Measurement Computing Micro 488/EX rev.2.1 - Manual (Page 116)

Section 5 Command Descriptions
5.46
MASK
The MASK command is used to mask the high bit (MSB) of serial input data. Some
serial host computers set the most significant bit when using eight bit serial data
lengths. When the MASK ON command is issued, each serial character received is
logically ANDed with &H7F (127 decimal).
MASK OFF is the power-on default. In default operation, all serial input data is
automatically masked with &H7F. The exception to this is any data which follows a
semi-colon (;), an apostrophe (') or a quotation mark ("). After a MASK ON command,
all characters are masked.
SYNTAX MASK {ON | OFF}
RESPONSE None
MODE Any
BUS STATES None
EXAMPLE PRINT#1,"MASK ON"