Null Printer Adapter

Copyright Tomi Engdahl 1996,1997

This circuit is a simple adapter for PC parallel port to think that there is a printer connected to computer. This adapter is useful for testing printing speed with different software and as a part of your own designs.

You can use this circuit to connect your simple circuits to the parallel port. Usually many home-made circuits use parallel port in a way which you have to use direct I/O commands to control the parallel port to be able to communicate to your circuit. Unfortunately this approach is not always possible, because some operating system prohibit the programs to directly control the computer hardware. In this situation you might want to use standard operating system printer printing functions for sending data to your circuit, but it does not work because your circuit does not generate the handshakes that are needed.

With this adapter you don't have to worry about generating handshaking signals in your own circuits. This simple circuit consists of only few wires connected to parallel port connector and it makes the computer to think that there is a real printer connected to parallel port and accepting the data.

Circuit operation

This circuit works using following principles: Bysy and paper out signals which normally come from printer have to be kept low, so they are tied to ground. Select and error signals which normally come from printer must be kept high, so they are connected to reset output in parallel port. Acknowledge signal is directly driven from strobe line.

Circuit diagram

Connecting diagram ASCII presentation:

1  -> 10
11 -> 12,18
13 -> 15,16

The circuit is very easy to built inside the printer connector shell. The circuit can be easily shown in ASCII and graphical presentation. All pin numbers are for 25 pin D connector (as in PC parallel port).

Another proposed circuit

I saw the following schematics was shown on Dontronics web pages. The circuit is simpler because it does not connect to all handshaking pins. The basic idea is the same as in the circuit shown above.

Make up a DB-25 plug as follows:

• Connect Strobe (1) to ACK (10)
• Connect Busy (11) to PE (12). Connect these via a 270 Ohm resistor to Ground (18 to 25).
• Connect SLCT (13) ro ERROR (15). Connect these via a 22K resistor to INIT (16).