The Walchem WDP320-54N controller is now obsolete and the closest equivalent is the WPH600 controller. The new WPH600 series has upgraded features; for reference, here is the description of the old model:
w/ 2 pulse proportional and 2 on/off control outputs and diagnostic alarm relay, hardwired, cable glands, single isolated analog (4-20 mA) output, no electrode