[GreenKeys] Why and When 7.00 vs. 7.42 unit code?

[David Burns]

The difference between a machine running 7.00 and 7.42 is the length of 
the stop-bit.  The data-bits are exactly the same length (22ms) on both 
machines (hence they can interoperate), but the stop-bit is about 1 1/2 
times (1.42) a standard data-bit time span on the 7.42 machines.

When I received a WU Model 100 being retired from service in 1966, I was 
told it was a "66WPM machine" but would run OK with a "60WPM machine".  
True.

It would seem a 31-millisecond stop-bit (on a 7.42 machine) affords a 
bit more latitude in synchronizing machines on a sloppy circuit at the 
expense of moving data at a slightly slower rate than a machine with a 
22-millisecond stop-but (7.0).

For the gory details, see Table II in Irv Hoff's introduction:

http://www.samhallas.co.uk/repository/telegraph/introduction_to_RTTY.pdf

-Dave in Boston