user41969: Objective: To get my USB keyboard to interface with an Arduino Uno board. 
Constraints: I can't afford a USB host shield for now for Arduino. Currently available is a keyboard with PS/2 connector. That example is available here. However, it is silly going for such a keyboard, or, buy a USB to PS/2 converter at the moment.
Challenge: USB has VCC, GND, RX/TX. (Rx/Tx also labelled D+/D- respectively). The parallel port connector has the following: 

Where will the D- pin go?

user4143: Many USB keyboards (certainly older ones, though almost certainly not wireless ones) are also PS/2 keyboards.  The wiring of the cable is all that differs, and the chip in the keyboard changes its behaviour depending on how it's wired.
A simple USB to PS/2 adaptor is merely a USB socket and PS/2 plug wired together in a certain way:

So that's:

5V to 5V
D- to Data
D+ to Clock
GND to GND

It may work with your keyboard, or it may not. All you can do is try it and see.

41969: <p>Objective: To get my USB keyboard to interface with an Arduino Uno board. </p>

<p>Constraints: I can't afford a USB host shield for now for Arduino. Currently available is a keyboard with PS/2 connector. That example is available <a href="https://playground.arduino.cc/Main/PS2Keyboard" rel="nofollow noreferrer">here</a>. However, it is silly going for such a keyboard, or, buy a USB to PS/2 converter at the moment.</p>

<p>Challenge: USB has VCC, GND, RX/TX. (Rx/Tx also labelled D+/D- respectively). The parallel port connector has the following: </p>

<p><a href="https://i.stack.imgur.com/jVlLN.gif" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/jVlLN.gif" alt="enter image description here"></a></p>

<p>Where will the D- pin go?</p>
 4143: <p>Many USB keyboards (certainly older ones, though almost certainly not wireless ones) are also PS/2 keyboards.  The wiring of the cable is all that differs, and the chip in the keyboard changes its behaviour depending on how it's wired.</p>

<p>A simple USB to PS/2 adaptor is merely a USB socket and PS/2 plug wired together in a certain way:</p>

<p><a href="https://i.stack.imgur.com/tXhcw.jpg" rel="noreferrer"><img src="https://i.stack.imgur.com/tXhcw.jpg" alt="enter image description here"></a></p>

<p>So that's:</p>

<ul>
<li>5V to 5V</li>
<li>D- to Data</li>
<li>D+ to Clock</li>
<li>GND to GND</li>
</ul>

<p>It <em>may</em> work with your keyboard, or it may not. All you can do is try it and see.</p>
