A PC based Analog Voltmeter. The Printer Port or Parallel Port Interface is used to acquire high resolution analog data using ICL7135. This is a method of getting analog data into your PC. This is only of Educational Value. Applications include Data Loggers and PC based Process Monitors, chart recorders. (USB, Wireless Interfaces and Tablet Computers are in vogue today
The four Digit Drive outputs and four BCD outputs of the Multiplexed Display Driver of 7135, along with A-D status output which is also four, are routed to four input pins on printer port via three 74HCT373 Digital Switches. This is because we have less Input pins on Port and 4 are shared by making one 74HCT373 transparent and others High Impedance when PC takes a reading. So the three sets of four outputs of 7135 can be read sequentially.
74HCT139 U9A which is controlled by the PC program can select U3, U4 or U5 for sending data to 4 pins on printer port. Data sent on the eight output pins of printer port can be latched by U10B into U6. This eight outputs of U6 can drive relays or lamps.
The ICL7135 details can be seen here 41/2 A/D with BCD ICL7135 . This Mixed device was far ahead of its times, a product of Intersil.