Multiplex Operation

In this diagram, S and S' and the segments with which these brushes make contact are the sending elements; and R and R' and the corresponding segments are for receiving from the distant station. Since the sending and receiving arms rotate in synchronism, the code combinations must be arranged and the proper segments energized at a constant rate so that when the arm rotates onto these segments they will be ready for sending the correct impulses. A paper tape, on which the message has previously been punched by a keyboard arrangement and fed into the transmitter at a uniform rate, is therefore used to actuate the transmitter equipment. The impulses pass over the line to the receiving magnets as in the start-stop system, and by an arrangement of code selecting bars the signals are translated into the proper letter combination, which is then printed.

Figure 15. Simplified multiplex sending and receiving arrangement.

With the arrangement just described, two simultaneous messages can be handled over a duplexed grounded line. It will be observed in Fig. 15 that only one-fourth of the face of the distributor plate is in use. It is therefore possible to add three other sets of transmitting segments and three other sets of receiving segments. Then, when the brushes are passing over one set of segments, the signal impulses are being arranged for the other three sets of segments, and with a duplexed line eight messages are handled in a sense "simultaneously" over one telegraph line.¹⁴