use a built-in oscillator, so no external crystal is needed.
The MCP2200 board uses a tiny 12 MHz ceramic
resonator.) Most modules have five output pins (RX, TX,
GND, + 3.3V, and +5V), giving you a choice for powering
other circuits. Some boards (like the one on the left) only
have four pins, with a jumper block for selecting Vdd
(+ 3.3V or +5V).
There is no standard pinout sequence. You can
temporarily power your PICAXE circuit from the module,
but for a final design use something that works when the
download cable is unplugged.
Nearly all modules have one or more LEDs to indicate
power, transmit, receive, etc. All of them use a USB Type
A connector which eliminates the need for a separate USB
cable. My favorite is the board at the far right of Figure 3,
which is supplied with a protective outer wrapper of clear
heat shrink tubing. It uses a Prolific PL-2303HX chip, is
well built, reliable, and has good stable drivers.
If you search Google for CEG004200 or search eBay
for PL-2303HX, you'll find it widely available for as little as
$1 or $2 including shipping. None of these boards have
the capability of inverting RX and TX. You must add a
couple of inverters powered by the module's +5V output.
That's easily done in several ways.
Figure 4 shows how you can use discrete transistors
and resistors for the inverters. Any type of NPN transistors
can be used, and the resistor values are not critical. Note
that the 22K and 10K resistors from Figure 2 are still
necessary to prevent noise on the serial input when the
cable is disconnected.
Transistor inverters are inexpensive and parts are
readily available. However, if you want to simplify your
cable with just one component instead of six, you can use
almost any digital IC with two or more inverters, such as
the 74HCT04 hex inverter or the 74HCT00 quad NAND
shown in Figure 5.
There are many choices of similar ICs in a 14-pin DIP
package with through-hole leads, and many hobbyists
prefer this type of package because it is easy to work with
when using perfboard or a solderless breadboard.
If you can deal with surface-mount components
and want something elegant, a good choice is the
SN74LVC2GU04DBVR in a tiny SOT- 23 package with just
six pins as shown in Figure 6.
To package the additional inverter circuitry, attach a
small perfboard extension to the end of the module board
■ FIGURE 5. Download cable
with DIP IC inverters.
■ FIGURE 6. Download cable
with SMD dual inverter.
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