Apple Express Fax/Modem - Manual (Page 135)

S registers
Default Range Units
S0 Number of rings to auto answer
0 0:255 Number of rings
The S0 register determines the number of times your telephone must
ring before your modem will automatically answer it. When S0 is set
to 0, the modem is in manual answer mode.
S1 Ring counter
0 0:255 Number of rings
The S1 register is cleared if no rings are detected throughout an
8-second interval. S1 is automatically set by the modem.
S2 Escape Character
43 <+> 0:127 ASCII Decimal
The S2 register determines the ASCII character to be used as the escape
character. The escape sequence is used to force the modem from the
on-line state to the local command state.
S3 Line-termination character
13 0:127 ASCII Decimal
The S3 register determines the ASCII character to be used as the
line-termination character. No matter what value S3 is set to, ASCII 13
is always recognized.
S4 Line-feed character
10 0:127 ASCII Decimal
The modem returns the ASCII character programmed in the S4 register
as the line-feed character when the modem sends response codes to the
computer.
S5 Backspace character
8 0:127 ASCII Decimal
The S5 register determines the ASCII character to be used as the
backspace character.
123 M O D E M C O M M A N D S E T