Process controller with interchangeable individual I/O units

A method of determining a type of a plurality of I/O circuits connected to a processor, said method comprising the steps of:

• (a) transmitting a code-request signal from a processor to an I/O circuit;
• (b) receiving a code signal from said I/O circuit in response to said code-request signal;
• (c) determining the type of said I/O circuit based upon said code signal received from said I/O circuit during said step (b), said type being one of the four following types:
• an analog input circuit,
• an analog output circuit,
• a digital input circuit, or
• a digital output circuit;
• (d) if the I/O circuit to which said code-request signal was transmitted during said step (a) is said analog input circuit, then communicating a first analog value from said I/O circuit to said processor;
• (e) if the I/O circuit to which said code-request signal was transmitted during said step (a) is said analog output circuit, then communicating a second analog value from said processor to said I/O circuit;
• (f) if the I/O circuit to which said code-request signal was transmitted during said step (a) is said digital input circuit, then communicating a first digital value from said I/O circuit to said processor;
• (g) if the I/O circuit to which said code-request signal was transmitted during said step (a) is said digital output circuit, then communicating a second digital value from said processor to said I/O circuit;
• and
• (h) continuously repeating said steps (a) through (c) and one of said steps (d) through (g), said steps (a) through (c) being performed prior to each performance of one of said steps (d) through (g).