Twin Tweaks ...
8. SERIAL CABLE SURGERY.
downside. Of course, there are USB
devices like keyboards and mice that
have a very slow data rate, but they
go all the way up to disk data
transfer, so USB devices cover a much
broader range of interface capability.
You might use that for a kit if you had
a video camera or something for a
robot; the data would come over just
fine on a USB connection. You would
never get anything other than very
slow scan video to come over on a
normal asynchronous serial interface.
So, it is an interesting question
certainly, but I have no idea why
anybody would sell anything anymore
with just a simple serial interface on it.
The UART of War
We thought that a little more
research was in order before actually
diving in and “making the switch.”
Firstly, we were ecstatic to hear that
the folks at Parallax were making the
much needed switch from a serial
interface to USB, but a quick review of
some of our robot kits revealed that
Parallax was in the enlightened
minority: Asuro from Arexx
Engineering (Figure 1); the Viper from
Microbric (Figure 2); the POB robot
from POB Technologies (Figure 3);
and even the Scribbler from Parallax
(Figure 4) are all afflicted by the
sinister serial port. We could list more
kits, but that would get a little bit
repetitive and depressing.
Secondly, we wanted to find
out more about the technology that
would allow us to make the switch —
64 SERVO 07.2009
an FT232R chip (Figure 5)
from Future Technology
Devices International (FTDI).
Professor Hodgkiss was
correct in his estimation of
the price of the chips — most
of the devices from FTDI cost
between two and six dollars,
according to Digi-Key.
Thankfully for us, there
are several modules that are
commercially available that
use an FT232R chip to make
the switch from serial to USB.
After a bit of searching, we
found a suitable part at the
Mark III Robot Store (www.junun.
org). The module itself is from
Acroname (though strangely hard to
find on their own site), and can be
purchased for a manageable $21
(Figure 6). The module appeared to
be fantastically simple. Except for the
USB connector and the pinouts that
connected to the serial cable, the PCB
was only populated by an FT232R chip
and a few capacitors.
We picked the Viper from
Microbric as a fine candidate for the
connector switch. The Viper’s serial
port was exposed and easy to remove
if need be, and the modular design of
the Viper made any unexpected additions a welcome challenge. As we
waited for the part to arrive in the
mail, we decided to get better
acquainted with the connections upon
which we would be practicing our surgical skills.
The offending connection on the
Viper is a D-subminiature, and we
think that the current inappropriateness of the name is matched by the
current inappropriateness of the
connector itself. The D-subminiature
was quite the cutting edge technology
when it was first introduced in 1952
by the Cannon ITT. The connection
was so named because of the D 1
shaped shield around the pins and it
was, in fact, very small for the times.
The specific serial connection on the
Viper (and the ones most often seen
on laptops of yore) are designated
DE- 9; so referred to for their nine pins
We could simply refer to
documentation about the RS232
pinouts for a DE- 9 connector, but
because of the apparent conspiracy
of the persistent serial connection we
felt compelled to verify some of the
pinouts ourselves. We did so with a
trusty multimeter using the continuity
The continuity setting is often
identified by a symbol meant to
resemble sound waves; when the
multimeter prongs touch two ends of
a continuous circuit, the multimeter
will emit a helpful beep. By touching
one prong of the multimeter to one of
the numbered pins on the D-sub and
then cycling through the wires on a
split open serial cable (Figure 8), we
were able to connect a wire color to a
pin number by the telltale beep.
Wire by wire we were able to
associate each pin with a color, and
we found that this was indeed consistent with the existing documentation.
The pin-outs of the connector are of
utmost importance for our proposed
surgery. They are given in Figure 9.
Now that we had the serial side of
things sorted out, we needed to
figure out the USB pinouts.
The USB 1.0 specification was
first introduced in 1994, and it had a
data rate specification of 12 Mbit/s.
The USB 2.0 specification was first
introduced in 2000, and standardized
by 2001. USB 2.0 has a data rate
specification of 480 Mbit/s — much
9. RS232 PINOUTS FOR A
DE- 9 CONNECTOR.
Data Terminal Ready
Data Set Ready
Request To Send
Clear To Send