Carriage returs password field

[Adrian Hopkins]

I am using a magnetic card reader reading cards encoded with an 11 digit serial number followed by a carriage return. When swiping cards into a password or single line text field, the carriage return has the effect of trying to post the data but returns an error 4 message. If I change the field to a multiline text field it reads the data into the field and I can then submit the data manually. Any ideas why the carriage return doesnt post properly?