[WriteLog] 5 bit mystery unraveled

[Bob Naumann - N5NJ]

OK - I think I have a complete explanation of what this is all about.  =


Here goes:

1) The problem is not really a problem, but rather a feature of most serial=
 ports, which is an holdover from the early days.  The UART (universal asyn=
chronous receiver-transmitter) device that is part of the serial port can p=
roduce properly timed RTTY characters on it's output, which is the TXD of t=
he serial port.

2) Newer devices, most notably, USB to serial convertors, do not have this =
"old" functionality.  I understand that Belkin USB to serial devices may be=
 an exception to this and have a UART in them.  The reason this has changed=
, is that this 5 bit capability is only run at 45 baud, and these new devic=
es are engineered to go so much faster than that, that they have left this =
capability out as unnecessary.  No one uses 5 bit baudot code any more - ri=
ght?

3) The input to a radio - the RTTY jack - is a simple input that receives a=
 logical off or on, open or closed, a 0 or a 1.  It does not receive the 5 =
bit "data", but instead receives logical open or closed input that follows =
the changes in state that produce the 5 bit baudot code.  There is no conve=
rsion of the data going on in the radio.  If the RTTY input is left open, t=
he carrier is output on the Mark frequency.  When the RTTY input is shorted=
, the frequency of the transmitter shifts by 170 Hz.  This is all that happ=
ens.  Open - the transmitter is on the Mark Frequency; closed - the radio s=
hifts the transmitted carrier to the Space Frequency offset.

4) The UART in the serial port converts the 5 bit code - not the radio.  Yo=
u could just connect a momentary pushbutton to the RTTY jack on your radio,=
 and it might be possible that you could produce 5 bit baudot FSK output fr=
om your radio.  The timing is critical enough that you probably cannot comm=
unicate with someone else.  =


5) It would be possible to use the DTR output of the serial port to provide=
 the logic necessary for driving the radio's RTTY input - if the functional=
ity of the UART was emulated in software.  However, the approach is to use =
the UART since it already has this capability.  Sort of a "why re-invent th=
e wheel" approach.  The other school of thought would be to move this to so=
me other external device rather than doing it in software.  We may see new =
devices that will provide this capability in the near future.

Summary:

The reason that I wanted to get to the bottom of this, is that I wanted to =
understand why we had to sacrifice the radio control capabilities of a seri=
al port in order to produce a simple switch closure to drive the radio's RT=
TY input.  The reason is because the UART has the intelligence to convert t=
he letters and characters to the precisely spaced output which can only be =
produced on the TXD output of the serial port.  It seemed logical to me (pr=
ior to knowing about the UART stuff) to seek to use the DTR output (CW norm=
ally) instead, as it is not used during RTTY operation.

For the time being, if you want to control your radio, and you want to run =
FSK, you will need 2 serial ports.  However, you can use any of the USB ser=
ial ports to control your radio, and use the built-in serial port (which li=
kely has a UART) to do the FSK stuff.

I am now doing this with one of the Microham USB devices to do CW, PTT and =
radio control, and the serial port on my computer to do the FSK.

73,
Bob N5NJ

> ------------Original Message------------
> From: Bob Naumann - N5NJ <n5nj at gte.net>
> To: "Nick Kail" <m3mlr at tiscali.co.uk>, "'FireBrick'" <w9ol at billnjudy.com>=
, "'Writelog List'" <writelog at contesting.com>, "'FT1000MP List'" <1000mp at ma=
ilman.qth.net>
> Date: Fri, Feb-27-2004 10:43 AM
> Subject: Re: RE: [WriteLog] microHAM USB interface
> =

> Can someone explain the 5 bit "problem"?
> =

> Why couldn't the CW output just be hooked up to the FSK connection on the=
 radio?  Is that connection on the serial port just not capable of switchin=
g fast enough to do rtty?
> =

> -N5NJ
> =