[GreenKeys] 7.00 vs 7.42 unit code

[Jim Haynes]

To the best of my knowledge all Teletype 5-level machines can receive
7.42 or 7.00 code perfectly well.  Same gear set for both, since the
baud rate is the same.  It's just that 7.42 gives the receiving selector
a little extra time to rest, as explained below.

A 7.00 keyboard or tape transmitter needs both different gears and
different cams from a 7.42 transmitter, to make the bit length the same
and thus the baud rate the same.

The reason for 7.42 is historic and somewhat amusing.  In the days of
rotating brush or cam sending and receiving distributors for both
transmitting and receiving.  Morkrum ran the transmitting and receiving
shafts at different speeds, receiving being a little faster, so that
the receiving shaft had time to stop before the end of the transmitted
character, at 7.00 code.  That is necessary for START-STOP synchronization
to work.  The receiving shaft has to get around and stop before the
transmitting shaft can begin the next character.

Western Electric, however, had made machines where the sending and 
receiving distributors were on the same shaft, which would cause START- 
STOP synchronization to fail.  To fix this, they put a clutch on the 
transmitting distributor that was operated by a relay.  This added a 
little delay to the transmitting stream by having the transmitting shaft 
stop between characters for at least the operate time of the relay and 
clutch.  They measured the time delay introduced by the relay and found it 
to be about 9.25 milliseconds, which at 45.45 baud works out to 0.42 of a 
bit duration.  So they mandated that the sending devices should send a 
1.42 length stop pulse; and this remained in effect until the end, long 
after the W.E. machines had all been retired and after they had increased 
speeds beyond nominal 60 wpm.

 	---

 	"Ya can argue all ya wanna, but it's dif'rent than it was."
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 		Meredith Willson, The Music Man