Multiplex Telegraph Apparatus

apparatus used in multiplex telegraphy, mainly in very long radiotelegraphy lines; it consists of a distributor with several segments, and also transmitters and receivers for sequential transmission and reception of telegram characters.

The first duplex apparatus used in wire communications was invented in 1872 by the French engineer J. Baudot. The principle of operation of multiplex telephone apparatus can be explained using Baudot’s, simplex apparatus as an example (see Figure 1). The distributor of the apparatus is a disk made of an insulating material with metal rings mounted on it. The outer ring of the distributor is divided into ten insulated contacts combined into two segments. In station A, five contacts of the first segment are connected to the transmitter (to its keys). The contacts of the second segment are connected to five electromagnets of the receiver. In station B, conversely, the contacts of the first segment are connected to the electromagnets of the receiver and the contacts of the second segment are connected to the keys. The inner ring is connected to the communications line. The distributor brushes of both stations rotate synchronously and cophasally at 200 rpm; the rate is limited by the inertia of the moving parts of the apparatus.

Figure 1. Simplex telegraphy: (M₁), .. . , (M₅) receiver electromagnets, (E_p) power sources for transmitter keyboard

As the brushes rotate through the first half-turn, they connect the keyboard contacts of station A sequentially with the receiver electromagnets of station B; during rotation through the second half-turn they connect the keyboard contacts of station B to the receiver electromagnets of station A. The telegraph operator presses the keys (as required by the combinations of pulses of the characters being transmitted) when the brushes are on the receiver segment, in accordance with a sound signal generated by a time tapper. The current pulses sent by the keyboard of station A arrive at the contacts of the first segment of the outer ring of the distributor and—through its brushes, a communications line, and the distributor brushes of station B—reach the contacts of the outer ring of the first segment and the electromagnets of the receiver. The receiver then prints the appropriate character on a paper tape. The transmission capacity of duplex apparatus is about 2,000 words per hour.

Improved Baudot multiplex telegraph apparatus was used until the mid-20th century. In the 1930’s apparatus for threefold, sixfold, and ninefold multiplexing was developed, thus increasing the transmitting capacity of telegraph communications to 20,000 words per hour (for ninefold multiplexing apparatus). In the 1960’s electromechanical multiplex telegraph apparatus were replaced by electronic apparatus equipped with devices for error detection and correction. Electronic multiplex telegraph equipment is being manufactured (as of 1974) in the USSR, the Netherlands, Switzerland, and the Federal Republic of Germany.