Tuning ZMODEM

MAXTRIES   has traditionally been set to 20.  A larger number may be
needed for extremely noisy channels.

Rxtimeout  has been traditionally been set to 600 for the sender, and has
not been changed for 20 plus years.  The traditional value for the receiver
is 100 which gives 10 seconds.  It may be set lower to speed up certain
forms of stress testing.  See the file "results".