical format. Standard math and logic functions may be coordinated with customizable
communication and interface functions. LD is a graphic language for discrete control and interlocking logic. It is completely compatible with FBD for discrete function control. ST is a text language used for complex mathematical procedures and
calculations less well suited to graphical languages. IL is a low-level language similar to assembly
code. It is used in relatively simple logic instructions. Relay Ladder Logic (RLL), or ladder diagrams, is the primary programming language for programmable logic
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controllers (PLCs). Ladder logic programming is a graphical representation of the program designed to look like relay logic. Flow Chart is a graphical language that describes sequential operations in a controller sequence or application. It is used to build modular, reusable function libraries. C is a high level programming language suited to handle the most complex computation, sequential, and data logging tasks. It is typically developed and debugged on a PC. BASIC is a high level language used to handle mathematical, sequential, data capturing and interface functions.
Programmable logic controllers can also be specified with a number of computer interface options, network specifications and features. PLC power options, mounting options and environmental operating conditions are all also important to consider.
2 。INTRODUCTION
For simple programming the relay model of the PLC is sufficient. As more complex functions are used the more complex VonNeuman model of the PLC must be used. A VonNeuman computer processes one instruction at a time. Most computers operate this way, although they appear to be doing many things at once. Consider the computer components shown in Figure 1.
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Figure 1 1 Simplified Personal Computer Architecture
Input is obtained from the keyboard and mouse, output is sent to the screen, and the disk and memory are used for both input and output for storage. (Note: the directions of these arrows are very important to engineers, always pay attention to indicate where information is flowing.) This figure can be redrawn as in Figure 2 to clarify the role of inputs and outputs.
Figure 2 An Input-Output Oriented Architecture
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In this figure the data enters the left side through the inputs. (Note: most engineering diagrams have inputs on the left and outputs on the right.) It travels through buffering circuits before it enters the CPU. The CPU outputs data through other circuits. Memory and disks are used for storage of data that is not destined for output. If we look at a personal computer as a controller, it is controlling the user by outputting stimuli on the screen, and inputting responses from the mouse and the keyboard.
A PLC is also a computer controlling a process. When fully integrated into an application the analogies become;
inputs - the keyboard is analogous to a proximity switch
input -circuits - the serial input chip is like a 24Vdc input card
computer - the 686 CPU is like a PLC CPU unit
output - circuits - a graphics card is like a triac output card
outputs - a monitor is like a light
storage - memory in PLCs is similar to memories in personal computers
It is also possible to implement a PLC using a normal Personal Computer, although this is not advisable. In the case of a PLC the inputs and outputs are designed to be more
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reliable and rugged for harsh production environments.
3 。 OPERATION SEQUENCE
All PLCs have four basic stages of operations that are repeated many times per second. Initially when turned on the first time it will check it’s own hardware and software for faults. If there are no problems it will copy all the input and copy their values into memory, this is called the input scan. Using only the memory copy of the inputs the ladder logic program will be solved once, this is called the logic scan. While solving the ladder logic the output values are only changed in temporary memory. When the ladder scan is done the outputs will be updated using the temporary values in memory, this is called the output scan. The PLC now restarts the process by starting a self check for faults. This process typically repeats 10 to 100 times per second as is shown in Figure 3.
Figure 3 PLC Scan Cycle
SELF TEST - Checks to see if all cards error free, reset watch-dog timer, etc. (A watchdog timer will cause an error, and shut down the PLC if not reset within a short period of time - this would indicate that the ladder logic is not being scanned normally).
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INPUT SCAN - Reads input values from the chips in the input cards, and copies their values to memory. This makes the PLC operation faster, and avoids cases where an input changes from the start to the end of the program (e.g., an emergency stop). There are special PLC functions that read the inputs directly, and avoid the input tables.
LOGIC SOLVE/SCAN - Based on the input table in memory, the program is executed 1 step at a time, and outputs are updated. This is the focus of the later sections.
OUTPUT SCAN - The output table is copied from memory to the output chips. These chips then drive the output devices.
The input and output scans often confuse the beginner, but they are important. The input scan takes a snapshot of the inputs, and solves the logic. This prevents potential problems that might occur if an input that is used in multiple places in the ladder logic program changed while half way through a ladder scan. Thus changing the behaviors of half of the ladder logic program. This problem could have severe effects on complex programs that are developed later in the book. One side effect of the input scan is that if a change in input is too short in duration, it might fall between input scans and be missed.
When the PLC is initially turned on the normal outputs will be turned off. This does not affect the values of the inputs.
3 。1 The Input and Output Scans
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When the inputs to the PLC are scanned the physical input values are copied into memory. When the outputs to a PLC are scanned they are copied from memory to the physical outputs. When the ladder logic is scanned it uses the values in memory, not the actual input or output values. The primary reason for doing this is so that if a program uses an input value in multiple places, a change in the input value will not invalidate the logic. Also, if output bits were changed as each bit was changed, instead of all at once at the end of the scan the PLC would operate much slower.
3 。2 The Logic Scan
Ladder logic programs are modelled after relay logic. In relay logic each element in the ladder will switch as quickly as possible. But in a program elements can only be examines one at a time in a fixed sequence. Consider the ladder logic in Figure 4, the ladder logic will be interpreted left-to-right, top-to-bottom. In the figure the ladder logic scan begins at the top rung. At the end of the rung it interprets the top output first, then the output branched below it. On the second rung it solves branches, before moving along the ladder logic rung.
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Figure 4 Ladder Logic Execution Sequence
The logic scan sequence become important when solving ladder logic programs which use outputs as inputs. It also becomes important when considering output usage. Consider Figure 5, the first line of ladder logic will examine input A and set output X to have the same value. The second line will examine input B and set the output X to have the opposite value. So the value of X was only equal to A until the second line of ladder logic was scanned. Recall that during the logic scan the outputs are only changed in memory, the actual outputs are only updated when the ladder logic scan is complete. Therefore the output scan would update the real outputs based upon the second line of ladder logic, and the first line of ladder logic would be ineffective.
Figure 5 A Duplicated Output Error
4 。 PLC STATUS
The lack of keyboard, and other input-output devices is very noticeable on a PLC. On
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the front of the PLC there are normally limited status lights. Common lights indicate;
power on - this will be on whenever the PLC has power
program running - this will often indicate if a program is running, or if no program is running
fault - this will indicate when the PLC has experienced a major hardware or software problem
These lights are normally used for debugging. Limited buttons will also be provided for PLC hardware. The most common will be a run/program switch that will be switched to program when maintenance is being conducted, and back to run when in production. This switch normally requires a key to keep unauthorized personnel from altering the PLC program or stopping execution. A PLC will almost never have an on-off switch or reset button on the front. This needs to be designed into the remainder of the system.
The status of the PLC can be detected by ladder logic also. It is common for programs to check to see if they are being executed for the first time, as shown in Figure 6. The ’first scan’ input will be true on the very first time the ladder logic is scanned, but false on every other scan. In this case the address for ’first scan’ in a PLC-5 is ’S2:1/14’. With the logic in the example the first scan will seal on ’light’,
until ’clear’ is turned on. So the light will turn on after the PLC has been turned on, but it will turn off and stay off after ’clear’ is turned on. The ’first scan’ bit is also
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referred to at the ’first pass’ bit.
Figure 6 An program that checks for the first scan of the PLC
5 。 MEMORY TYPES
There are a few basic types of computer memory that are in use today.
RAM (Random Access Memory) - this memory is fast, but it will lose its contents when power is lost, this is known as volatile memory. Every PLC uses this memory for the central CPU when running the PLC.
ROM (Read Only Memory) - this memory is permanent and cannot be erased. It is often used for storing the operating system for the PLC.
EPROM (Erasable Programmable Read Only Memory) - this is memory that can be programmed to behave like ROM, but it can be erased with ultraviolet light and reprogrammed.
EEPROM (Electronically Erasable Programmable Read Only Memory) – This memory
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can store programs like ROM. It can be programmed and erased using a voltage, so it is becoming more popular than EPROMs.
All PLCs use RAM for the CPU and ROM to store the basic operating system for the PLC. When the power is on the contents of the RAM will be kept, but the issue is what happens when power to the memory is lost. Originally PLC vendors used RAM with a battery so that the memory contents would not be lost if the power was lost. This method is still in use, but is losing favor. EPROMs have also been a popular choice for programming PLCs. The EPROM is programmed out of the PLC, and then placed in the PLC. When the PLC is turned on the ladder logic program on the EPROM is loaded into the PLC and run. This method can be very reliable, but the erasing and programming technique can be time consuming. EEPROM memories are a permanent part of the PLC, and programs can be stored in them like EPROM. Memory costs continue to drop, and newer types (such as flash memory) are becoming available, and these changes will continue to impact PLCs.
6 。 SOFTWARE BASED PLCS
The dropping cost of personal computers is increasing their use in control, including the replacement of PLCs. Software is installed that allows the personal computer to solve ladder logic, read inputs from sensors and update outputs to actuators. These are important to mention here because they don’t obey the previous timing model. For example, if the computer is running a game it may slow or halt the computer. This issue
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and others are currently being investigated and good solutions should be expected soon.
7 。 SUMMARY
• A PLC and computer are similar with inputs, outputs, memory, etc.
• The PLC continuously goes through a cycle including a sanity check, input scan, logic scan, and output scan.
• While the logic is being scanned, changes in the inputs are not detected, and the outputs are not updated.
• PLCs use RAM, and sometime EPROMs are used for permanent programs.
8 。 PRACTICE PROBLEMS
1. Does a PLC normally contain RAM, ROM, EPROM and/or batteries?
2. What are the indicator lights on a PLC used for?
3. A PLC can only go through the ladder logic a few times per second. Why?
4. What will happen if the scan time for a PLC is greater than the time for an input
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pulse? Why?
5. What is the difference between a PLC and a desktop computer?
6. Why do PLCs do a self check every scan?
7. Will the test time for a PLC be long compared to the time required for a simple program?
8. What is wrong with the following ladder logic? What will happen if it is used?
9. What is the address for a memory location that indicates when a PLC has just been turned on?
9 。 PRACTICE PROBLEM SOLUTIONS
1. Every PLC contains RAM and ROM, but they may also contain EPROM or batteries.
2. Diagnostic and maintenance
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3. Even if the program was empty the PLC would still need to scan inputs and outputs, and do a self check.
4. The pulse may be missed if it occurs between the input scans
5. Some key differences include inputs, outputs, and uses. A PLC has been designed for the factory floor, so it does not have inputs such as keyboards and mice (although some newer types can). They also do not have outputs such as a screen or sound. Instead they have inputs and outputs for voltages and current. The PLC runs user designed programs for specialized tasks, whereas on a personal computer it is uncommon for a user to program their system.
6. This helps detect faulty hardware or software. If an error were to occur, and the PLC continued operating, the controller might behave in an unpredictable way and become dangerous to people and equipment. The self check helps detect these types of faults, and shut the system down safely.
7. Yes, the self check is equivalent to about 1ms in many PLCs, but a single program instruction is about 1 micro second.
8. The normal output Y is repeated twice. In this example the value of Y would always match B, and the earlier rung with A would have no effect on Y.
9. S2:1/14 for micro logy, S2:1/15 for PLC-5
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中文翻译
1．PLC介绍
PLCS（可编程逻辑控制器）是用于各种自动控制系统和过程的可控网络集线器。他们包含多个输入输出，输入输出是用晶体管和其它电路，模拟开关和继电器来控制设备的。PLCS用软件接口，标准计算器接口，专门的语言和网络设备编程。
可编程逻辑控制器I/O通道规则包括所有的输入触点和输出触点，扩展能力和最大数量的通道。触点数量是输入点和输出点的总和。PLCS可以指定这些值的任何可能的组合。扩展单元可以被堆栈或互相连接来增加总的控制能力。最大数量的通道是在一个扩展系统中输入和输出通道的最大总数量。PLC系统规则包括扫描时间，指令数量，数据存储和程序存储。扫描时间是 PLC需要的用来检测输入输出模块的时间。指令是用于PLC软件（例如数学运算）的标准操作。数据存储是存储数据的能力。程序存储是控制软件的能力。
用于可编程逻辑控制器的输入设备包括DC，AC，中间继电器，热电偶，RTD，频率或脉冲，晶体管和中断信号输入；输出设备包括DC，AC，继电器，中间继电器，频率或脉冲，晶体管，三端双向可控硅开关元件；PLC的编程设备包括控制面板，手柄和计算机。
可编程逻辑控制器用各种软件编程语言来控制。这些语言包括IEC61131-3，顺序执行表（SFC），动作方块图（FBD），梯形图（LD），结构文本（ST），指令序列（IL），继电器梯形图（RIL），流程图，C语言和Basic语言。IEC61131-3编程环境能支持五种语言，用国际标准加以规范，分别为SFC，FBD，LD，ST和IL。这便允许了多卖主兼容性和多种语言编程。SFC是一种图表语言，它提供了编程顺序的配合，就能支持顺序选择和并列选择，二者择其一即可。FBD用一种
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大的运行库，以图表形式建立了一些复杂的过程。标准数学和逻辑运行可以与用户交流和接口运行相结合。LD是适用于离散控制和互锁逻辑的图表语言。它在离散控制上与FBD是完全兼容的。ST是一种文本语言，用于复杂的数学过程和计算，不太适用于图表语言。IL是与组合编码相似的低级语言。它用在相对比较简单的逻辑指令。继电器梯形图或梯形图是适用于可编程逻辑控制器的重要的编程语言。梯形图编程是设计成继电器逻辑程序的图表表示法。流程图是一种图表语言，用于在一个控制器或应用软件中描述顺序操作，它用于建立有标准组件的可循环使用的运行库。C语言是一种高级编程语言，适用于处理最复杂的计算，连续的数据采集任务。它典型地在PC机上运行调试。BASIC语言是用于处理数据的连续的数字采集和接口运行的高级语言。
可编程逻辑控制器也规范了许多计算机接口设备，网络规则和特色。PLC能源设备和运行环境也是非常重要的。
2．指令
对于简单的编程，继电器型PLC是有效的。随着功能的复杂化，复杂的VonNeaman型PLC就必须被采用。一个VonNeaman计算机一次只能执行一个指令，他们是这样运行的，尽管许多计算机看上去一次在做许多事情。正如图1所示的计算机组成。
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图 1 简化个人计算机结构图
输入是通过键盘和鼠标得到的。输出被送到屏幕。磁盘和存储器用于输入和输出存储（注意：这些箭头的方向对于设计者是非常重要的，要注意表明信息是流向哪里的。）这个图表可以像图2那样能被重新拟订来阐明输入设备和输出设备的作用。
在这个图表中数据通过输入设备进入左边。（注意：大多数设计图表都是左边输入，右边输出的。）在进入CPU之前，它穿过缓冲电路。CPU通过其他回路输出数据。存储器和磁盘用语存储要输出的数据。如果我们把个人计算机看作一个控制器，它通过在屏幕上输出激励和输入
来自鼠标和键盘的响应来控制用户。
PLC也是一个控制过程的计算机。当与应用程序完全结合起来时，类似之处变成：
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输入设备—键盘与接近开关相类比。
输入电路—连续输入芯片就像一个直流24V的输入卡。
计 算 机—686CPU就像一个PLC的CPU模块。
输出电路—图形卡就像一个三相开关输出卡。
输出设备—监控器就像指示灯。
存 储 器—PLC的存储器与个人计算机的存储器相似。
用普通个人计算机可以运行PLC，虽然则并不被提倡做。就PLC来说，输入和输出
设备设计得更加可靠，更加粗糙，更适合恶劣的制造环境。
3．运行顺序
所有的PLC系统有每秒钟重复多次的四种基本运行阶段。最初被第一次接通时，它会检测它的硬件和软件是否有错误。如果没有错误，它会把所有输入和输入值复制到存储器，这叫输入扫描。只用复制了输入值的存储器，梯形逻辑图将被解决一个，这叫逻辑扫描。在解决梯形图期间，输出值只在临时存储器中被改变。当梯形图扫描完成后，输出将用存储器中临时值修正，这叫做输出扫描。PLC此时将从自我检测开始重新启动这个过程，这个过程很明显地每秒钟重复10到100次，正如图3所示
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自我检测—检测是否所有的卡没有错误，把时间继电器复零等。（如果在很小一段时间内没有复零，时间继电器会引起错误，关闭PLC系统。—这会表明梯形图没有被正常扫描。）
输入扫描—从芯片上的输入卡读取输入值，并把输入值复制到存储器，这能使PLC更快速地运行，并且避免从程序开始到结束输入变化。（例如：意外停止）有一些特殊的PLC功能，能直接读取输入值，避免了输入表格。
逻辑处理/扫描—基于存储器的输入表格，程序被一次执行一步，同时输出值也被修正，这是其它节的集中。
输出扫描—输出表格从存储器复制到输出芯片，这些芯片然后驱动输出仪器。
输入输出扫描经常会令初学者感到迷惑，但是他们是很重要的。输入扫描是输入值的快照，并且解决逻辑关系。在一个梯形图扫描期间，如果一个输入在梯形图的多个地方被用到，它就会起变化，潜在问题就可能发生，而输入扫描却避免了这些问题。这个边境效应是如果在一段持续时间内如果一个输入变化太短，它可能在输入扫描之间会减少或者丢失。
当PLC最初被启动时，通常的输出会被关闭，这不会影响输入值。
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3．1 输入输出扫描
当输入值被扫描到PLC时，自然输入值被复制到存储器。当输出值被扫描到PLC时，他们将从存储器复制到自然输出设备。当梯形图被扫描时，它将用存储器中的值，并不是实际的输入输出值。这样做的主要原因是如果一个程序在多个地方用一个输入值，那么输入值的变化将使其逻辑关系无效。而且，如果随着每块的变化，输出模块也变化，在扫描结束时PLC的运行速度将大大减慢。
3．2 逻辑扫描
梯形逻辑程序图是模仿继电器逻辑图的。在继电器逻辑图中，程序的每个元件将尽可能快地开关。但是在一个程序中，元件只能按固定的顺序一次检测一个。如图4所示，梯形图将按从左到右，从上到下的顺序被解释。在图中，梯形逻辑扫描将从最高层开始。在底层，它将先解释高层输出，然后输出它下面的分支。在第二层，沿着梯形逻辑图移动之前，将先解释分支。
图 4 梯形图逻辑执行顺序
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解决梯形逻辑程序时，逻辑扫描顺序会变得非常重要。梯形图输出作为输入，考虑输出应用时，它也变得非常重要。如图5所示，梯形图第一行将检测输入并把输出X置1，得到相同的值。第二行将检测输入B并把输出X置1，得到相反的值。因此，直到梯形图的第二行被扫描时X值才能与A相等。在逻辑扫描期间，输出值只能在存储器中被改变，只有当梯形逻辑扫描完成时，实际的输出才能被修正。因此，在第二行的基础上，输出扫描将修正实际输出值。并且梯形图的第一行将无效。
4．PLC状态显示
在一个PLC中，缺少键盘和其他的输入输出设备是非常值得注意的。在PLC前端通常有一定数量的状态指示灯。通常指示灯表明：
电源启动—只要PLC带电，它将被启动。
程序运行—这将指示是否程序正在运行或是否没有程序正在运行。
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错误显示—当PLC有大的硬件或软件错误时，这将有显示。
这些灯通常用于调试。一定数量的按钮也将提供给PLC的硬件。最普通的按钮是一个运行/编程选择开关，当在保持状态时，它将被调到编程；当在生产状态时，它将被调到运行。一个PLC系统几乎没有一个启动关闭开关或复位开关在前面。这需要被设计到系统剩余部分。
PLC的状态也能被梯形逻辑图检测。检测程序是否第一次被执行是非常普遍的。如图6所示。‘first scan’输入在梯形图被第一次扫描时，将是对的，而在其余的每次扫描时是错误的。这种情况下，PLC—5的‘first scan’的地址是‘S2：1/14’。根据例子中的逻辑关系，第一次扫描将封上‘light’，直到‘clear’被启动。因此灯将在PLC被启动之后变亮，但在‘clear’被启动之后，它将关闭并且保持在关闭状态。‘first scan ’模块在‘first pass’模块中被提到。
图 6 核验PLC第一次扫描的程序
5．存储器类型
有几种基本的现在经常使用的计算机存储器类型：
RAM（随机存储器）—这种存储器速度很快，但是当没电时，它的内容将被丢失。这是一种不稳定存储器，每个PLC在运行时，都用这种存储器作为中央处理器 。
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ROM（只读存储器）—这种存储器是永久性的不可擦除的。它通常用于存放PLC的操作系统。
EPROM（可擦除可编程只读存储器）—这是一种像ROM一样可编程的存储器，但是它能用紫外线光擦除并且可以重新编程。
EEPROM（电可擦除可编程只读存储器）—这种存储器能像ROM一样存放程序。它能被编程并且用电压擦除，因此它正变得比EPROM更加普遍。
所有的PLC系统都用RAM做CPU，用ROM存储PLC的基本操作系统。当有电时，RAM的内容被保存，但是问题在于当供给存储器的电源失去时会发生什么。原先PLC卖主用带有电池的RAM，这样如果不失电，存储器的内容就不会丢失。这种方法现在仍被使用，但变得不那么受欢迎。EPROMS也是PLC编程的比较好的选择。EPROM在PLC外部编程，然后被放入PLC。当PLC被启动时，在EPROM上的梯形逻辑程序被下载PLC并且运行。这种方法非常可靠，但是擦除和编程技术都是很消耗时间的。EEPROM存储器是PLC的永久部分，程序能EPROM一样被存放在他们中。存储器的价钱一直在下降，新类型正变得可被利用，这些变化将继续对PLC系统发生影响。
6．基于软件的PLC系统
个人计算机持续下降的价格增加了他们在控制系统中的应用，包括PLC的替代品。安装了软件就能用个人计算机解决梯形图逻辑.从传感器中读取输入，修改输出送到激励。这些对于维持是很重要的，因为他们不用遵守以前的计时模式.例如，计算机正运行一个游戏，就可能减慢或停止计算机.这个以及其它问题现在正被研究，好的解决方案不久就会出现。
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7．概要
PLC系统和计算机与输入设备，输出设备,存储器等很相似。
PLC系统不断地执行系统检查，输入扫描,逻辑扫描和输出扫描这个循环。
当逻辑图被扫描时，输入的变化没有被发现,输出也没有被修正。
PLC系统用RAM，有时用EPROM存放永久程序。
8．实际问题
一个PLC系统通常包括RAM，ROM，EPROM和/或电池吗？
PLC的指示灯用于什么？
为什么一个PLC系统每秒钟只能扫描梯形图几次？
如果一个PLC系统的扫描时间比输入脉冲长，会发生什么?为什么？
一个PLC系统与一部台式计算机的不同是什么？
为什么PLC系统每次扫描要做自我检查？
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PLC检测时间会比简单程序所需时间长吗？
下面的梯形逻辑有什么错误？如果它被用会发生什么？
当一个PLC系统已经被启动时，表明启动的存储器地址是什么？
9．实际问题解答
每个PLC系统包括RAM和ROM，但是他们也包括EPROM或电池。
诊断和保持。
尽管程序是空的，PLC系统仍需扫描输入和输出，做自我检测。
如果在两次输入扫描之间发生，脉冲就会丢失。
主要的区别包括输入设备输出设备和应用。PLC系统是为工厂设计的，因此它没有鼠标键盘之类的输入设备。(虽然，一些较新型PLC能够达到)他们也没有屏幕声音之类的输出设备，.取而代之，他们有电压,电流这样的输入设备和输出设备。PLC使用户为专门的任务设计程序,然而在个人计算机上给系统编程是不常见的。
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这能帮助检测硬件和软件错误。如果一个错误发生了，PLC还继续运行,控制器就可能以一种不可预见的方式运行，这对人和机器是非常危险的。自我检测则帮助检查出这些错误，并且安全地关闭系统。
是的，在许多PLC系统中，自检大约需要1ms，但一个单一程序需1mms。
通常输出Y被重复两次。在这个例子中，Y的值将总等于B。前一层的A将对Y没有影响。
S2:1/14用于微机，S2:1/15用于PLC—5。
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