Design Ideas: July 6, 1995
You can use a desktop or laptop PC to monitor physical variables such as temperature or pressure. A convenient way to do this is to connect an A/D converter through the ubiquitous Centronics printer port (Fig 1). The current requirement of the ADC0804LCN used here is less than 1.5 mA at 5V supply voltage with a 640-kHz clock. Tristate-buffer 74HC244 multiplexes 8 bits of A/D-converter data through 4 bits (54 through 57) of the status port of the printer adapter.
This design does not require an external power supply, because it uses the RS-232C port of the PC to satisfy its meager power requirements. Most RS-232C ports are capable of delivering a current of about 10 mA. Of this current budget, the circuit shown here consumes about 3 mA. You could use the remaining available current to power additional circuitry. The 5V for the A/D converter and the tristate switch derives from the RS-232C port's DTR and RTS signal lines. Because the power requirement is low, you can use a pair of LM-336-2.5V reference diodes to set the voltage and to generate the 2.5V reference for the A/D converter.
Control-port bits C0 and C1 control the tristate switch, IC2. The power-on reset level of C0 and C1 is logic one, so you run no risk of shorting IC2's outputs. The other 2 control bits, C2 and C3, generate the read and write strobes for the A/D converter. The INTR pin of the A/D converter, which status-port bit S3 monitors, indicates end of conversion for the converter. With the component values shown, the converter has a 200-µsec conversion time on a 16-MHz AT, running the C code in Listing 1. A Turbo C version 2.01 compiler compiled this source code. (DI ##1728)