A system for transferring electronic funds data includes portable modules (20) each having a display (24) and a keyboard (26) together with an optical interface (36) adapted to transfer electronic funds data to other modules either directly or over the telephone lines through resident units (62). The modules (20) are operable in either a credit mode or a debit mode using a selector switch (30). Prior to establishing communication, the module determines whether it is in communication with another module or with a resident unit, then undergoes a "handshaking" protocol to establish half-duplex, bidirectional communication, before funds are actually transferred. Transaction records are retained in a random access memory (44) within each module. The transaction records are periodically downloaded to a central computer, or printed locally to generate a hard copy. Other applications of the system include monitoring physiological data, maintaining work-time records and monitoring radiation dosage.