PLC programming need to pay attention to what issues? - - - - - - - programmable logic

Programmable controller ( The PLC controller or PC) Applications, we often encounter the problem of the relay control system upgrading, we tend to refer to the original relay control circuit to prepare the PC application. In programming, therefore, we should pay attention to the PC control system and relay control system some different on the way to work.

let's look at an example: a relay control circuit is shown in figure 1.

by relay control system is a 'parallel' way to work, and its contact on-off needs certain action time. So when the starting circuit, time relay KT delay time, KT whether can continue to keep the current state, 'parallel' two actions need to be considered at the same time process: KT normally closed delay off contact, KA1, losing electricity KA1 normally open contacts disconnect; KT normally open delay contact closure, KA2 electricity, KA2 normally open contact closure. The results of the two process function, to determine the status of KT. At the same time, the existence of the action time of the contact, makes the circuit in time sequence. So the circuit can not reliable work. If join the empty boxes in the loop, and as shown in figure 1 the KA2 normally open contacts for KA3 normally open contacts ( As shown in figure 1 in parentheses) 。 As a result, KT, the KT itself lost electricity, will not continue to maintain current state.

the same circuit, we use PC to realize, ladder diagram as shown in figure 2.

PC is a 'serial' way to work, that is, in the form of scanning cycle, continuously, sequentially, to perform a detailed work. At the same time, the PC, the action of soft contact can be considered to be instantaneous, and it can keep the memory of this action until the next time scanning operation. With memory function. In such an order 'serial' style of work, ladder diagram action sequence is as follows: when in a scan cycle TIM00 delay to the back, is:

1. TIM00 normally closed off contact ( 关闭) ,0000;

2。 TIM00 normally open contact closure ( ) ,0001;

3。 Normally open contacts OFF 0000, 0001 normally open contacts ON, TIM00 keep current state. And no matter how much we between 0001 and TIM00 plus level in front of the relay circuit and the circuit of gridlines, 0001 normally open contacts and convert into the final relay and circuit by the normally open contact, TIM00 still can continue to maintain current state.

the same circuit, the relay control system and the differences of PC control system works, the two have different results. Take note, when we in programming, can avoid some should not have mistakes.

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