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PacketSerial
An Arduino Library that facilitates packet-based serial communication using COBS or SLIP encoding.
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SurveyJS
Open-Source JSON Form Builder to Create Dynamic Forms Right in Your App. With SurveyJS form UI libraries, you can build and style forms in a fully-integrated drag & drop form builder, render them in your JS app, and store form submission data in any backend, inc. PHP, ASP.NET Core, and Node.js.
You can write test programs to send very specific messages to simulate errors, or simulate entire components that aren't written yet. There are also free programs like MQTT Explorer that will let you browse the message traffic, generate messages manually, log whatever you cant, and even graph your values if you happen to send numerical values (that is really cool when you do some long-term testing).