The GL-Hell System

The Siemens GL-Hell machine, model 72c, was introduced in 1950, to compete with start-stop teleprinter machines, which by then were becoming widely used. Start-stop operation allowed the machine to be left unattended, as the paper feed only ran when a character is received. It also reduced the timing accuracy required, and so the GL-Hell machines did not need to print the text twice.

The GL-Hell machine prints on 9mm wide tape, and prints only a single line of text. Notice how, in the print-out below from an actual Siemens GL machine, only one row of text is printed, and the text is exactly horizontal, although individual letters may be sloped.

The drawing on the right is reproduced from a Siemens GL-Hell manual. The Start-Stop technique was introduced at the transmitter by adding a vertical bar to column I, on the left side of every character transmitted. Note in the drawing in column I the hatched area to the left of each character, which represents the "Start Pulse". Reception of this key-down period before each character at the receiver started the rotating mechanism, by releasing an electromagnetic latch. The white space at the completion of the character, in column VII, enabled the mechanism to latch again.

In other respects the font used by the GL system is identical to the Feld-Hell system, including the "two pixel rule" and the offset pixel pairs, clearly visible here on the "6".

The Start-Stop technique means that any character scan rate error only lasts for the duration of the one character, considerably reducing the accuracy required. The following example shows what happens when the receiving system (a) runs much too fast or (b) much too slow:

At the time the asynchronous communications protocol was introduced, other changes were also made. The GL-Hell machines uses a different tone frequency, 1000 Hz or 3000 Hz, and operate at 300 baud (6.1 characters/sec). The unit was arranged to operate on a public telephone line as a normal subscriber (like a modern FAX machine), with the motor starting automatically and a tone returned to the sending equipment to signal that the unit was "online". If the tone frequency was set to 3000 Hz, the phone line could be used for normal conversation between the two ends of the call, concurrent with the Hell transmission.


The Siemens 72c GL-Hell unit (Picture courtesy of Dick PA0SE)

The Siemens 72c machines were widely used by the German Railway (the Bundesbahn) and the German Army (the Bundeswehr), and were eventually released to the surplus market from about 1974. Since that time, they have been widely used by Amateurs on VHF, and they are still the most common Hell machines. Unfortunately the asynchronous system is very noise prone.

The GL-Hell system is not a system, since the hardware makes crucial decisions on when a character starts. As a result, the printed image is frequently garbled by noise and therefore irretrievable. GL-Hell operation in Europe was widespread for a time on VHF, using FM transmissions, and a Sunday morning net on 80m still operates in GL-Hell mode.

The keyboard is a normal four row QWERTY type (upper case only), and an ITA2 punched tape reader option was offered.


The 72c equipment with tape reader option (Picture courtesy of Arie PE1AQB)