Convert your PK-232 or PK-900 to USB.

This article details how to do a completely reversible modification to allow you to use USB to directly access your PK-232 or PK-900 TNC. A FTDI USBMOD3 development board makes this very easy to do. FTDI has standard drivers that create a virtual comport so any software may access it. The cost is slightly higher than just buying one of the cheap USB to RS-232 cables but the advantage to using the FTDI USBMOD3 board is that it has an EEPROM for enumeration data.

All the cheap USB to Serial cables do not have an enumeration data EEPROM. This means you can't have more than one of them on the computer your using, and if you plug it into a different USB port or buss the operating system will set up another comport for it. You also have the option of writing information to the EEPROM on the USB board that will show your call and custom description. An example is what I loaded in the USB board for my PK-900.

When you plug the now USB enabled PK-900 or PK-232 in the first time the operating system will open a window telling you a new USB device has been found and is asking for a driver location. For my PK-900 it reads, "N4ZOU PK-900 TNC". This can be reprogrammed at will. You don't even need to open up the case! Just plug the USBMOD3 into a computer with Mprog loaded on it and simply erase the EEPROM and rewrite whatever you want it to read IE: URCALL PK-232 TNC. You're not limited to this. You can write any description you want in it including, “I Hate Sound Cards”. I actually do use a sound card and this was just an example. I built this project to allow more available serial ports for sound card use like MMTTY running in FSK mode. Ok, enough on why to do it. Lets move on to how to do it!

First of all lets look under the hood of a PK-232 and PK-900. There are two 14 pin DIP chips for converting TTL level data used in the TNC to RS-232 level data used by the standard serial port in your computer (if you have one). They are located just behind the DB-25 serial/parallel port connector. This is J1 on the PK-900 and J2 on the PK-232.

They are 1488 and 1489 line receiver and transmitter chips. These two chips will be removed and two 14-pin DIP sockets will be installed with jumper wires for all internal connections made in the TNC. To reverse the mod you simply remove the two sockets and reinstall the 1488 and 1489 chips. Order two of these sockets http://www.jameco.com/cgi-bin/ncommerce3/ExecMacro/Jameco/searchResult.d2w/report?sort=BPA&search=42657caWhile . These come with a cover so you can mark 1488 and 1489 on them. While your making your order you will need a DB-25 male connector http://www.jameco.com/cgi-bin/ncommerce3/ProductDisplay?prmenbr=91&prrfnbr=690&cgrfnbr=742&ctgys=503;524;742; and a hood http://www.jameco.com/cgi-bin/ncommerce3/ProductDisplay?prmenbr=91&prrfnbr=689&cgrfnbr=501&ctgys= Now for the most expensive part of the project, the FTDI USBMOD3 board. http://www.hobbyengineering.com/SectionIN.html The Hobby Engineering part number is Item: 1516. You will want to use a 32 pin socket for this board http://www.jameco.com/cgi-bin/ncommerce3/ProductDisplay?prmenbr=91&prrfnbr=2735&cgrfnbr=501&ctgys=

You will also need a 33nF (.033uF) capacitor for the board as per the data sheet. I have found that a .047uF capacitor works just as well http://www.jameco.com/cgi-bin/ncommerce3/ProductDisplay?prmenbr=91&prrfnbr=700&cgrfnbr=501&ctgys=. This is part of the setup of the USBMOD3 along with jumpers between two sets of pins. You will need a prototyping board that will accept the 32-pin connector for the USB board. This board should work fine http://www.jameco.com/cgi-bin/ncommerce3/ProductDisplay?prmenbr=91&prrfnbr=2721&cgrfnbr=501&ctgys=. You should enclose it in a project box.

Below is a drawing of how it All hooks up.

Look familiar? Yes, it's also the USB to TTL interface used for transceiver control from another article here on eHam.net. http://www.eham.net/articles/8192. You can use it for both and there is no need to worry about the enumeration data in the EEPROM. The programmed USBMOD3 board just looks for the USB comport driver in the operating system. It could care less about the TTL level device hooked up to it. Below is how to jumper the pins on the 14 pins dip sockets that replace the 1488 and 1489 drivers. Also shown is the DB-25 male connector wiring. The 1/8-inch stereo jack will fit in the DB-25 hood. The only cable required is a standard shielded stereo cable with 1/8-inch stereo plugs on each end.

You could also simply make a long cable with one end connected to the DB-25 connector and a male stereo plug on the other end wired up to plug into the USB <-> TTL converter box. Another idea is to make a special circuit board with holes to match the 1488 and 1489 socket pins, which would also be large enough, allow mounting the USBMOD3. This board would just plug into the 1488 and 1489 sockets and the DB-25 connector would have a USB cable soldered to the correct pins and have a USB-A male plug on the other end. You will need a special board, as the two sockets on the motherboard do not line up exactly with standard .100 spacing between them. You would also need different boards for both the PK-900 and PK-232. This would require that a large number of them be built and sold.

Be sure that you post a Warning Label on the TNC that only the USB to TTL device be plugged into the TNC. Use the supplied screws to attach the DB-25 with 1/8-inch jack to the TNC. I simply printed a Label with this information and stuck it over the RS 232 I/O lettering on the rear of the TNC.

Now you could get cheap and just use a peace of wire to jump the 1488 and 1489 pins in the sockets mounted on the TNC motherboard. You really don't want to do that. If the wire hangs up when you pull it back out of the socket it will destroy the socket contacts. This would require replacing the socket you damaged. That's no fun at all!

You might also ask why the hand shaking lines are not used. There really not required with the PK-232 and PK-900 TNC's. XON/XOFF software control is the default setup and the USB board is more than fast enough to handle the 9600 maximum baud rate. It also keeps the cost down and you can use the same USBMOD3 board and case for other TTL controlled projects. Also internal circuits place +12 volts on the chips that in turn drive +5 volt TTL level hand shake lines. This would damage your TNC! You also only need to solder one jumper to two pins on each socket. Again, this is a 100 percent reversible modification. If you want to sell the TNC or go back to RS-232 simply remove the jumper sockets and plug the 1488 and 1489 chips back in, remove the warning label, and remove the special DB-25 connector.

One note here, no more DOS terminal programs! Using USB will preclude using old DOS terminal programs as the USB comport is not accessible by these old programs. There are many very good modern terminal programs that run under operating systems that will operate with USB serial ports. Logger32 is one and I like the terminal program in the older Logger 16 programs.