If you read voltage (RS485 + / - Port from back of DVR) difference from voltage meter while DVR is sending signal (focus, zoom, etc), you will read around 2.2v - 2.5v.
Normally, RS485 signal is measured by Oscilloscope which is much accurate. The voltage range is around 4 - 4.5 v from oscilloscope.
If you can't read voltage range from DVR, you need to contact tech support.