nt end. Further, telex can be operated with the element followed by 5 information elements and then one stop distant end completely unattended, which makes it ideal for inter- element (see Figure 59.2). With the pulse durations shown in Figure national working where businesses operate in differing time zones. 59.1, the telex operates at 50 bits per second. As telex transmits the written word, then it also makes it ideal for A variety of line signalling systems exist for telex operation, using communication between people of different languages, as time can voltage and current signalling and mainly designed around the be spent on translating the message and then formulating the reply. needs of electromagnetic systems, where the transmit and receive Since most countries that adopted telex also had a telephony net- elements were contacts and electromagnets respectively. Later work, they naturally used the telephony network as the basis for the single channel voice frequency signalling was added to the telex Start Stop Signal Code D Who Are You Carriage Return Figures Letters Line Feed Space Key:- Mark Signal Space Signal Figure 59.1 CCITT International alphabet number 2 59/4 Telex communications SPACE START CHARACTER Figure 59.2 Start stop telex signal system. There are therefore three major signalling systems in use transmitter to the receive path in the machine and hence making this throughout the world all with minor country differences depending system effectively half duplex. Telex machines operate at the fol- on the type of telex exchange operating in the local country. These lowing voltages and currents at 20mA, 40mA, and 60mA Line are double current signalling, single current signalling, single chan- currents for negative mark or positive space: (Figure 59.4) nel voice frequency signalling, as described in the following sec- tions. In addition some telex machines are connected to private 1. 80-0-80 Volts. networks or point to point private wires using similar signalling 2. 60-0-60 Volts. systems. The mode of