1747-ACN15
1747-ACN15
1747-ACN15
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控制系统中常用的串行通信一般采用RS-232C串行总线标准。RS-232C是电子工业协会(EIA)推荐的一种普及型异步通信总线标准,目前已广泛应用到了计算机系统中外围设备与主机的连接中。 但是RS-232C应用于计算机系统时存在如下问题: (1)传送距离过近:由于受负载电容容许值的限制,其传送距离一般不得超过15M; (2)电平偏移:由于信号地公用且通信双方的信号地需连结在一起,RS-232C应用于长距离通信时信号地线上会存在较大的电流,使得电平发生偏移,容易造成传输错误; (3)潜在的接地问题:RS-232C总线有一个信号地和一个屏蔽保护地,通信中,双方的信号地需连在一起,但是屏蔽保护地却不一定允许接在一起,同时DTE的信号地与屏蔽保护地之间又不一定绝缘,这就存在一个潜在的接地问题,使得系统的抗干扰能力较差。 为了扩大串行通信的有效传输距离,提高通信传输速率,增强抗干扰能力,EIA制定了RS-422/485串行总线通信标准,也叫双端接口电气标准或平衡传输电气标准。RS-422/485与RS-232C的关键不同之处在于将单端输入改为双端差分输入,信号地不再共用,通信双方的信号地不再连在一起。 RS-422/485的优点在于: (1)采用双端差分输入,总线的抗干扰能力强; (2)互连双方的信号地分立,两条信号线形成回路,避免了电平偏移,同时也避免了RS-232C中的潜在接地问题; (3)输出端采用双端平衡驱动,其信号放大倍数高,信号传输距离远。RS-422支持全双工串行通信,RS-485支持半双工串行通信。
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