Typically PC visualization software packages are made for PLC-s。 Some special devices have also PC software packages。 Wide range of communication options between PLC-s makes it possible to gather all information from field devices into one central control point。
Communication lines between PLC-s allow using information collected from other parts of the system in local process control。 Modern communication technologies enable remote diagnostics and configuration (Jack, 2003)。 These two significantly reduce overall maintenance costs of the system。
There are other methods for programming PLCs。 One of the earliest techniques involved mnemonic instructions。 These instructions can be derived directly from the ladder logic diagrams and entered into the PLC through a simple programming terminal。 An example of mnemonics is shown in Figure 1。6。 In this example the instructions are read one line at a time from top to bottom。 The first line 00000 has the instruction LDN (input load and not) for input A。 。 This will examine the input to the PLC and if it is off it will remember a 1 (or true), if it is on it will remember a 0 (or false)。 The next line uses an LD (input load) statement to look at the input。 If the input is off it remembers a 0, if the input is on it remembers a 1 (note: this is the reverse of the LD)。 The AND statement recalls the last two numbers remembered and if the are both true the result is a 1, otherwise the result is a 0。 This result now replaces the two numbers that were recalled, and there is only one number remembered。 The process is repeated for lines 00003 and 00004, but when these are done there are now three numbers remembered。
Modern communication options for PLC-s include standard protocols for example Ethernet。 It is tempting to use existing office networks also as data carrier for automation system communications。 Time has shown that it is better practice to keep these two separated if there is a need for constant online communication。 Hardly traceable temporal network overloads can cause problems also in automation system a communications。
All PLC-s need be programmed。 All programming works include risk for accidental errors in control algorithm。 Special devices are well tested and generally free of this kind of problems。 If available, it’s economically thoughtful to use special devices。
Safety applications that require highest degree of reliability should contain simplest devices and circuits possible。 There is a rule that every new link in chain decreases overall reliability。
The oldest number is from the AND, the newer numbers are from the two LD instructions。 The AND in line 00005 combines the results from the last LD instructions and now there are two numbers remembered。 The OR instruction takes the two numbers now remaining and if either one is a 1 the result is a 1, otherwise the result is a 0。 This result replaces the two numbers, and there is now a single number there。 The last instruction is the ST (store output) that will look at the last value stored and if it is 1, the output will be turned on, if it is 0 the output will be turned off。
Figure 1。6 An Example of a Mnemonic Program and Equivalent Ladder Logic
The ladder logic program in Figure 1。6, is equivalent to the mnemonic program。 Even if you have programmed a PLC with ladder logic, it will be converted to mnemonic form before being used by the PLC。 In the past mnemonic programming was the most common, but now it is uncommon for users to even see mnemonic programs。
Sequential Function Charts (SFCs) have been developed to accommodate the programming of more advanced systems。 These are similar to flowcharts, but much more powerful。 The example seen in Figure 1。7 is doing two different things。 To read the chart, start at the top where is says start。 Below this there is the double horizontal line that says follow both paths。 As a result the PLC will start to follow the branch on the left and right hand sides separately and simultaneously。 On the left there are two functions the first one is the power up function。 This function will run until it decides it is done, and the power down function will come after。 On the right hand side is the flash function, this will run until it is done。 These functions look unexplained, but each function, such as power up will be a small ladder logic program。 This method is much different from flowcharts because it does not have to follow a single path through the flowchart。。 PLC自动化制造系统英文文献和中文翻译(3):http://www.youerw.com/fanyi/lunwen_97058.html