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USB to TTL Transceiver Control!

Do not contact (N4ZOU) on May 5, 2004
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USB to TTL Interface.

Here's another Interface for software control of your Radio. The difference with this one is that it connects to a USB port! Not only that it's so easy you might not think it will work properly. I have it connected to my ICOM IC-756 PRO II on a prototyping board with jumpers hanging out and it works fine even when transmitting! I tested it with Logger32, Logger ver 8.07, Zakanaka, MMTTY, MMSSTV, Hamscope, and Commander.

It should work fine for any transceiver that has a TTL level computer control jack. Only one capacitor is required for the board. No other level converter or inverter chips are required. There is an EEPROM located on the board so that a serial number and description can be loaded into it. This way there are no conflicts with more than one of these USB boards connected to the same computer. The software for programming it is free as are the required drivers.

I got fancy and loaded my call in the description window while writing the data to the USB board. Now the first time the device is plugged in and Windows asks for a driver location my call is shown in the device window! This can be reprogrammed at will so it's not stuck in there forever.

Now some of you with Transceivers or a TNC with RS-232 serial ports can add a MAX232 chip and have RS-232 level out but it would be much better to buy a USB to RS-232 serial cable that's ready to go. The FTDI data sheets show how to add a MAX converter chip for true RS-232 levels. The USB boards are available from and at this time cost's $33 plus shipping. You might want to wait until late April 2004 when will also start selling a new updated version with two TTL level serial ports for the same cost! This would allow you to have transceiver control on one port and run a TNC with a MAX232 chip on the second port. The driver configures the USB serial device to show up as a regular serial port.

After you program the USB board on one computer you may then install it on another. If you have only one computer then program your board and then remove that software and driver. Then plug the USB board back in and load the virtual comport drivers. Now go to Start, Settings, and Control Panel and double click on System. Click on Device manager and then Ports (Com and LPT). Here you will find out what comport number your new USB Comport has been named by Windows. Simply select it in your software when setting it up for transceiver control. You will find it works just like a normal serial port.

The link for the FTDI Home Page where you may download drivers and Mprog 1.0 for writing information to the EEPROM on the board is You can buy a USB board without the EEPROM but it's the same cost so you might as well get the better board with the EEPROM chip. It will also work fine without writing a serial number and description in the EEPEOM but you will have trouble if you connect a second empty EEPROM USB board to the same computer. In this project I ordered the USB plug and play module (Rev2) USBMOD3 board with the Hobby engineering part number 1516. Below is a drawing on how to hook it up.

0x01 graphic

YES! It is that simple. The TTL serial connector shown above is a 1/8 stereo phone jack with RXD pin 27 connected to the ring, TXD pin 28 connected to the tip and Ground pin 29 is connected to the sleeve. This allows connecting to almost any transceiver using TTL level signals with only the cable being required to be custom made.

For an example an ICOM CI-V cable can be made with the 1/8 inch stereo plug with tip and ring shorted and the shield connected to the sleeve. Then the single wire with shield cable would have a 1/8-inch mono plug with the tip connected to the single wire and shield connected to the sleeve. For Yaesu you would use shielded stereo cable and TXD to the DATA IN line and RXD to the DATA out line of the radio. Same thing for Kenwood except you might need to have RTS and CTS lines, which are also available on the USBMOD3 board. CTS is pin 25 and RTS is pin 26. In this case you might want to change the 1/8-inch stereo jack for the same plug used on the transceiver. Pins 1, 2, 16, 29,and 32 are all common to ground and pins 4 and 13 are common to VCC. You might want to hook the capacitor leads to pins 11 and 16 and not to 2 as shown. I show it this way for simplicity of the drawing. The capacitor is not all that critical to operation. A .047 uF capacitor also works fine.

I would like to thank the following operators with my on the air test! HR1RMG, Rene in LSB mode with MMTTY. VE2CFS, USB mode with MMSSTV. W6JL, Don Logger32 and a PK-900 TNC in ARQ Amtor. K7UIR, Dave USB mode with Logger 8.07 and Zakanaka in PSK-31.

The interface was used to control the transceiver PTT with the sound card modes and automatic Logging information when using the PK-900 TNC. This interface will allow you to use a computer with no serial ports for PTT and Logging information as well as Transceiver control. You also will need a sound card interface for Audio but there will be no PTT circuit required in it. I have not tried hooking up RXD to a 4N33 chip or transistor for direct FSK control with MMTTY. FTDI lists 300 baud as the lowest rate available. This may preclude it from being used this way.

73, Scott N4ZOU.

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USB to TTL Transceiver Control!  
by K0BG on May 5, 2004 Mail this to a friend!
Nice article. There is a siimilar article published in the May/June issue of QEX. It was written by Dick Lichtel, KD4JP. Both articles bear reading by those folks who want to control their interfacable transceivers with modern computers.

Alan, KBG
RE: USB to TTL Transceiver Control!  
by AA4PB on May 5, 2004 Mail this to a friend!
I might add FTDI offers Microsoft certified WindowsXP virtual com port drivers for their chips. I've had problems with the non-certified XP drivers for some other USB/Serial converters making Windows unstable. I looked for a converter using one of the FTDI chips so that I could use their XP certified drivers and all the problems went away.

Thanks for a great article. You might want to consider sending that in to QST for a "Hints and Kinks" article.
USB to TTL Transceiver Control!  
by AA6YQ on May 6, 2004 Mail this to a friend!
Scott, the Icom CI-V bus requires open collector drivers, since multiple devices - a transceiver, and the PC interface - can simultaneously attempt to pull the bus in opposite directions. Can the FTDI chip's output handle this?


Dave, AA6YQ
RE: USB to TTL Transceiver Control!  
by N4GI on May 7, 2004 Mail this to a friend!
These devices are 25 bux in my latest Mouser catalogue. (no affiliation)

Blake N4GI

USB to TTL Transceiver Control!  
by N4ZOU on May 7, 2004 Mail this to a friend!
YES! No problem, If your going to run several CI-V devices from the FTDI USB board simply replace the 1/8 inch stereo jack with several 1/8 inch mono jack's hooked up in parallel and use the 1uH chokes shown in the Icom CT-17 circuit. Simply hook one end of each choke to RXD and TXD and hook the other ends to the jack TIP connection and ground to the SLEEVE connection on the 1/8 inch jack. Label the box as a USB <-> CI-V device. Follow the manual for the CI-V device you plan on hooking up to the USB board as for setting the diffrent address required. It will also allow using a second CI-V receiver with the address set up so the receiver will track with the CI-V capable Transceiver. The USBMOD3 board will handle up to 499 mA (MProg will not allow 500 mA to be set) current supply when set up as self powered as shown in the drawing. MProg defaults to 100 mA self power supply. This should be plenty of current for +5 volts and 0 TTL level DATA. This will prevent using this USB <-> TTL converter for other TTL driven devices. I just uploaded another Article on using the exact same setup in this Article with a PK-232 or PK-900 TNC. It will take a few weeks for it to show up if it gets approved. No MAX232 chip is used in that project. You can switch it around at will between transceiver control, or the PK-232/PK-900 link to USB. There really is no limit to what you can convert that uses TTL level serial commumition. If it were me I would build the project as shown and put the 1uH inductors in another project box with a 1/8 inch stereo jack to match the one on the USBMOD3 board as shown and then connect to the 1/8 inch mono jacks for the CI-V system.
USB to TTL Transceiver Control!  
by KJ5XX on May 7, 2004 Mail this to a friend!

Seems like an awful lot of trouble to go to when you can now buy simple Serial to USB interfaces that plug directly into the port of the radio.

I'm using one with my Kenwood TS-570G and it only cost me about $15 at the local electronics store.


Ron - KJ5XX
RE: USB to TTL Transceiver Control!  
by AA6YQ on May 8, 2004 Mail this to a friend!
Let me try again.

The CI-V bus allows collisions. That means that the PC interface might be trying to pull the bus to "TTL high" at the same time the transceiver is trying to pull the bus down to "TTL low".

Classic TTL devices use a totem pole output structure, in which one transistor is used to pull the output pin to TTL high via a connection to Vcc (+5v), and another transistor is used to pull the output pin down to TTL ground. Obviously a device enables either its pull-up transistor or its pull-down transistor -- never both.

During a bus collision with such an output structure, however, there's a direct path from Vcc through one device's pull-up transistor across the CI-V bus through the other device's pull-down transistor to ground. Over time, the pull-up transistor fails due to thermal overload.

Normally, one drives things like the CI-V bus with open-collector drivers, in which the pull-up function is achieved with a resistor to Vcc rather than a transistor; a current-limited driver would also work.

So my question, again: "is the USB device's TTL output open collector or sufficiently current-limited to not self-destruct over time if directly connected to the CI-V bus?". If not, then a non-inverting open-collector driver, e.g. a 7407 section, should be interposed between the USB device's TTL output and the CI-V bus.


Dave, AA6YQ
RE: USB to TTL Transceiver Control!  
by N4ZOU on May 8, 2004 Mail this to a friend!
This is the data sheet for the USBMOD3 board. This is not a "Cheap" board as is the case of a lot of the stuff showing up from China. The board is also populated with the standard passive parts to protect the UART in the chipset from damage. It is more robust than the Maxium MAX232 family of level converters. MAX232 chips have been driving Icom CI-V devices for many years without problem. Anyway I am sure with the technical background as shown from your post you can check the data sheet listed here and also the FTDI site for more reference material to make your on determination as to required driver chips you might think is required. If you feel they are then simply put them into your own design.
RE: USB to TTL Transceiver Control!  
by AA6YQ on May 8, 2004 Mail this to a friend!
Thanks for the data sheet. It shows that the module's TXD output (pin 28) is directly sourced by pin 25 of an FT232BM -- with no buffering, current limiting, or external protection.

The voltage/current specification for pin 28 is limited to Vol and Voh. Voh is specified as 4.4 - 4.9 volts at 2 ma. That sounds like an active pull-up to me, but one can't tell from the information provided whether current limiting for use with busses like CI-V has been provided. No schematic of the FT232BM's output structure is provided.

Since this device doesn't incorporate a MAX232 -- and in fact doesn't do anything comparable to what a MAX232 does (which is level conversion), I don't understand why you'd cite the MAX232's robustness as a way of increasing our confidence in this unit's ability to safely drive the CI-V bus directly.

If the spec doesn't plainly indicate that the module can drive an open-collector bus, I'd spend the extra 50 cents and interpose an open-collector driver.


Dave, AA6YQ

RE: USB to TTL Transceiver Control!  
by AA4PB on May 13, 2004 Mail this to a friend!
AA6YQ is correct. In order for the interface to work properly with an Icom CI-V it will need an open collector driver circuit between the chip and the radio. This is true whether you have one radio or several connected. Without the open collector buffer it is possible for one device (the radio) to pull the single data line in one direction while the other device (the computer) pulls it in the oposite direction. This amounts to a short circuit that could cause a failure in the USB chip or the radio.
RE: USB to TTL Transceiver Control!  
by KB3CO on January 1, 2005 Mail this to a friend!
That works very nicely for radios with 232 interfaces like the 570. Read the first article again. CI-V is a TTL interface. What you suggest will not work for Icom's.
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