Parallel Port Digital Probes

As a digital logic scope all basic hardware that is needed to get started already exists in the hardware of your parallel port if you are using a PC. is a common ground line (pins 18-25) and the sampling lines to be tied to the data inputs of the parallel port (pins 2-9). This is all you will need to get a two(2) to eight(8) line digital logic scope started to help debug slower digital designs on your computer screen.

The above design directly connects your computer's parallel port to the outside world and any circuits you try to test. It only safe to be used with signals that range from ground (0 volts) and Vcc (5 volts). This leaves the danger that if there are any high or negative voltages in the circuit to be tested that damage to your computer's hardware could result. If you accidently connect to the wrong voltage point then you could blow your computer's printer port or even worse damage your basic computer itself.

The simplest protection to add is a 1K resistor in series to the port's input. This limits any current flow and it's possible damaging affects. Most interface circuits used in computers today already have protection diodes built-in, the addition of the resistors should make it safer if you happen to touch a negative five(5) volt or a positive twelve(12) line - two common voltages found inside most computers.

Another way to limit possible damaging signals is the use of blocking diodes, the diodes directly in-line to the parallel port prevents high positive voltages reaching the inputs of the port. If you are sure only positive voltages exist in the circuits to be tested that is all you will need for protection. However, if there is also the possibility of negative voltages existing in the test items then additional diodes connected to ground will prevent negative voltages reaching the same inputs.

Used in combination with resistors that limit current flow you should get better protection. Note: the combination the capacitance of the diodes with the resistors results in a RC time delay that limits the speed the probes will respond to signal changes. If faster sampling code is developed it is probably better *not* to use this design as it may be too slow for high speed sampling.

Finally, you can use buffer circuits to protect your port. There are a number of octal buffers you can use. The 74LS245 is readily available in most electronic parts stores and does not load TTL circuit's fan-out heavily. If other models of buffers are used, try to get the LS versions.

GamePort Joystick Digital Probes

Game ports are available on most computers either separately or as part of the SoundBlaster compatible sound cards found in most PC computers. The basic pin-out of a standard game port supports four buttons that are accessed as belonging to one of two joysticks as is shown here.

In most designs the buttons are feeded into a TTL compatible gate, with a pull-up resistor (about 2.2K to 5v) added to each input. In theory we can just probe with the button inputs directly to our circuit to be tested. To be safe I suggest adding a 330 ohms resistor in series to the probe to prevent damage from possible shorts since not all game ports are designed the same way. Below is the basic design suggested.

To use as a reliable digital input that does not load down the circuits we are testing we need a way to convert digital signals to openning and closing a connection of the buttons to ground. Either the 74LS06 or the 4066 chips are ideal for this job. Both will convert a high input to a grounded output. Below is a design using a 74LS06 hex invertor chip.

One problem with the open collector design is it can be more sensative to outside signal noise, an active driving chip like the 7404 can be used instead as it is a pin for pin match to the 7406.

And this is another design using the 4066 analog switching chip.

Future improvements will probably be by user request.