代码收藏家 NodeRED学习：构建MQTT/Modbus网关（第七部分）
简述：
用Node-RED 搭建一个MQTT网关，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。
网关：
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。
虚拟串口软件VSPD配置两对虚拟串口：
Modbus Slave模拟两个传感器：
配置MQTT代理：
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：
添加注入(inject)节点和延迟(delay)节点配置轮询：
添加函数节点，为消息添加主题：
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