Automatic dispatch system for half-duplex teletypewriter lines

HALF-DUPLEX multistation teletypewriter lines efficiently meet the needs of several points that require record communication with each other. In such cases it is important to be able to send messages to selected stations only. This ability is necessary to save paper, to provide privacy, and to avoid the need to scan all messages at each station to find those addressed to it. Direct interstation communication avoids sending all messages to a relay center for retransmission to other stations. This saves line and attendant time. To facilitate sending messages to addressee stations only, manually operated selective calling devices have been used. By present standards these are relatively slow. In addition, until recently it was necessary for attendants to initiate transmission of messages only when the line was idle. On a busy line this frequently required waiting for the line to become idle as shown by a line lamp. The attendant could then seize the line, operate the selecting equipment, and send the message. Sometimes attendants at two or more stations attempted to seize the line simultaneously. Unsuccessful attendants, unfortunately, had to resume waiting. On a busy line this procedure required appreciable amounts of nonproductive line and attendant time. Furthermore, there was no way to give an urgent message priority over other messages.