If you have an application that would adapt well to computer control or would just like to learn about interfacing PCs to the real world, this project is for you.

Connection couldn’t be simpler; just plug it in to the parallel printer port on your PC, hook up DC power and you’re ready to begin experimenting.

Fig.1: the Windows-based software is easy to drive, with everything controlled and displayed via this dialog box.

The card’s eight digital outputs can be used to control devices such as relays, solenoids, motors and lamps. Ten analog inputs are provided too and these can be easily interfaced to a multitude of devices like temperature, pressure, light and position sensors. In addition, two variable voltages can be generated using the analog outputs.

Software examples are available if you want to write your own control programs or you can download Windows software, written by James Rickard, from SILICON CHIP to get you off the mark right away.

Low power consumption means that it can be battery powered or it will operate from any DC power source from 7.5V to 25V.

In the following text, we take a brief look at the PC parallel port and how it connects to the interface board. We then look at how data is transferred from the parallel port to the interface board. Next, we examine how that data is used to generate the digital and analog outputs. Last but not least, the analog-to-digital section gets the treatment.

PC parallel port basics

Fig.2: this dialog box provides the setup options.

Software control of the interface board is carried out via the standard PC parallel printer port. Table 2 lists the function of each signal on the PC parallel port as related to its usage on the interface board. For reference, we also show the function of each signal when the port is used for its "normal" purpose – driving a printer!

All signal lines in and out of the PC parallel port are at TTL or CMOS (0-5V) voltage levels. The port occupies three sequential addresses in the PC’s I/O memory map. The first address is called the "base" address.

PCs support up to three parallel ports, commonly referred to as LPT1, LPT2 and LPT3. Generally, the first two ports are mapped to base addresses 378H and 278H, respectively.

For example, to read the IC4 data out and EOC pins when the interface board is connected to LPT1, the software would read I/O address 379H (base +1).