Sounds like a BAUD mismatch. Just to confirm, have you adjusted the port setting in the Windows control panel?
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StSomewhere:I'm assuming the Vytec was already configured with imperial units and you are just trying to get the SDM software configured, correct?
Run SDM 2.2.1, create a new username/password and login with that username.
Select the new username in the logbook frame (on the left, at the top).
Now in the bottom section under the User heading in the Settings section, pick the imperial settings (ft, psi, °F, cu ft, lbs). What's not obvious at first is that the selected settings will be blue. You can set up cylinders including a default cylinder here, too. There are also Personal, Dive Log and Printing buttons, Dive Log is where you can specify the custom field names.
After you've set your default units for the dive logs, then pick the download wizard to pick your computer type, connection port, etc.
HTH
John