[RTTY] USB serial port adapters

[Joe Subich, W4TV]

> One article I read was saying that 45 baud rtty is too slow for the
> adapter while another said that they don't handle 5 bit characters.

Modern USB to serial adapters are designed (since the advent of USB 2.)
to support data rates to 1 MBaud or more.  Typically the maximum data
rate is 3 to 5 MBaud, for example the FTDI products support data rates
to 3 MBaud.

Unfortunately, since the data rate is set using a 14 bit integer, the
ratio of maximum data rate to minimum data rate is fixed at 2^14 or
16,384:1.  With a 3 MBaud maximum data rate, the minimum available
rate becomes 183 Baud ... the lowest "standard" data rate is 300 Baud.

Note: FTDI's specifications only list support for 7 or 8 data bits in
their current products.

> Anyway, I'm writing a C program under Linux and I have successfully
> got an adapter with a PL2303 chip to send FSK RTTY at 45 baud. I had
> to write code for a custom bit rate since 45 baud isn't a standard
> rate.

Yes, some UART/Bridge chips provide for custom baud rates (typically
by bypassing the "baud rate divisor").  However, doing that generally
limits one to that particular manufacturer/chip as custom data rates
are generally not supported by any of the OS drivers.

73,

    ... Joe, W4TV

On 4/24/2016 12:16 PM, Lee wrote:
> Now for my next question: why is FSK RTTY difficult when using USB
> serial port adapters? One article I read was saying that 45 baud rtty is
> too slow for the adapter while another said that they don't handle 5 bit
> characters. From my research POSIX allows you to specify 5 bit
> characters.   Anyway, I'm writing a C program under Linux and I have
> successfully got an adapter with a PL2303 chip to send FSK RTTY at 45
> baud. I had to write code for a custom bit rate since 45 baud isn't a
> standard rate. Anyway, my project is still WIP, though. I've noticed
> that there's EXTFSK for MMTTY and I've tried it - works for me - but I
> want a program that runs on Linux. If the developers of FLDIGI ever
> decide to incorporate FSK into their code or if another developer puts
> out a native Linux FSK RTTY program, I'll drop my project. I don't want
> to build an adapter to convert audio tones (AFSK) to FSK (pseudo-FSK).
> Hopefully nobody thinks I'm being critical of any developers - I just
> want to know what's going on and why.
>
> N0SQ
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