Communications of the ACM

The delay after clicking the mouse and the door opening is less than one second, making the system fast and convenient, compared with similar systems based on telephone calls.

The data lines are designed to send information to external peripherals like a printer, so they were specified as output-only in older SPPs, though today all computers support bidirectional data flow on these lines. Status lines are used for handshaking and status indicators—paper empty (S5), busy signal (S7), or device error (S3)—act as input lines. Finally, control lines are used as interface control and handshaking, acting as input and output lines, although in newer ports they may be output-only. All the lines are accessible through consecutive registers beginning at I/O address 0x378 for the LPT1 port or 0x278 for LPT2 port (these are the typical I/O addresses, but they vary in some PCs).

In order to make an action, like opening the door or turning on the light, a set of relays is used. In some cases the relay must close a 220v circuit, so all relays used in the circuit board are designed for 220v and controlled by a 5v primary circuit. According to the IEEE 1284 standard, voltage levels at the signal lines are 0v or 5v (maximum 5.5v without charge and minimum 2.4v at 14mA). The current taken from the parallel port is lower than the current needed to operate the relay, so a simple circuit based on a transistor was used. This circuit uses an external voltage supply of 5v taken from the power supply of the PC.

It is possible to manage eight electrical devices using one circuit for each data line of the connector (pin 2 through pin 9). A program that writes a value to the I/O address 0x378 will set 5v or 0v values for these pins and therefore will control which relays open or close.