This is an interesting read from MOAM Industries and might enlighten you to consider ways of finding serial data where it seemingly doesn’t already exist.
Initially we considered our own interface for a blood pressure cuff. Just run the pump and take the readings with our own processor and pressure sensor, how hard can it be. Rather difficult it seems, the processing and knowledge required to develop a device to perform even rudimentary readings would have completely blown the time budget. Instead we looked to hack an existing device, enter the Omron RS8.
Working through the ICs on the top of the PCB and searching for data sheets showed the main IC to be something somewhat obscure and difficult to source information for. A Similar scenario again for the NFC chip, little available information in data sheets or examples on the web. Reviewing the product documentation gave the impression the NFC interface was designed to upload logs and not stream real time data. The rear side of the PCB however held promise. A separate IC was used to drive the segmented LCD display built into the housing, the BU9795A. Whilst still an obscure component a data sheet was available and it listed an SPI interface with a nice clear protocol.