Technical Tips


A project of connecting analogue signal to LPT printer port. Included a VB6 software and a circuit diagram. The ADC0804 is an 8 bit analogue to digital converter. 74HC4052 multiplexes the 8 bits output of the ADC into 2 data lines, it is needed because most LPT's have only 5 input lines. 74HC4052 can be replaced by 4052B CMOS.
You can read more about LPT interface in

The VB6 software includes a freeware DLL (inpout32.dll) from
VB6 code (30KB). Keep the .exe file and .dll in the same folder. You are free to use the circuit diagram and the VB software with no limitations.

The VB6 application is a combined digital and analogue voltmeter.

Circuit Description
See also Technical Tips

5 volt supply can be an external power supply, using the hosting computer 5V is much preferred because variation between the computer 5V and the external 5V can lead to undesirable currents through the data lines. 5 volt supply is available at a USB port or the 4 way peripherals connector from the internal power supply unit.
10k resistor and 470pF are for setting the internal clock of the ADC.

Input at pin 6 of the ADC is 0 to 5 volts full range positive voltage only. The chip over voltage protection is 30V max.
Pins 2 and 3 of LPT select the 2 out of 8 bits to be read. Pins 15 and 13 read the selected 2 bits. Pin 1 initiates the ADC.
74HC4052 can be replaced by 4052B CMOS 4000 series.