--- In digitalradio@yahoogroups.com, "W5XR" <w...@...> wrote: > > I'm asking. :) > > Bob, W5XR.
O.K. For START-STOP synchronization to work the receiving shaft (selector or distributor) has to stop between characters. The Morkrum Co. (ancestor of Teletype Corp.) had the sending and receiving distributor shafts running at different speeds, so the receiving distributor completed its rotation part way through the STOP pulse and was held there until the next START pulse. the STOP pulse was the same length as all the others, so it was 7.0 unit code. Western Electric had built some teleprinters of their own, and in theirs the transmitting and receiving distributors were on the same shaft. So to give the receiver a chance to stop they had to stop the transmitter between characters. They did this by adding a relay to operate the transmitting distributor clutch. At the speed they were running at the time, something like 45 wpm, the relay added a delay in milliseconds that was equal to 0.42 of a pulse duration. When Western Electric discontinued making their own teleprinters and started buying from Morkrum they insisted on interoperability with the W.E. machines. Morkrum didn't want to use a relay in the transmitter clutch, so they simply elongated the STOP segment on the transmitting distributor to 1.42 times the length of the other segments and changed the shaft speed to keep the pulse duration the same. So we got 7.42 code; and this continued as speeds were increased and after the Western Electric equipment had all been phased out. Western Union didn't have the problem of interoperability with old Western Electric designs, so they insisted on 7.0 unit code because of the slightly higher speed that gives, roughly 65 wpm instead of 60. For many years Teletype had to make equipment that could transmit either way, by supplying the appropriate transmitter cam and gear. The printers all had no trouble copying 7.0 unit code. At 100 wpm, 7.42 unit code gives a speed of 74.2 baud. At some point the U.S. military decided to round that up to 75 baud, and then to standardize on speeds that are 75 multiplied by a power of two, so we got 75,150, 300, 600, 1200, etc. for our terminals and modems. For some reason Europe standardized on 50 baud and 7.50 unit code; I can only assume that some equipment manufacturer had trouble with a unit-length STOP pulse and needed extra time to get the receiver stopped.
