Yes the footprint could be smaller but I want to make a Library with all function available. Special1 and Special2 needs to be coded.
If you want to debug example code use GitHub - gremlinc5/VirtualInstrumentsM9803R: An easy to use virtual M9803R Mastech Multimeter to be used on a Serial Port
In the Publish folder there is a setup to install a M9803R Virtual Instrument. With that you will be able to send to your arduino M9803R packets with a simple USB to TTL converter.
Your PC with Virtual M9803R sends packets to your arduino.
Remeber to set RS232C CheckBox in the program or you will not get packets.
With the example code you will get the led set high if the value of the packet is greater than 10.0 (absolute value, not the four digits value that are relative to the range in use).