The Serial Port is Dead! Long Live the Serial Port!
FIGURE 7. DB9
connector wiring.
section of smileymicros.com.
• Plug your USB cable into your PC and the
BBUSB, following the previous instructions to
install the FTDI drivers.
• Browse to the directory where you
downloaded Simple Term.exe and click on it.
You should see the form shown in Figure 5.
New Hardware” balloon pop up for a moment.
• Surprisingly, the Wizard will reappear and you will have
to repeat the entire process since two drivers are installed.
• Click the “Setting” menu item and you should
see the Settings form shown in Figure 6, which
will list all your serial port devices. In this case,
we select COM10. Your case will probably be
different. If you have multiple choices and aren’t
sure which to choose, unplug your BBUSB, then look at
the Settings again to see which one disappeared. Then,
plug it back in and select that port.
With the hardware set up as shown and the drivers
loaded, you can now do your loop back test with a
terminal program such as Hyper Terminal or the Simple
Terminal, which follows.
• Select the device and the baudrate, and then click the
Okay button.
• Type ‘Hello world!” into the Send window and you
should see ‘Hello world!’ appear in the Receive window.
Using Simple Terminal
You can find Simple Terminal in the downloads
• Now you know the hardware and software work.
USB RS-232 Level Conversion
FIGURE 8. RS-232 voltage
level conversion.
60
June 2008
Let’s create a USB to RS-232
converter on a breadboard that
functions exactly like a pre-made USB to
RS-232 converter cable. Why would you
want to go to the trouble of building
one on a breadboard when you can
buy one already made for about the
same price as rolling your own? Good
question, and about all I can say is that
this way, you learn how that cable works
and get the opportunity to use the extra
pins on the BBUSB for other projects.
You also get the opportunity to make a
dozen frustrating mistakes and do a lot
of debugging (but that’s part of the fun,
isn’t it?). Figure 7 shows the pin-outs of
the two ends of an RS-232 cable. We
will use a Null Modem cable, meaning
that the RxD and the TxD lines cross so
that the TxD of one end goes to the
RxD of the other. This just means that
what you transmit from one device is
what you receive on the other device.
We are using an ST202EBN (or a
MAX202EBE or similar compatible IC),
RS-232 voltage level converter. Since
this project is kind-of hard to wire
properly, I built this on three separate
