Nuts and Volts - March 2015

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■ FIGURE 3. Pinout of the PL2303HX cable.

■ FIGURE 4.

Prolific adapter schematic.

unconnected end of the jumper wire into either the +5V rail or the ground rail.) The jumper wire provides an opportunity to observe an interesting aspect of the circuit, as we’ll soon see.

After you type this code into the PICAXE Editor, download it to your breadboard setup. Whenever you insert the loose end of the long jumper wire into the +5V rail, the When you’ve completed your breadboard setup for the experiment, we can turn our attention to the program we’ll use to test the inverter. It’s so short that I’m including it here; just type it into the PICAXE Editor. The program simply displays the real time state of the input to pin C. 3 on the LED. If the input is high, the LED is on; if the input is low, the LED is off.

LED should be off; whenever the loose end of the jumper wire is inserted into the ground rail, the LED should be on.

because it’s known to work reliably with PICAXE processors (as long as the two data signals are inverted — which we’re about to do); it’s readily available; and it sells for less than half the price of the FTDI cable.

Also, note what happens whenever you disconnect the jumper wire from the +5V rail — the LED immediately lights. In other words,

Figure 3 presents the pinout of the cable that we’ll be using in this experiment. A cable with the same color-coding is available on my website ( www.JRHackett.net) and elsewhere. (If you use a different PL2303HX cable, the color coding of the four wires may differ.)

The schematic for Experiment 2

(and 3) is presented in Figure 4.

‘‘=== InverterTest.bas ========
‘‘Program tests the 2N3904
‘‘inverter.

+5V is the only input level that results in a low input to pin C. 3. If the input to the transistor switch is at ground level or if it is entirely disconnected, the input to pin C. 3 is high.

‘‘=== Directives ========
  #com 6
 #picaxe 08M2
  #terminal off
  #no_data

When you’re finished testing your breadboard setup, we can move on to applying what we’ve learned in Essentially, it consists of two NPN transistor inverter circuits: one for each of the two data signals. Both inverter circuits function identically to the single inverter circuit of Experiment 1, and all six current-limiting resistors are again 10K.

‘‘=== Constants =========
  symbol LED = C.2
‘
‘‘=== Variables =========
  symbol data_In = pinC.3

Experiment 1. We’re going to invert the Tx and Rx data signals on a USB-to-serial cable, so that it can be used to power and program our PICAXE projects.

In this experiment, we can ignore the two headers in the schematic and just focus on the required connections. (When we get to

‘‘=== Begin Main Program ======
do
  if data_In = 1 then
 high LED
  else
 low LED
  endif
loop

Experiment 2: Inverting the Data Signals on a USB-to-Serial Cable

Experiment 3, we’ll discuss the two headers.) As you can see in Figure 4, the output signal from the Prolific I chose to use the Prolific PL2303HX cable for Experiment 2