Why build my own
USB to RS232 converter when there are adapters already available
from Keyspan and Belkin?
- The Answer is
simple:
- Neither the Keyspan's
or the Belkins actually worked *properly* with all the devices
I tried. They did not seem to support ALL the RS232 lines.
-
- That is a problem
if you want to, as in my case, program your Stamp Microcontroller
on a USB based Mac using Virtual PC.
Does
it work with other devices?
Not only does
this adapter work with the Stamp IDE natively under Virtual PC,
but I have also tested it OK on the Macintosh with
The adapter also works with MacBS2
under OS X. Note, make sure you have installed the latest OS X
driver, and are running the latest version of MacBS2.
The
Solution
A USB to RS232
converter using the FTDI FT8U232AM and a Maxim-IC
MAX235.
The FT8U232AM
requires a small number of external components to produce a device
that converts USB to TTL level RS232 signals. All you need to
add is a TTL to RS232 converter to provide the +/-12V RS232 logic
levels.
The FT8U232AM
and supporting components is also available as a complete module
- called a USBMOD1 - in a 32pin DIL IC package, making the whole
project very easy to build.
Distributor information
for the USBMOD1 is mentioned later.
Below is a photo
of my finished Adapter:
OK, this is what
you will need:
Hardware:
- Veroboard
- DB9 male socket
- 2 x 100ohm resistors
- 1uF Electrolytic
Capacitor (16VW or higher).
- 3mm RED LED
- 3mm GREEN LED
- MAX235 Intergrated
Circuit
- USBMOD1 Hybrid
Module
Computer:
These are the specs
as used on my system. Higher spec systems should work fine, but
I can not be sure about lower spec systems.
- Macintosh:
- Processor: G3
@300MHz
- RAM: 128MB
- OS 9.2.2
- A USB port (I'm
using a Belkin USB PC Card on my Powerbook Wall Street PDQ)
- Virtual PC:
- Virtual PC v3.0,
with RAM setup for 64MB.
- Windows 98
- Stamp IDE v1.098,
or v1.33 (also tested OK with v2.5b1), also works with the Javelin
Java IDE
- Stamp:
- Tested OK with
a BS2 (24pin DIP and OEM), BS2sx OEM, and Javelin Stamp
Circuit
Diagram:
The circuit is
powered from the USB Bus. No external capacitors are needed with
the MAX235 (except for the decoupling capacitor), as they are
all on the chip.
Construction:
- Cut your veroboard
to 31 tracks x 18 holes.
- Use the diagram
below to determine where to break the tracks, and where to place
the components.
- Install the jumpers
first (the red ones go underneath), then the resistors, decoupling
capacitor, MAX235, LEDs, and USBMOD1.
- Lastly, using
the circuit diagram above, wire up the DB9 connector and solder
the wires to the appropriate tracks for the MAX235.
Software
installation:
Some important
information:
FTDI's
site has
drivers for Macintosh and Windows for their USB controller chips.
If you want to use this adapter for Macintosh devices and Windows
devices, you will need to set up different extension sets in
the Extensions manager.
- One with the
Macintosh Extension for the FTDI chip enabled, and one with it
disabled.
If you don't,
and you start up Virtual PC with the FTDI Macintosh extension enabled, you will
cause Virtual PC (and your Mac) to crash.
Installing
the software:
- Don't plug the
adapter in just yet.
- Click here to download the required
Windows drivers. You will not need the Macintosh Extension in order for
the USBMOD1 to work under Virtual PC. However, you will need
it if you want to use the USBMOD1 on the Mac side. See note above
re Extension sets if you want to install the Mac driver.
- The zip archive
contains drivers and associated documentation to support FTDI's FT8U100AX, FT8U232AM
and FT8U245AM USB controller products.
- Unzip the archive,
and copy the files to a folder on the "C:" drive. Note: In order for the
Windows software to install, you need to make sure the install
files from the zip archive are on the "C:" drive.
- Make sure you
have selected the USB <-> serial converter in the VPC preferences,
as below (remember - disable the Mac FTDI extension)
- Now, when you
plug in the USB adapter, Windows should see it and look for the
install .inf file. Browse to the location on the "C:"
drive where you have saved the Unzipped archive, and let Windows
install the drivers.
- If all is present
and correct, you should be able to open the Stamp IDE, and with
the Stamp connected, when you press the ID button, you should
get a confirmation message like the one shown below.
Windows Driver | Mac OS 8.6 to
9.2.2 and Mac OS X drivers
Where
to get the parts:
- Veroboard - Any
Electronics Hobby Store
- DB9 male socket
- Any Electronics Hobby Store
- 2 x 100ohm resistors
- Any Electronics Hobby Store
- 1uF Electrolytic
Capacitor (16VW or higher) - Any Electronics Hobby Store
- 3mm RED LED -
Any Electronics Hobby Store
- 3mm GREEN LED
- Any Electronics Hobby Store
- MAX235
- USBMOD1