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USB-to-Serial Adapters

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This info might seem off-topic for a ham blog, but (1) this is my blog and isn’t restricted to ham radio just because it bears my callsign and (2) there is relevance in my presented data.

For those of you who want to use a USB-to-Serial adapter to work FSK RTTY, this article is for you. To operate FSK RTTY, your serial interface must support 5-bit data and low baud rates [45.5 / 75 baud]. Well, I’ve got bad news for you — many USB-to-Serial adapters fail on one or both counts.

USB-to-Serial adapters generally rely upon a chip provided by FTDI ( http://www.ftdichip.com ) or Prolific ( http://www.prolific.com.tw ). You may find one using a Cypress programmable chip. I haven’t found the specs on what the Cypress option can or cannot do, but below is some information regarding the FTDI and Prolific products.

Current generation FTDI products

None of the current FTDI products will support direct-keyed FSK on USB-to-Serial adapters that employ these chips. FTDI products do not support 5-bit data and do not support baud rates below 300 baud.

FT2232D / FT232R

  • 7-bit and 8-bit data
  • 300 bps to 3 Mbps

FT2232H / FT4232H

  • 7-bit and 8-bit data
  • 183 bps to 12 Mbps


  • 7-bit and 8-bit data
  • 300 bps to 1 Mbps

Current generation Prolific products

USB-to-Serial interfaces utilizing the Prolific PL-2303 series do support 5-bit data, but according to product literature they are limited to a low of 75 baud.

PL-2303HX / PL-2303HX-Edition(Rev D)

  • 5-bit, 6-bit,7-bit and 8-bit
  • 75 bps to 6 Mbps (12 Mbps or the -Edition(Rev D))

There are rumors [and I say this because I can’t verify the claim] that Belkin sells a USB-to-Serial adapter F5U103EA that some RTTY folks claim work for FSK. The data sheet provided by Belkin matches, almost verbatim that of a PL-2303HX, which leads me to believe that the Belkin adapter uses the Prolific PL-2303HX or some older Prolific chip for which they don’t have product information on their website.


Based upon specs provided by these two manufacturers, I have come to the conclusion that you’ll never get a USB-to-Serial adapter based upon one of the FTDI chips above to support FSK RTTY. But, you may very well get an adapter employing a Prolific chip to do FSK RTTY. I will attempt to test this.

I have a couple USB-to-Serial cables with a Prolific chip. One of them definitely “gets farther” when I try to run FSK RTTY with it, as compared to an FTDI-based adapter that i have. With the FTDI-based adapter that I have, when I try to do FSK RTTY my software claims to not even be able to find the comport. With the Prolific-based adapters the software I’m using sees the serial port, keys, and I can hear the RTTY data. The FSK signal doesn’t sound the same as if I were running AFSK RTTY or FSK RTTY through a bonafide serial port, and when I try to actual send data [vs just diddling], it locks up the computer. Further testing is needed.

I tested the Prolific-based adapter on 45.5 baud RTTY. Based upon product documentation for the Prolific chip, it would only go down to 75 baud. So I’ll test and see if I am successful at running 75 baud.

NOTE: I’ve been able to get every USB-to-Serial adapter I have tried to run FSK RTTY when using EXTFSK in MMTTY. EXTFSK is a DLL that is loaded by MMTTY to perform software-generated FSK. It’s very nice to have this available. However, EXTFSK only support 45.5 baud RTTY, not 75 baud. BUT, if my tests bear out that the Prolific-based adapters will run 75 baud FSK RTTY, then you can run FSK RTTY direct from within your RTTY software using a USB-to-Serial adapter and then, if you want to run 45.5 baud FSK you can just use EXTFSK inside MTTY. I don’t use any other FSK RTTY software, so I can’t comment on what other software may be able to do. Ham Radio Deluxe doesn’t support FSK RTTY, and when I want to run RTTY outside of a contest I simply use HRD for AFSK RTTY. When I contest, I use N1MM / MMTTY and run FSK when possible, AFSK when not.

ADDENDUM: I have tested my Prolific-based USB-to-Serial adapter with an FSK keying circuit and was able to operate 75 baud FSK RTTY. I also tested 45.5 baud FSK, but I didn’t test it thoroughly and it seemed kind of “iffy” — meaning that sometimes it would lock my computer up. Prolific documentation states that the low limit for baud rates is 75, so if people are actually using a Belkin and the Belkin uses a Prolific chip, then I would question if they actually have reliable 45.5 baud FSK RTTY comms. Outside of contests I do not do much RTTY, and at the times I’m available to try it out there usually isn’t anybody on RTTY 20m to test with. But, this test is positive nonetheless.

With a Prolific-based adapter, you should be able to do 75 baud FSK RTTY with a certain degree of confidence. For 45.5 baud FSK RTTY using a Prolific-based adapter, your mileage may vary. Give it a try, but be prepared for trouble because you may experience some.

These are good findings though. At least with the Prolific-based USB-to-Serial adapter that I’m using, I can operate 75 baud FSK RTTY and 45.5 baud FSK RTTY (with EXTFSK for sure, possibly without EXTFSK). So I can run FSK for both baud rates on my FT-950 and take advantage of the better RTTY mode filtering on the radio.

Here is a link to the exact Prolific-based USB-to-Serial adapters I have been using and testing with:


Written by Mike

October 7th, 2010 at 9:21 pm

G4ZLP Digimaster PRO+ and N1MM / MMTTY


Now that I have received my Digimaster PRO+ and have connected it to the radio and PC and have the PC and interface communicating with each other, it is now time to set up N1MM.

In my previous post I mentioned that the first comport [used for FSK / CW / PTT keying] is COM6 and the second comport [used for CAT control] is COM7 and that my Yaesu FT-950 is configured with a 38400 baud rate on the RS-232 CAT interface.

In N1MM, my goal is to have CAT control available to me as well as to be able to automatically key CW. I’m also going to be configuring N1MM and MMTTY for FSK operation.

Config -> Configure Ports, Telnet Address, Other
Hardware tab

SOV1 is selected (I have one radio, single VFO)

COM6 : None : checkmark Digital : checkmark CW
– click Set : DTR=CW, RTS=PTT, Radio Nr=1
– PTT Delay=30, Digital Wnd Nr=1, no checkmarks
– Two-Radio Protocol=None, Footswitch=None
– CW/PTT Port Address : left as what it had default, 3E0 for COM6

COM7 : FT-950
– click Set: Speed=38400, Parity=N, Databits=8, Stop Bits=1
– DTR=Always On, RTS=Always On, Radio Nr=1
– no checkmarks, no footswitch

Config -> Configure Ports, Telnet Address, Other
Digital Modes tab

DI-1 MMTTY Setup
– Digital Interface 1 TU Type: Soundcard
– Digital Interface 2 TU Type: None
– MMTTY Path: C:\Program Files\MMTTY\mmtty.exe

(MMTTY was previously downloaded and installed in the default location of C:\Program Files\MMTTY\mmtty.exe)

– MMVari RTTY Mode: AFSK

Config -> Configure Ports, Telnet Address, Other
Audio tab

1-Zero or Single Card, One Radio, No Sound Card SO2R is selected

Select Device: USB Audio (this is the USB Audio stick that came with the Digimaster PRO+)
Select Input Line: Microphone
Recording Bits: 24
Recording Sample Rate: 11025
Radio Input Port: Microphone

I don’t believe the above Audio section was even relevant for me since I don’t set/use SSB macros or process my transmit voice audio through the PC.

Config -> Configure Ports, Telnet Address, Other
Mode Control tab

– Mode Recorded in the Log: Use Radio mode (default)

(if you are running AFSK RTTY, which you have to do if you want to run 75 baud since the EXTFSK driver in MMTTY won’t TX 75 baud, only 45.5 baud, then you’d want to set this to RTTY during RTTY contests to make sure that N1MM properly logs the contact as a RTTY contact. If you’re using FSK and have the radio already set in RTTY mode, then the option I have selected above works fine)

– Mode Sent to Radio
– RTTY to RTTY (sets radio to RTTY mode, LSB)
– PSK to AFSK-R (sets radio to PKT mode, USB)

With the above setup, N1MM controls the radio perfectly and keys CW without a hitch. Remember, I had previously set up DI-1 (digital interface 1) to operate FSK in N1MM. Now we have to go into the MMTTY setup (via N1MM) to set some additional parameters so MMTTY knows to do FSK. But, before we do this we have to install some additional software, EXTFSK. This is needed to allow MMTTY to do the FSK keying over the USB-to-Serial interface supplied by the Digimaster PRO+. If you don’t install / set up EXTFSK within MMTTY, you won’t get FSK RTTY to work.

This assumes that you already have MMTTY installed and that the installation directory for MMTTY was C:\Program Files\MMTTY .

Download EXTFSK from the MM HamSoft page, currently at http://mmhamsoft.amateur-radio.ca/pages/extras-add-on/extfsk.php. Unarchive this zip archive to some folder, and then copy EXTFSK.dll into the directory where MMTTY.EXE is. In my case, I copied EXTFSK.dll into C:\Program Files\MMTTY\ folder.

After you’ve installed EXTFSK, you should be ready to configure MMTTY (via N1MM).

In N1MM, click Window at the top and select Digital Inteface. You’ll notice that a few new MMTTY-related windows are now visible.

Now go to the DI1 RTTY Mode window that is open and select Setup from the top of that Window and choose Setup MMTTY.

Under the Soundcard tab:

Set USB AUDIO as the Transmission and Reception audio selections since USB AUDIO is the USB audio stick that was installed from the Digimaster PRO package and since this will be used when running MMTTY for AFSK RTTY.

Under the TX tab:

Set the PTT & FSK Port to EXTFSK

Under the Misc tab:

Set the TX Port to COM-TxD(FSK) and then click on USB Port and set the option C. Limiting speed and then press OK to close that USB Port window.

Then press OK in the Setup window.

You should now notice another new Window open, which is probably listed on your taskbar. It would be labeled EXTFSK 1.06 (your version may be different – 1.06 is the latest as of this writing). Click on this so that it brings up the window pane for configuration of EXTFSK.

Port: COM6 (because this is the Digimaster PRO+ comport that is used for FSK/PTT keying)
FSK Output: TXD
PTT Output: RTS
– no checkmarks

Status should show as OK. If it does, minimize the EXTFSK window. If it doesn’t show OK, you’ve got some troubleshooting to do.

Assuming you’ve set up everything else up properly thus far and the interface is communicating with the PC and the radio, you should be ready to test MMTTY. You can test it by clicking on TX (remember to keep your wattage low, preferably running into a dummy load while testing). When you click on TX, you should hear the diddles if you have the means to monitor the audio. On my FT-950, I have MONI turned on and the Moni level turned up high enough so that I am able to hear the FSK in action.

As mentioned earlier above, I have not been able to get FSK to work with the Digimaster PRO+ without the use of EXTFSK. And, EXTFSK only supports 45.5 baud FSK RTTY. So, if you are wanting to run 75 baud RTTY, such as in the BARTG 75 contest, you’re going to have to set up DI-1 in N1MM for AFSK instead of FSK, you’ll need to specifically tell N1MM that the logged mode needs to be RTTY instead of the default radio mode, and you’ll need to configure MMTTY for AFSK and get your audio levels adjusted properly.

I’m going to mention this to Neil, since I was thinking that since his USB-to-Serial interface is based upon the FTDI chipset and drivers it would probably do FSK without the need for EXTFSK. It may be that I just didn’t configure things right. I was in the hurry to set this up and test it. But so far, I could only get FSK RTTY to function with EXTFSK installed and configured in MMTTY.

Written by Mike

October 5th, 2010 at 2:36 pm

G4ZLP Digimaster PRO+ CW / PTT / FSK / CAT / Digital Modes Interface

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Wanting to operate FSK RTTY to make better use of the available filtering in my radio as well as to eliminate the need for constantly adjusting my audio drive to keep the ALC meter happy, I had been searching for an FSK interface. I already own a CW keying interface from ZLP Electronics and have always had good dealings with Neil, G4ZLP, so I figured I’d check out his site for something new. Sure enough, he has thhe Digimaster PRO+. It claims to handle all digital modes including FSK RTTY and to provide CW keying / PTT and CAT control. This sounded great to me. I placed an order.

The Digimaster PRO+ arrived yesterday [within seven days]. It was packaged very efficiently. There is nothing specular about the look of this interface. It is a small black box with an LED on it and adjustments for TX and RX audio levels. If you were looking for eye candy, you won’t find it here. [to be honest, I have a Signalink USB as well, and there is nothing spectacular about its look either].

The packages included all of the cabling I would need to hook this up immediately to my Yaesu FT-950. When you order the unit, you specify the radio you are using. There was the interface box, a cable with audio in/out jacks to plug into your audio card, a CAT cable that plugged directly into the RS-232 port on the back of my Yaesu, a CW interface cable with a 3.5 mm jack that plugged directly into the back of the radio [after using a 3.5mm to 1/4″ stereo adaptor], a data cable which plugged directly into the PKT/DATA/RTTY jack on the back of my radio, and a cheap and simple USB sound device. The USB sound device is similar to ones I had previously purchased on Ebay from China or Hong Kong for $1.99 a piece plus shipping. Of course, the only purpose for this sound device is to support the AFSK modes, and it has everything you need to do that.

Wiring up the interface to my radio/computer was a no-brainer. All cables that you would have any doubt about were labeled / color coded. Here is the process i used:

  1. make sure the radio / interface are off
  2. plug all cables into the interface
  3. With the PC on, plug the USB sound device and the interface USB cable into two available USB ports on my computer
  4. turn on the Digimaster PRO+ interface

A few seconds later the PC reported that it saw some new hardware, and it found the appropriate drivers for the USB sound device automatically. It did not find the drivers for the USB-to-Serial chipset in the Digimaster automatically. It did give the option to Search the Internet. I chose that option, and it went out and find appropriate drivers and installed them. The LED then blinked red/green a few times, indicating that the drivers were installed and the interface was talking to the computer fine.

I then went into the Device Manager on the computer to check what numbers were given to the comports. In my case, the first comport (the one used for CW / PTT / FSK keying) was COM6 and the second comport (the one used for CAT control) was COM7. [NOTE: On your PC the Digimaster PRO serial ports would likely be assigned different comports than 6/7. This is dependent upon many things, such as what other comports are already in use as well as what USB port on the computer you have the interface plugged into.]

So far so good. The interface is talking to the computer. It’s now time to get N1MM, MMTTY and Ham Radio Deluxe set up to work with this new interface.

A few things I needed to know before setting up software to use the Digimaster PRO:

a. what comport number was assigned to the first [CW/PTT/FSK] interface? COM6
b. what comport number was assigned to the second [CAT control] interface? COM7
c. what is the baud rate set to in my FT-950’s radio settings? 38400

I’ll leave the setup info to my next post.

Written by Mike

October 5th, 2010 at 1:35 pm