a combination electrosensitive reader - printer able to perform both functions by sensing the condition of a recording medium in the read mode and by altering the condition of the recording medium in the printing mode . the change of mode is accomplished manually or automatically . the data to be read or written may be coded optically or digitally for interpretation by a processor . the recording medium is a metalized paper or similar material with an ink under or over lay . the resistivity , conductivity or capacitance change of the medium is sensed or altered by the reader - writer . appropriate timing tracks and registration marks are normally placed to enable reading and writing by different devices . the styli for reading and writing normally are disposed in an array , straight or angled relative to the paper path . the styli have tips which are cylindrical , rectangular or oval in cross section . control circuitry , including an analog to digital converter , control input to and output from the reader - printer .