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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