[GreenKeys] [External] How were teletypes connected across long distances?

[Jones, Douglas W]

From: Chuck Robertson [hootsk at gmail.com] -- Friday, November 24, 2023 8:58 AM

> If they didn’t have modems or routers, and the machines send and receive only by 110v current, does that mean there were dedicated wires across long distances between cities,

You need to remember that Teletypes were developed in an era when there was a developed telegraph network.  Teletype signalling was designed to be compatible with the existing Telegraph network.

Now, go back an extra 75 years or so to what Samuel Morse actually invented.  Morse and Joseph Henry traveled back from Europe on the same boat, and fell into discussions of the whole idea of telegraphy.  Henry knew more about electromagnets than just about anyone, and Morse was inspired by the relay stations of the French optical telegraph, where a man with a telescope would read the signals sent from a distant hilltop and work the levers to relay that signal to the next hilltop.  Read the Count of Monte Cristo for a cute story about hacking that communications system.

Anyway, out of that sea voyage came the electromagnetic relay, the key to the success of Morse's new telegraph network.  Every five miles or so, the telegraph line came to a relay station with a battery of crows-foot cells providing power for the loop to the next relay station.  At the heart of each relay station was one of Henry's newly invented relays, sending the signal onward to the next station.  That system was essential to Morse's first long distance telegraph line from Baltimore to Washington DC.

Had he tried to do it with just one battery of cells somewhere along the line, getting the signal through would have required thousands of volts and risked the lives of the telegraphers.  With relays, the local battery at each relay station could be modest, only a few tens of volts.

Properly adjusted relays can easily send data at 50 or 100 bits per second.  The telegraph network relied on patch panels for switching, to the extent that switched signals were used.  That continued to be the case with early teletype networks.  Most telegraph lines were unswitched, with signals taken down by a telegrapher on paper and handed to the telegrapher on the outgoing line.  When teletypes began replacing telegraphers, paper tape could be torn from a receiving teletype and carried to an outgoing teletype for transmission.  No change in the wiring was required.

               Doug Jones
               jones at cs.uiowa.edu