另外要注意，一些高密度输入点的模块对一起接通的输入点数有约束，一般一起接通的输入点不得超越总输入点的 60 ％； PLC 每个输出点的驱动才干（ A/ 点）也是有限的，有的 PLC 其每点输出电流的大小还随所加负载电压的不同而异；一般 PLC 的允许输出电流随环境温度的升高而有所降低一级。在选型时要考虑这些问题。
PLC 的输出点可分为共点式、分组式和阻隔式几种接法。阻隔式的各组输出点之间能够选用不同的电压品种和电压等级，但这种 PLC 均匀每点的价格较高。假如输出信号之间不需求阻隔，则应挑选前两种输出办法的 PLC 。
First, clear the total i/o points of the control system, and then reserve 15 ~ 20% of the total points required in practice (leaving room for the transformation of the system), and then confirm the number of PLC points required.
In addition, it should be noted that some modules with high-density input points have restrictions on the number of input points connected together. Generally, the input points connected together shall not exceed 60% of the total input points; The driving ability (a/ point) of each output point of PLC is also limited. The output current of some PLCs varies with the applied load voltage; Generally, the allowable output current of PLC decreases by one level with the increase of ambient temperature. These problems should be considered when selecting models.
The output points of PLC can be divided into common point connection, grouping connection and blocking connection. Different voltage varieties and voltage grades can be selected between each group of output points of barrier type, but the price of this kind of PLC is higher. If there is no need to block between the output signals, the PLC of the first two output methods should be selected.
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