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1. 干接点和湿节点/ Definition of dry contact

(1) 干节点：無源開關；具有閉合和斷開的2種狀態；2個接點之間沒有極性，可以互換；

常见的干接点信号有：

a) 各种开关如：限位开关、行程开关、脚踏开关、旋转开关、温度开关、液位开关等；

b) 各种按键；

c) 各种传感器的输出，如：环境动力监控中的传感器：水浸传感器、火灾报警传感器、玻璃破碎、振动、烟雾和凝结传感器；

d) 继电器、干簧管的输出；有干接点就有湿接点。

(2) 湿接点：有源開關；具有有電和無電的2種狀態；2個接點之間有極性，不能反接；

常见的湿接点信号有：

a) 如果把以上的干接点信号，接上电源，再跟电源的另外一极，作为输出，就是湿接点信号；工业控制上，常用的湿接点的电压范围是DC0～30V，比较标准的是DC24V；AC110～220V的输出也可以是湿接点，尽管这样做比较少；

b) 把TTL电平输出作为湿接点，也未尝不可；一般情况下，TTL电平需要带缓冲输出的，例如：7407、245、244等，与VCC等构成回路；244、245也可以跟gnd构成回路；才能驱动远方的光耦。

c) NPN三极管的集电极输出和VCC；

d) 达林顿管的集电极输出和VCC；

e) 红外反射传感器和对射传感器的输出；

2. UART、RS232、COM/ UART、RS232、COM

UART是通用異步收發器（異步串行通信口）的英文縮寫，它包括了RS232、RS499、RS423、RS422和RS485等接口標准規範和總線標准規範，即UART是異步串行通信口的總稱。

RS232、RS499、RS423、RS422和RS485等，是對應各種異步串行通信口的接口標准和總線標准，它規定了通信口的電氣特性、傳輸速率、連接特性和接口的機械特性等內容。實際上是屬于通信網絡中的物理層（最底層）的概念，與通信協議沒有直接關系。而通信協議，是屬于通信網絡中的數據鏈路層（上一層）的概念。

COM口是異步串行通信口的簡寫，串口通常是9針，也有25針的接口，最大速率115200bps，串口的接口標准規範和總線標准規範是RS-232，有時候也叫做RS-232口。

3. RS232与RS485的主要区别/ Main differences between RS232 and RS485

規定	RS232 (MAX232)	R485/RS422（MAX485）
工作方式	單端	差分
節點數	1收、1發	1发32收(1发128收,MAX487)
最大傳輸電纜長度	15m	1220m
最大傳輸速率	20Kb/S	2.5Mb/s (1m以内)
最大驅動輸出電壓	-5V～+5V	-5V～+5V
驅動器負載阻抗(Ω)	300	27
接收器輸入電壓範圍	+/-15V	-7V～+12V
接收器輸入門限	+/-3V	+/-200mV
接收器輸入電阻(Ω)	7K	12K(48K,MAX487)
驅動器共模電壓		-1V～+3V
接收器共模電壓		-7V～+12V

4. 异步串口通讯协议的工作原理/Asynchronous serial communication protocol

其中各位的意义如下：

a) 起始位：先发出一个逻辑”0”的信号，表示传输字符的开始。

b) 数据位：紧接着起始位之后。数据位的个数可以是4、5、6、7、8等，构成一个字符。通常采用ASCII码。从最低位开始传送，靠时钟定位。

c) 奇偶校验位：数据位加上这一位后，使得“1”的位数应为偶数(偶校验)或奇数(奇校验)，以此来校验资料传送的正确性。

d) 停止位：它是一个字符数据的结束标志。可以是1位、1.5位、2位的高电平。

e) 空闲位：处于逻辑“1”状态，表示当前线路上没有资料传送。

f) 波特率：是衡量数据传送速率的指针。表示每秒钟传送的二进制位数。例如数据传送速率为120字符/秒，而每一个字符为10位，则其传送的波特率为10×120＝1200字符/秒＝1200波特。

5. 差分信号及差分信号抗干扰原理/Differential signal and anti-interference principle

差分傳輸：一種信號傳輸的技術，差分傳輸在這兩條線上都傳輸信號，區別于傳統的一根信號線一根地線的做法，差分信號，這兩個信號的振幅相等，相位相反。在這兩根線上的傳輸的信號就是差分信號。信號接收端比較這兩個電壓的差值來判斷發送端發送的是邏輯0還是邏輯1。在電路板上，差分走線必須是等長、等寬、緊密靠近、且在同一層面的兩根線。

傳輸差分信號的線纜必須是雙絞線，如圖所示：

差分信號抗幹擾原理：由于雙絞線是2根緊密殘繞在一起的一股線，當有外部幹擾的時候，2根線纜受到的幹擾信號是一樣的，産生同樣的波動，這幹擾信號在接收端進行“差”運算後，結果等于0，相當于沒有幹擾産生。

6. RS485通信接地问题/ RS485 communication grounding problem

RS485通信也需要處理接地問題：電子系統接地是很重要的，但常常被忽視。接地處理不當往往會導致電子系統不能穩定工作甚至危及系統安全。RS-422與RS-485傳輸網絡的接地同樣也是很重要的，因爲接地系統不合理會影響整個網絡的穩定性，尤其是在工作環境比較惡劣和傳輸距離較遠的情況下，對于接地的要求更爲嚴格。否則接口損壞率較高。很多情況下，連接RS-422、RS-485通信鏈路時只是簡單地用一對雙絞線將各個接口的“A”、“B”端連接起來。而忽略了信號地的連接，這種連接方法在許多場合是能正常工作的，但卻埋下了很大的隱患。

7. RS232接口的调试/ Debugging of RS232 interface

測試卡

測試：RJ45#1，RS45#2，和NMS2056等主機接口，直連。RJ45#3，與PC連接，用于跟蹤RJ45#1#2的數據通信。

8.RS485通訊的設備調試注意事項/ Precautions for equipment debugging of RS485 communication

a) 确定接口的TX+、TX-，使用万用表测试一下，确定电压在+5V以上。

b) 如果有GND，调试的时候最好将GND和监控主机的GND连接。

c) 如果TX+、TX-之间电压是+3V的，通信距离不要超过300米。

d) 如果通信距离超过300米的，确认2件事情。

e) 先使用电脑+转换器进行近距离调试，确认通信成功后在延长线路。

f) 波特率尽可能低，例如有2400、4800、96000三种波特率可设置，一定选择2400。

g) 主机和智能设备接口的地方并联120欧姆电阻，防止信号反射。

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