The Walchem WDP420-12NN controller is now obsolete and the closest equivalent is the WPH620PSSAN-NN controller. The new WPH600 series has upgraded features; for reference, here is the description of the old model:
pH/ORP controller, 2 pulse proportional relays & 2 on/off dry contact relays & 1 dry contact dedicated alarm, 120 VAC, pre-wired with 20 ft. cables for E Class pumps, two isolated analog (4-20 mA) outputs, no electrode, no USB