How do I hook up a +24V supply to a loop-powered DMS-20LCD-4/20S?
You don't! The two input terminals on DMS-20LCD/4/20S can only be connected to the modulated 4-20mA transmitter output current. The system power supply's two "+" and "-" output leads should never be connected directly to a loop-powered device's "+" and "-" input terminals; doing so will place the full supply voltage (sometimes in excess of 36Vdc!) across the monitor's input resistors. These input resistors typically measure less than 30 Ohms, and can easily be damaged by the resulting excessive power dissipation.