The Walchem WDP420-1NNU controller is now obsolete and the closest equivalent is the WPH620PSSNN-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, no 4-20 mA output, no electrode, integrated datalogging and config file import/export via USB flash stick.