Servo - July 2009

faster than its predecessor. USB 2.0 is the most widely used, even though a USB 3.0 specification was released in 2008. The 3.0 specification has a data rate of up to ten times that of the 2.0 specification. A nice caveat about the USB connections is that they are backwards compatible — machinery that works with USB 2.0 will also work with USB 1.0 devices, and so forth.

the USB connection over the serial connection is that USB can transmit power in addition to data streams. The last pin on the USB connector was a +5V pin, and there was no corresponding pin on the serial connector. What could possibly be done?

USB connectors only have four pins, and the connectors themselves have a rather whimsical range of shapes and sizes. The classic rectangular connector is Type A, and this compact unit is the beast of the USB connectors. The connector on the serial-to-USB converter that we ordered — and one often seen connecting to peripherals like printers — is Type B. There are also Mini-A, Mini-B, Micro-AB, and Micro-B connectors, with the most prevalent of those seeming to be the Mini-B connector. This one can be found on digital camera cables and cell phone chargers.

The answer is pleasantly simple. All we had to do is connect the +5V pin from the serial-to-USB interface to a 5V source. That 5V source could come from another place on the robot PCB, and the Viper in particular offered plenty of places to tie in. Every set of solder pads available for one of the Viper modules included a 5V pad, and putting a jumper cable between the power pad and the corresponding pin on the adapter was certainly a manageable task.

In short, USB is way better than RS-232. It offers better data rates, backwards compatibility, and a much greater range of applications (in other words, everything that Professor Hodgkiss mentioned). One of the only remaining stumbling blocks would be if the interface was difficult to implement, but we felt that this wouldn’t be the case.

In addition to the physical aspect of wiring up the adapter, we had to make sure we had the correct drivers. The drivers are available for free on the FTDI website, and the available selection covers tons of different operating systems. The drivers are for what FTDI terms their Virtual Com Port (VCP), so named because it effectively acts as a serial com port for all of those newfangled laptops that don’t have any. All we needed now was for our part to arrive in the mail.

Conspiracy Theory

Implementing the adapter appeared to be an easy task – we simply had to match the pins on the adapter to the necessary pins on the DE- 9. But what were those pins that needed to be matched? The serial cable has nine pins and the USB has only four — which ones would we use?

But alas, it would seem that the persistence of the serial connection is surely some part of a vast conspiracy. The connector we received from the Mark III store was not the Acroname part we ordered. Instead, we were given the Devantech I²C-to-USB connection (Figure 10). The modules may have looked similar — both had a prominent Type B USB connection mounted on a tiny PCB (Figure 11). Both also used an FT232R chip from FTDI, and several tiny capacitors.

The easy choice was ground.

Our first clue to the case of mistaken identity was the fact that the

The ground on the serial would correspond to the ground on the USB. But what else? The Data + and

FIGURE 12. USB PINOUTS.

Data – pins on the USB correspond to Pin received data and transmitted data;

1

also fairly intuitive. The last pin,

2

however, was not so simple.

3

One of the distinct advantages of

4

Name
VCC
D-
D+
GND

Color Red White Green Black

Description
5V
Data-
Data+
Ground

Universal Serial Bust

FIGURE 10. A USB-TO-I²C INTERFACE FROM DEVANTECH.

part we actually received had an extra chip that we were fairly sure was absent from the module from Acroname. Our second clue was the fact that the Devantech module had a different connection on the side opposite the Type B USB. From the picture on the Mark III Robot Store website, the Acroname module appears to only have solder pads. The Devantech module, on the other hand, has five long angled pins.

The final clue was printed on the circuit board itself. The board is emblazoned with the words USB and I²C, and it rang a bell with something that Evan had heard in his Sensor Networks class. We looked back to the website and our fears were confirmed — we had been sent the wrong part.

I²C is indeed a type of serial communication; an Inter IC bus, to be exact. The problem is that we were looking for a UART connection – Universal Asynchronous Receiver/ Transmitter. The main difference between I²C communication and UART is that I²C is synchronous; the master and slave devices operate using a common clock. UART, as the name would suggest, is asynchronous