BRX Do-more PLCs MQTT Part 1: MQTT Basics from AutomationDirect

mqtt iot stands for message cueing telemetry transport and is a protocol in the popularly growing system of iot or the internet of things this system consists of interrelated devices automatically exchanging data with one another however devices that use mqtt do not directly exchange data with one another instead they utilize an mqtt broker all kinds of devices utilize mqtt like coffee makers, thermostats, temperature sensors, cell phones, and yes brix plc's some or all these devices might make some of their data available to other devices by publishing that data to the broker other devices might need that data and so these devices will subscribe to those data via the broker usually devices do one or the other the brx plc can do both what does the broker do it receives published topics it distributes topics to subscribers and it sends the last will and testament or the lwt to subscribers if a client ungracefully disconnects that is dies so to speak what does the client do a client provides the broker with his own keep a lifetime and qos or quality of service optionally he can publish his own topics their associated retain bit and his own last will and testament optionally he can also subscribe to topics so what is a topic very simply a topic is the name you give to the data like temperature or psi or whatever how about payload payload is the value the topic has in other words if the topic is temperature then the payload would be say 79 degrees f message this is the topic plus the payload this is actually what's exchanged between broker and client what is the qos or quality of service there are three values for the quality of service zero one and two zero is at most once and it means that a message is sent once it relies on tcpip to get the message to the interested device one is at least once and this means that a message will be transmitted until an acknowledgement is received from the receiver so it could be transmitted multiple times but guarantees the receiver gets it at least once two is exactly once and this means that the message needs not only an acknowledgement from the receiver but other qualifying exchanges as well the brx plc does not use a qos of two retain this bit is associated with a published topic and it tells the broker whether or not to retain this topic even after sending it to the subscribers if it is set to not retain then the broker will delete the topic after it has sent it to this to the subscribers keep alive time this time value tells the broker how often he should hear from the client before considering him to be dead if publishing subscribing and updating is happening between broker and client faster than to keep a lifetime then all is well if however for some reason there is no activity between them then the brx plc will purposefully ping the broker at about 75 percent of this value to let him know that he is not dead if he does not do this and to keep alive time expires the broker knowing this value will consider the client to have died last will and testament now this is a funny term but it's appropriate for this use this is a topic published to the broker to tell him essentially this if you haven't heard from me within the keep a lifetime then consider me dead and send this particular topic to all those other clients who are interested in my well-being if the client therefore dies subscribers will get his last will and testament if the client disconnects gracefully that is he sends an mqtt disconnect then the broker will delete his last will and testament without sending it to anyone session type there are two kinds clean and persistent clean first let's say the connection is disrupted and the client disappears the broker then deletes or cleans this client's subscriptions when the connection is restored the client must re-subscribe to the topics that he's interested in persistent means that he will not clean up after the client disappears so once again let's say the connection is disrupted but this time the broker keeps the client's subscriptions in other words the subscriptions persist and even cues up messages for the missing client thus when the connection is restored the client does not have to resubscribe and even receives any queued up messages mqtt was introduced in do-more technology version 2.3 in june of 2018. it had no tls or ssl encryption support only supported a qos of zero only a clean session type was allowed and there was no user specified client id allowed and the only data that you could publish was text also whatever was subscribed to was stored in text in the bricks this means if the data you needed was a numeric you had to use other do-more instructions to convert it from a string to a numeric also of course the mqtt pub and mqtt sub-instructions were added to facilitate using mqtt in a brix plc version 2.52 came out in april of 2019. in this version tls and ssl encryption was added also a qos 01 was supported and also a persistent session type was added also two new instructions were added that can be useful when using mqtt the reason being many times data you get from a topic comes back in the popular json data format so to aid you in this data format both sending and receiving instructions json build for building your own data in json format and json parse for interpreting data that you receive in this format were added version 2.62 came out in august of 2019. this version changed the mqtt pub and mqttd sub instructions so that they can send and receive numeric data instead of text only version 2.74 came out in april of 2020. this added a tool for viewing json data in a pretty format json data comes back in a single string yet it can have many nested topics or objects viewing it as a single string is very difficult the new tool allows you to view the json data string in such a way that it's easy to view and to pick out what you're interested in also the client id that in previous versions was generated automatically by do-more was now allowed to be user specified in a 64-byte string this concludes the overview of mqtt for bricks to see how to avail its use in the briggs plc check out our simple mqtt example video