If by 'digital' you mean a communication protocol like Profibus or Foundation Fieldbus, buy a replacement DP pressure transmitter that covers the same DP pressure range and is made of the same materials. Replace the pneumatic DP cell with the Fieldbus model and connect to your digital Fieldbus. Or you could use a gage pressure Fieldbus transitter that monitors the 3-15psi/20-100kPa pneumatic output of the Foxboro.
If by 'digital' you mean a discrete electrical change (on-to-off or off-to-on) at some pressure value, buy a pressure switch and pipe it to the 3-15psi/s0-100kPa output signal from the Foxboro and set the electrical pressure switch setpoint at the appropriate trip point.
