物联网AIoT之MQTT与MODBUS在开源低代码物联网平台的相爱相杀
现在我们应用Node-RED 搭建一个MQTT网关，把一个水箱的水位和水温发送到物联网平台。网关通过Modbus总线连接一个水位传感器和一个温度传感器，采集水位和温度数据，再通过MQTT客户端把水位和温度信息发布到MQTT代理。
1.0 网关用途
为了便于理解，我们这个示例非常简单，在日常的应用场景中，会比较复杂。这个网关的主要用途是协议转换和信息汇聚。
2011年客户定制的4路无线DVR通过CANBUS走MODBUS协议读取瓦斯等传感器数据
https://www.besovideo.com/detail?t=1&i=141
协议转化：
网关可以通过Modbus总线把多个485串口的传感器，以轮询的方式采集，通过MQTT协议发布给信息平台。
信息汇聚：
网关可以把多个传感器数据，聚合为一个对象，为数据附加语义描述，使信息具有结构化，易可识别。
2.0 网关配置
2.1 模拟两个传感器
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用虚拟串口软件VSPD配置两对虚拟串口com3-com4，com5-6。
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用Modbus Slave模拟两个传感器分别使用com3和com5。
2.2 实现网关
2.2.1 配置MQTT代理
拖拽一个Aedes MQTT broker 节点到工作区，双击节点打开节点编辑器：
这里使用了默认的配置，端口号为1883。您可以根据需要修改配置。
2.2.2 通过Modbus分别采集水箱的水位和温度，合并为水箱参数，由MQTT客户端发布
2.2.2.1配置MQTT客户端
拖拽两个Modbus-Getter节点到工作区，双击节点打开节点编辑器，如下图所示配置：
双击右下角的铅笔图标，编辑Sever(服务端)：
这里串口端口分别使用com4和com6，波特率与传感器端一致。
2.2.2.2 配置轮询
如下图所示，在Modbus-Getter节点左侧添加注入(inject)节点和延迟(delay)节点：
双击打开注入节点编辑器，按下图配置节点周期注入消息：
双击延迟(delay)节点，设置信息延迟10毫秒：
2.22.3 为消息添加主题
在Modbus-Getter节点右侧，添加函数节点，为消息添加主题：
打开函数编辑器，编写函数体，为消息添加节点：
水位的函数体：
msg={topic:"WaterLevel",payload:msg.payload[0]}return msg;
水温的函数体：
msg={topic:"WaterTemp",payload:msg.payload[0],complete:true}return msg;
2.2.2.4 聚合并发布消息
两个函数节点的消息都传递到后边的连接(join)节点：
打开连接节点编辑器：
连接节点把消息组合成键值对对象输出给mqtt out节点，下面配置发布主题，如下图：
到这里网关功能完成。
2.3 测试
下面配置一个mqtt in节点，测试消息：
双击打开mqtt in节点，配置订阅主题，如下图：
完成后部署，会在右侧调试面板看到收到的消息：
3.0 完整源码
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"47697d9d8a564c16","name": "","props": [{"p": "payload"},{"p": "topic","vt": "str"}],"repeat": "5","crontab": "","once": true,"onceDelay": "10","topic": "","payload": "","payloadType": "str","x": 150,"y": 220,"wires": [["67b13033ddc0d223","cb911d4fa59973ac"]]},{"id": "16aa2453141586b1","type": "debug","z": "47697d9d8a564c16","name": "debug 4","active": true,"tosidebar": true,"console": false,"tostatus": false,"complete": "payload","targetType": "msg","statusVal": "","statusType": "auto","x": 420,"y": 380,"wires": []},{"id": "cb911d4fa59973ac","type": "delay","z": "47697d9d8a564c16","name": "","pauseType": "delay","timeout": "10","timeoutUnits": "milliseconds","rate": "1","nbRateUnits": "1","rateUnits": "second","randomFirst": "1","randomLast": "5","randomUnits": "seconds","drop": false,"allowrate": false,"outputs": 1,"x": 210,"y": 280,"wires": [["844ad47ff9474f12"]]},{"id": "d4cba5fe6569d8c3","type": "function","z": "47697d9d8a564c16","name": "水位","func": 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