Data communications over telegraph circuits 1934

Another early exception was the "recorder connector", a black box on the wall with a connector that allowed you to connect a wire or tape recorder to the telephone. It inserted a tone every 15 seconds or so to let the parties to the conversation know it was being recorded - this was required by the FCC at the time. A lot of people ignored the requirement and the telephone company box and simply used a magnetic coil pickup to connect the recorder to the telephone by induction.

Something I didn't mention previously is a system developed for Delta Air Lines that was a forerunner of Dial TWX. The immediate impetus was some development at the airport in Atlanta that required demolishing a building that housed Delta's leased message switching system. What Delta wanted was therefore a replacement store-and-forward message switching system, with mnemonic routing codes. What AT&T proposed and delivered was a circuit switched system operating over the voice switched network. The time frame is about 1959.

AT&T supplied a modem and autodialer, which occupied a cabinet about 4 feet high and three feet wide and a foot thick. (Same kind of cabinet as used for some of the old key telephone installations.) Teletype supplied the most complicated ASR set (automatic send-receive, meaning a teleprinter with paper tape facilities) that one could imagine. The message-originating set used 5-level Teletype code, but punched and read 8-level tape, the extra levels being used for control purposes. Telephone dial numbers were punched into the tape and used to control the auto-dialer. Relays and stepping switches in the base of the machine generated the beginning of message and end of message coding, so that the operator had only to type the text of the message. The modem was connected to the nearest #5 crossbar office, using whatever it took to do that.

To handle the mnemonic addressing there was a machine called the Codomat strapped on the side. This contained a wheel card file. The mnemonic addresses were on the tabs sticking up from the cards. To prepare a message the operator would rotate the wheel to bring up the card with the desired address on its tab. She would swing the card sideways into the machine, which had a crawling five-level reader that would read the phone number from the card and copy it to the punched tape of the message being prepared. She would repeat this for each addressee of the message, and then type the text and push the end of message button. Another part of the Codomat was a clock that generated the time of day in TTY code, so that the operator didn't have to do that.

The tape reader would read through the first telephone number in the tape and dial that number. If it obtained a connection it would forward step over any other dial numbers ahead of the message and send the text and end of message. Then it would read backward and mark the tape to show that number had been delivered to, and would dial the next number. This continued until the message had been delivered to all the addressees. If it failed to make a connection on one number it could skip that and deliver to the rest of the numbers in the tape and then go back and try again. After a certain number of failures it would dial a wired-in intercept number and deliver the message there. There was also a large amount of equipment involved in exchanging messages with other airlines that were using conventional store-and-forward message switches.

The system was considered successful. A second one was built for United Air Lines. However the whole scheme depended on FCC approval of the WADS - Wide Area Data Service tariff. I gather that was to allow the system to be leased to the customer at a fixed monthly rate rather than charging by the call. The tariff was disapproved and the systems were pulled out of service and replaced by computer-based message switching systems. --

jhhaynes at earthlink dot net