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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