The UT 803 is a low cost yet precise lab multimeter.
The Serial Port (RS232) obviously has no output ...
unless you supply it via Pin 4 and 8 of the SUB-D jack.
The supply of the RS232 Board may be done with the handshake signals, but if you build your own measurement setup,
this is likely to be overseen. (The manual forgot to mention that)
As you may see on the (reverse engineered) schematics, a positive supply (+5V to +12V) must
be applied to PIN 4, whereas a negative voltage (GND or -12V) must be applied to Pin 8.