SM202 HollySys main control unit, Shanghai Sijian Intelligent Technology Co., Ltd. is engaged in technical development, technical consultation, technical transfer in the fields of intelligent technology, information technology, and automation technology, as well as the sales of electrical equipment, automation equipment, electronic products, communication products, and electromechanical products. Industrial automation products: PLC, DCS, sensors, frequency converters, encoders, relays, servo drives, soft starters, touch screens. The brands involved in the products include: Eurotherm, Schneider, Siemens, ABB, SICK, P+F, E+H, Yokogawa, Yamakawa, Mitsubishi, Emerson, Honeywell, FANUC, GE, Eaton Electrical, CT. Blindly selecting a model with a high number of input/output points for the control system can lead to waste. First, determine the total number of I/O points required for the control system, then add a 15–20% margin (to allow for system modifications, etc.) to determine the required number of PLC points.
SM202 HollySys main control unit. Additionally, it should be noted that some high-density input point modules have limitations on the number of simultaneously activated input points; generally, the number of simultaneously activated input points should not exceed 60% of the total input points. The driving capacity (A/point) of each output point of the PLC is also limited; for some PLCs, the output current per point varies with the applied load voltage. Generally, the allowable output current of a PLC decreases as the ambient temperature increases. These issues should be considered during model selection. The output points of a PLC can be connected in several ways: common point, grouped, or isolated. Isolated output points can use different voltage types and levels among groups, but the average cost per point for such PLCs is higher. If output signals do not require isolation, then a PLC with one of the first two output methods should be selected. Selection of storage capacity
The user storage capacity can only be roughly estimated. In systems that only control switching values, the estimation can be made by multiplying the total number of input points by 10 words/point + the total number of output points by 5 words/point; counters/timers are estimated at (3–5) words/each; when there is operational processing, estimate at (5–10) words/quantity; in systems with analog input/output, estimate about (80–100) words of storage capacity per analog input/(or output) channel; when there is communication processing, roughly estimate at least 200 words per interface.
Field control or detection components input various control signals to the PLC, such as limit switches, push buttons, selector switches, and other sensors outputting switching or analog signals. These signals are converted by the input interface circuit into signals that the CPU can receive and process. The output interface circuit converts the weak control signals from the CPU into strong electrical signals required by the field, to drive actuators of controlled equipment such as solenoid valves and contactors.