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PLC在选型的时候，要遵循一定的原则，避免小马拉大车的现象，同时还要避免PLC的容量小无法满足使用要求的想象。在满足控制要求的前提下选型时应选最佳的性价比，一般可以从以下几个方面考虑：
1.I/O点数估算
I/O点数是PLC的一项重要指标。合理选择I/O点数既可使系统满足控制要求有可使系统总投资最低。PLC的输入输出点总数和种类应根据被控对象的模拟量、开关量、输入/输出设备状况（包括模拟量、开关量、输出类型）来确定，一般一个输入输出元件要占用一个输入输出点。考虑到今后的扩充，一般应估计的总点数再加上15%～20%的备用量。
一、本设计所占用的I/O点数计算：
输入信号：开始按钮，需要一个输入点；停止按钮，需要一个输入点；计数值加1按钮，需一个输入点；计数值减1按钮，需要一个输入点。以上共需要4个输入信号点，考虑以后对系统的调整与扩充留有20%的备用点，即用4×20%=1，取1个点，这样共用5个输入点。
输出信号：一共要用十七个LED数码管，段选码需要使用8个输出点；位选通信号如果使用74LS138译码器则需要4个输出点；以上共需要13个输出点考虑以后对系统的调整与扩充留有20%的备用点，即13×20%=2.6，取3个点，这样共用16个输出点。
ALLEN BRADLEY 1785-L46C15
ALLEN BRADLEY 1775-L3
ALLEN BRADLEY 1756-L1M2
ALLEN BRADLEY 1756-L1M3
ALLEN BRADLEY SP-119471
ALLEN BRADLEY 8520-MPR0
ALLEN BRADLEY 5250-LP3/B
ALLEN BRADLEY 1747-L20N
ALLEN BRADLEY 8520-CPUX
ALLEN BRADLEY 1785-L30B/E
ALLEN BRADLEY 1747-L532
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ALLEN BRADLEY 1785-LT
ALLEN BRADLEY 1772-LSP
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SDRAM 6189-DIMM64
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450 PENTIUM 6189-450CPU3
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ALLEN BRADLEY 1772-LC
ALLEN BRADLEY 1747-L524
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ALLEN BRADLEY 1756-L1M2
ALLEN BRADLEY 1756-L1M3
ALLEN BRADLEY SP-119471
ALLEN BRADLEY 8520-MPR0
ALLEN BRADLEY 5250-LP3/B
ALLEN BRADLEY 1747-L20N
ALLEN BRADLEY 8520-CPUX
ALLEN BRADLEY 1785-L30B/E
ALLEN BRADLEY 1747-L532
ALLEN BRADLEY 1772-LH
ALLEN BRADLEY 1756-M13A
ALLEN BRADLEY 1756-M12A
ALLEN BRADLEY 1747-L532
ALLEN BRADLEY 1747-L40B
ALLEN BRADLEY 1785-L30B
ALLEN BRADLEY 1770-M11
ALLEN BRADLEY 1785-L30B
ALLEN BRADLEY 1761-L32AWA
ALLEN BRADLEY 1785-LT2
ALLEN BRADLEY 1747-L541
ALLEN BRADLEY 1772-LF
ALLEN BRADLEY 1772-LXP
ALLEN BRADLEY 1772-LWP
ALLEN BRADLEY 1774-LB2
ALLEN BRADLEY 1772-LZ
ALLEN BRADLEY 1785-LT
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ALLEN BRADLEY 1785-LT2
ALLEN BRADLEY 1785-L11B/E
ALLEN BRADLEY 1764-LSP
ALLEN BRADLEY 1775-L3
ALLEN BRADLEY 1784-KTP
ALLEN BRADLEY 1785-L11B
ALLEN BRADLEY 1772-LZP
ALLEN BRADLEY 1785-LT4
ALLEN BRADLEY 1775-L2
ALLEN BRADLEY 1747-L511
ALLEN BRADLEY 1785-LT4
ALLEN BRADLEY 1745-LP10
ALLEN BRADLEY 1745-LP101
ALLEN BRADLEY 1785-LT2
ALLEN BRADLEY 1775-CAP
ALLEN BRADLEY 1745-LP103
ALLEN BRADLEY 1785-LT
SDRAM 6189-DIMM64
ALLEN BRADLEY 1772-LXP
ALLEN BRADLEY 1774-LB2
450 PENTIUM 6189-450CPU3
ALLEN BRADLEY 1772-LW
ALLEN BRADLEY 1772-LX
ALLEN BRADLEY 1772-LH
ALLEN BRADLEY 1772-LB
ALLEN BRADLEY 1772-LN2
ALLEN BRADLEY 1772-LN3
ALLEN BRADLEY 955095-33
ALLEN BRADLEY 1772-LC
ALLEN BRADLEY 1747-L524
