P_TW39

TW39 / TWM

Legend:

Symbol	Description
AT	User presses the button 'AT' (German: 'Anruf-Taste') on the FSG to start a call
ST	User presses the button 'ST' (German: 'Schluß-Taste') on the FSG to end a call
Nx	User dials the number x with the number switch on the FSG
✠	User presses the key '✠' (WRU, German: 'Wer da?') to retrieve the ID from the other teletype
✦	User presses the button '✦' on the teletype or on a ✠-request the teletype sends automatically its ID
T	User presses any key on the keyboard to type the message
WB	If the central office is ready-for-dialing (after pressing button 'AT'), it sends a pulse to signal this state. Here it is shown with symbol 'WB' (German: 'Wahlbereitschaft')
A	When the connection to the remote teletype is established, the central office switches polarity to signal the FSG to switch on the power of the teletype
Z	When the connection to the remote teletype stopped, the central office switches polarity back to signal the FSG to switch off the power of the teletype

Note: In software the symbols 'WB', 'A' and 'Z' are used as escape sequences to trigger the state.

Outgoing Call

If the system is in standby ('idle mode') the FSG reduces the line current by switching a resistor in series to the teletype. A typical current is +5...7mA.

Note: In the software this is detected as permanent low.

Begin a Call

To start a call the user presses the button 'AT' on the FSG. The FSG switches back to full current of 40mA and the central office prepares for connection. If the central office is ready-for-dialing, it sends a pulse to signal this state. The FSG switches relays to turn on the WB-bulb and release (electrically) the number switch.

Dialing

TW39: The user can dial the destination number with the number switch on the FSG. The power of the teletype is still off.

TWM: The power of the teletype is switched on. The user can dial the destination number with the numeric keys on the keyboard. The digits are coded in Baudot-Murray-Code. Some teletypes require the key 'Zi' to be pressed first.

The user dials digit by digit. If the number is complete and valid the central office establishs a connection.

Connecting

If the connection fails (wrong number or remote teletype is busy), the central office switches polarity to turn on the power of the teletype and after a second switches back for ending the call.

On success, the central office switches polarity to turn on the power of the teletype. During the whole following connection the current is negative.

Transmitting Content

The user can now start typing the message. The characters are sent as they are typed, at a maximum speed of 6.6 characters per second (400 per minute).

If using a punch tape for sending (or typing fast enough) the characters are transmitted without a pause with maximum speed.

---

Ending a Call

Ending by User

To end a call, the user presses the button 'ST' on the FSG. The FSG reduces the current and switches off the 'WB'-bulb. The central office reacts by switching back the polarity again. On that the FSG switches off the power of the teletype. The system is now back in 'idle mode'.

Ending by Central Office

If the central office wants to end the call it switches polarity again. The FSG reduces the current and switches off the 'WB'-bulb and the power of the teletype. The system is now back in 'idle mode'

---

Incoming Call

The central office signals an incoming call with switching of polarity. The FSG switches on all bulbs and the power of the teletype.

Some (modern) central offices send automatically a date and time string and request the ID of the teletype by sending '✠'.

The user can end the call by pressing the button 'ST' at any time.

---

✠ WRU (Wer da?)

Note: 'WRU' is the english short form of 'Who are You'

On an established connection, every side can send the '✠' character and the remote machine responds automatically with its ID in text form. The ID is coded in Baudot-Murray-Code.

The user can press the button '✦' to send the ID of his own machine.