Originally published by Paul Stenning in Everyday Practical Electronics, January 1996

• Diagrams and Schematics

Many households are now likely to have more than one PC-compatible computer. Ideally it would be useful for each to have its own printer, though budgets may not stretch to this. In many cases, though, it may be rare for both computers needing access to the printer at the same time.

Since plugging and unplugging a printer between computers soon becomes tedious, a switch or a multiplexed printer sharing unit can provide a better solution. A switch works fine, of course, but it is not ideal as it requires manual intervention - and inevitably there is a tendancy to forget to operate it. The Printer Sharer described here does not need to be remembered.

What it Does
The Printer Sharer will allow two PC's to share one parallel (Centronics) printer without manual intervention. Users simply print from their software applications as usual, and the printer sharer lets the data through on a first-come first-served basis. The computer that comes second receives a "busy" signal until the first computer has finished. Most software, including Windows 3.1 Print Manager, will happily wait for the printer to become available.

Unlike some commercial printer shaeres, this one allows one of the computers to be turned off, yet still allowing the other to use the printer. The unit is designed to overcome the shortcomings of some commercial printer sharers tried by the author.

The timeout period is individually adjustable for each computer to allow for programs which print slowly and send data infrequently in bursts. Adjustment is by two internal presets, and ranges from ten seconds to one minute approximately. Two LEDs indicate which computer has control - these can probably be ignored once the timeout delays have been set-up.

If one computer is off, the other computer can still access the printer. It is even possible to switch one computer on and off while the other is driving the printer, without effecting the printing.

Some recent printers are "Energy Star Compliant". In practice, this means that if the computer it is connected to is switched off, the printer goes into standby mode. This printer sharer will allow this feature to operate only when both computers are switched off.

In most cases this unit can be powered directly by the printer it is controlling - using the +5V which is normally available on pin 18 of the Centronics connector. A few printers omit this, so a separate power supply will be needed for use with these printers. A voltage regulator is included on the PCB, so the unit can be powered by nothing more fancy than a mains adaptor (battery eliminator) delivering around 9 Volts DC.

This unit is designed for parallel printers only, and will not work with serial (RS232) printers. Also, it has only been tested with IBM PC-compatible computers. Although the Centronics Parallel Interface is supposed to be a standard, it cannot stated for certain that this unit will work with other types of computer.

Parallel Interface
Definitive information on the parallel interface is not easy to come by. The author has examined the manuals for several different printers, and found that different manufacturers interpret the standard in slightly different ways. However, by gleening information from the manuals, it was possible to establish a reasonable understanding of how the Centronics interface works, and its possible variations.