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Re: [TenTec] Using Omni VI with MMTTY on FSK

To: <tentec@contesting.com>
Subject: Re: [TenTec] Using Omni VI with MMTTY on FSK
From: "Mark Erbaugh" <mark@microenh.com>
Reply-to: tentec@contesting.com
Date: Thu, 28 Aug 2003 08:39:00 -0400
List-post: <mailto:tentec@contesting.com>
> Then I bought a Rascal interface from K4ABT at www.buxcommco.com, along
> with his OmnI VI/V cable. The unit was advertised to work on FSK but the
> cable he sells only provides the AFSK connections. His instructions are
> unclear and he would not take the cable back. So I modified it to
> provide a MARK/SPACE input to the Omni using a diagram he sent me. It
> received FSK input from DB9 pin 3, just as in the AA5AU diagram. This
> way, his Rascal is very similar to the homemade one I had tried earlier,
> and did not work on FSK either.

I'm using a Rascal Mark V RTTY interface (I just checked the website and I
think it's now called the Mark V Plus). Is that what you have? The original
Rascal (still sold as a kit) and the Mark V (w/o the RTTY) do not have the
circuit for keying the FSK input.

>  >I've had no problem running FSK with my Omni VI - although I did
>  >notice that my tones seem to be more like 130 (rather than 170) Hz
>  >apart. I suspect that my rig needs some alignment.
> If I have an alignment problem, it's with two different rigs.

Alignment wouldn't cause you to get a single tone (unless it were WAAAY

Here's a simple explanation of how the serial port interfaces, such as your
homebrew and the Rascal are supposed to work. Maybe this will help

On a standard DB9 serial port, pin 4 is the DTR output, pin 7 is te RTS
output and pin 3 is the TXD output. On these ouputs a negative (-3 .. -12)
voltage represents a 1 and a positive (+3 .. +12) voltage represents a 0.
These voltage levels are commonly referred to as RS232 levels.

All the interface does is to take these voltage levels and convert them to
TTL levels (typically 0 - 5v) used to control the transceiver.

The computer program can turn DTR and RTS on or off (negative or positive)
and these signals are usually used for PTT and CW keying. The TXD signal is
controlled by a dedicated chip in the computer (a UART) and is used to send
data. As such, the computer program loads a byte of data into the UART and
the UART will turn the TXD line on or off with the proper timing to generate
the signal with appropriate baud rate, number of bits, parity, etc. This
output is used to generate Mark/Space inputs to the transceiver for FSK

In AFSK operation, the computer soundcard generates the tones which are fed
to an audio input of the transceiver operating in SSB.

Since most radios don't generate Mark/Space outputs giving the state of the
received signal, to receive RTTY, the computer sound card is used decode the
received tones. This is the same whether you are in FSK or AFSK for


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