FIGURE 3. This is an export of the
schematic from the ExpressPCB software.
for in the design, however, I had an insufficient number of
them to complete the prototype but had plenty of the
1/2W type so I used what I had. Normally, eight 1/4W
resistors side by side are cramped, so to get the 1/2W
resistors on the board I had to stagger them within the
same space. This is done by alternating which hole gets the
bent-over lead and which one gets the body of the resistor.
It just goes to show what can be accomplished in minimal
space with through-hole components.
The 3.9K resistor value was chosen to protect the
Propeller chip I/O pins from excessive current under many
conditions, including interfacing to 5V circuitry. Once all 16
I/O pins were completed, two additional three-pin SIP
headers and an LED were connected to P27 and P26. P27
also has a 3.9K series resistor and will serve as the serial
I/O pin for sending data back and forth with the controller.
P26 is connected through a 150 ohm resistor to an LED
Prototype Parts List
Propeller proto board
Three-pin SIP header
3.9K, 1/4W resistor
150 ohm, 1/4W resistor
Red T1 LED
Header shorting block
44 SERVO 09.2008
for indicating error, busy or serial data indication.
On the prototype, there is also a two-pin SIP header
shown to the left of the LED. This was connected to P25
through a 1K resistor and P25 was also pulled high by the
10K resistor. This jumper was going to be used to emulate
the ID selection used on the servo controller, however, David
has decided that since the proto board has an additional 32K
of EEPROM available we should make use of this for certain
settings and some features he will cover in the next article.
Because of this, the two-pin SIP header, 1K, and 10K
resistors in Figure 2A are not included in the reference
design schematics and can be omitted. Note that there was
already a VSS bus for P0 through P7 on the proto board,
however, I did have to run power down from the servo
header pads on the board to each bus as well as VSS for
the top row. I also had to install a three-pin SIP header to
select 5V or VIN to go to those pads.
Using the propeller proto board allowed me to prove
and test the design, but it was not exactly as I had planned.
My original goals were to isolate the servo supply from the
controller supply. The proto board made it more convenient
to test by running 6V into the DC power connector and
setting the servo jumper to VIN, but be warned! If you use
a wall adapter on the power input, you may be putting too
much voltage into the servos connected to your headers.
At the same time, using the 5V setting limits the voltage.
The onboard regulator will not be able to provide sufficient