[GreenKeys] ASCII <-> Baudot algorithm

[tony.podrasky]

Hello dr;

Well - it gets even a bit worse than that:

Some Teletype Machines have a feature called "unshift on space"
and some have "stayshift on space".

"Unshift on space" will take the machine out of the FIGS when you
receive a SPACE character and downshift it into LTRS.

"Stayshift on space" will leave it in FIGS when you
receive a SPACE character.

The difference: you type   QWE 123 123 QWE

If you don't add the appropriate FIGS or LTRS key after the SPACE
here's what you're going to see:

UNSHIFT: QWE 123 QWE QWE

STAYSHIFT: QWE 123 123 123

Both have a reason for existence:

STAYSHIFT was used for PIREPS (Pilot Reports) and was lines of
groups made of 5 numbers with about 10 groups per line.

UNSHIFT was used especially for RTTY because when there was static
or fading, the machine might get stuck in FIGS and you'd have to
translate the numerical stuff back to text stuff.

SO: when you write your program, you have to take (that) into
account - so no matter how the data is coming in or going out,
the message will appear the same.

If you were working with C I could give you some pointers, but I
don't know anything about a C-64 anymore.

UE,
K2EAA - TONY
NNNN
ZCZC


On 08/01/2015 05:19 PM, drlegendre . wrote:
> Still slogging away at my C-64 TTY driver project. Only now beginning to
> look into the ASCII / Baudot conversion issues, and would certainly
> appreciate some input on the matter.
>
> While ASCII has 128 possible values, Baudot is limited to 32 which
> doesn't even allow for the full US/Latin alphabet plus the ten numerals
> - much less punctuation. So it seems that all the numerals and
> punctuation are 'shifted' characters, using the LTRS and FIGS scheme.
>
> So when a TTY sends the string "ABC123DEF" it actually sends:
>
> LTRS+"ABC"+FIGS+"123"+LTRS+"DEF"
>
> With each printing character represented in it's 5-bit binary Baudot form.
>
> So to implement a conversion routine, I must not only translate between
> the ASCII & Baudot values for the chars, I must also know which set the
> char is from (FIGS or LTRS) and send the correct preamble to toggle the
> char set prior to the character value?
>
> Do I have this correct? Are there standard routines / algos for this,
> that I could study? An example in BASIC would be grand, as that would
> make it so much simpler on the C-64.
>
> Thanks in advance for any and all comments, you guys have always been
> great for this stuff..
>
>
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