TTY - Teletype Terminal

[Luciano]

Hi,

Use a microcontroller with large flash memory and SRAM.
(The 16F84 should be considered obsolete).

Start with the decoder part HW and SW.
(Receiving data is the difficult part of this project).


1. Using the PC freeware, generate the TTY/TDD tones with
the PC sound card at 45.45 bps. (Baudot @ 45.45 baud = U.S. TTY/TDD).

2. Decode the TTY/TDD tones from the PC with your HW decoder chip.
(Example of decoder chip: XR2211, Exar Corporation).

3. With the microcontroller take the output of the decoder chip
and display the data in ASCII format on the LCD.

Best regards,

Luciano

[Foxx373]

Thats a good idea. I was trying to tackle this project head on. I guess breaking it down piece by piece and getting each part to to work first is the key. Im not too familliar with the PIC MCUs. Any advice on which one to use? The 16F87 perhaps?

[mister_e]

PIC16F628 or 16F88 could be great ones to start.

PIC16F819, 16F877 are also really popular

[NavMicroSystems]

@ Dwayne

isn't this something we are "already" working on ?

;-)

[Foxx373]

ok, im trying to figure out how to at least transmit the TTY tones... the only way i can think of doing it is using a crapload of If-Then statements. Kinda like this...


IF key="A" THEN
FREQOUT PORTB.1,40,1800
FREQOUT PORTB.1,40,1800
FREQOUT PORTB.1,40,1800
FREQOUT PORTB.1,40,1400
FREQOUT PORTB.1,40,1400
end if


then have the same for the rest of the characters. But i think it would be better to put whatever they keyboard data that comes by into memory, that way there is a buffer... but i have no idea how to do that.

Any other suggestions on how to do this?

[Luciano]

Hi,

The standard used by TDDs is the Baudot code implemented
asynchronously at 45.5 (USA), 1 start bit, 5 data bits,
and 1.5 stop bits.

In your example I see only the 5 data bits.

From the PBP manual:

FREQOUT generates tones using a form of pulse width modulation.
The raw data coming out of the pin looks pretty scary. Some kind
of filter is usually necessary to smooth the signal to a sine
wave get rid of some of the harmonics that are generated.

* * *

Two questions when you use the FREQOUT command + filter:

1. What happen during transitions with the output signal?
2. Is the the output phase-continuous during transitions?

The output of the sinusoidal FSK Generator has to be
phase-continuous during transitions.(Mark and space transitions).

See page 2 of this PDF for Frequency shift keying (FSK) signal.
http://www.cs.sfu.ca/CourseCentral/3...s/lecture7.pdf

* * *

What about using the HW tone generator XR 2206 from Exar Corporation?
(The input for the tone generator XR 2206 is serial data).

XR-2206 monolithic function generator
Datasheet Rev 1.03
http://www.exar.com/products/XR2206v103.pdf

Page 9

FSK Generation
Figure 13 shows the circuit connection for sinusoidal FSK
signal operation. Mark and space frequencies can be
independently adjusted by the choice of timing resistors,
R1 and R2; the output is phase-continuous during
transitions. The keying signal is applied to Pin 9.


Again, start with the decoder part of your project.
The encoder part is easy.

Best regards,

Luciano

[Dwayne]

Hello Ralph,

Ralph>>@ Dwayne

isn't this something we are "already" working on ?<<

<chuckle> Yep... I was working more on letting the PIC do all the work, instead of using PLL chips to detect the tones like I did in 1990.

Dwayne