I interfaced a 40-year-old 8-bit computer to a Mendix application using Web Serial in the browser. To do this, I wrote a custom JavaScript action. If your multimeter and barcode reader expose a serial port or a Bluetooth port then you could use the same approach. I wrote up my work on Medium and you may find that useful.
https://medium.com/@robert.price_68862/making-mendix-talk-to-a-40-year-old-computer-72fa037a6323
I hope this helps.