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I'm working on a fingerprint sensor that is known as FPM383. It is much like the well known R503 sensor. Their datasheet and serial communication manual are almost identical.

I was trying to control the sensor module from an MCU like ESP32 and got it working well. But I also have found a test software for the module that runs under Windows:

enter image description here

So I used a logic analyzer to eavesdrop module's communication and to my surprise I found out the software is not communicating with the rules explained in the manual!!!

For example, in the official manual, all messages begin with 0xEF01 and followed by the module address: 0xFF 0xFF 0xFF 0xFF. Otherwise, they are disregarded as garbage:

enter image description here

But when the software communicates the module through a USB to serial converter, all messages start with 0xF11F 0xE22E 0xB66B 0xA88A

enter image description here

Is that normal for modules to have two sets of communication rules? How can I find the manual for the other method of communication?

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  • \$\begingroup\$ Capture a comprehensive set of communication sequences using the logic analyzer. Compare the sequences to the documented protocol and look for patterns or specific commands that might reveal the protocol structure. If the fingerprint sensor does not work after all your efforts, you can switch to the generic one. theengineeringprojects.com/2023/04/… \$\endgroup\$
    – liaifat85
    Commented Jun 18 at 8:23
  • \$\begingroup\$ @liaifat85 Thanks for your comment. I have done so but there was absolutely no common pattern with the known protocol. Good news is that I have found a manual regarding that alternative (apparently the main and recommended) protocol. Bad news is that it is in Chinese! \$\endgroup\$
    – AKTanara
    Commented Jun 18 at 20:24

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