3Com 56K Command Reference Manual page 185

Register Default Function
S15 0 Bit-mapped register. (See Chapter 1, Using the AT
Command Set, for instructions about how to set
bit-mapped registers.)
Bit
0
1
2
3
4
5
6
7
Value Result
1 Disable the Business Modem's extra
high-frequency equalization if it
causes problems on shorter-link calls
(for HST modulation only).
2 Disable online fallback.
4 Disable 450 bps back channel* HST
only.
8 Reset non-ARQ mode Transmit buffer
from 1.5K bytes to 128.* (for the
convenience of BBS operators taking
calls from remote users of slower
modems).
16 Disable MNP Level 4; retransmitting
the larger Level 4 data blocks may be
a problem if you expect a great
number of errors during a call.
32 Set Backspace key to delete.
64 Some earlier 2400 bps MNP modems,
not made by 3Com, U.S. Robotics, or
Microcom, were not fully compatible
with the MNP protocol. If you have
difficulty making a successful 2400
bps MNP connection with a remote
MNP modem, it may be because of
this incompatibility. Set S15 to 64 and
try again to make the connection.
128 Custom applications only.
* The default 1.5K byte non-ARQ
buffer allows data transfer with X-
and Y-modem-type File Transfer
Protocols without using flow control.
The 128-byte option allows remote
users with slower modems to stop
your transmitted data from scrolling
off their screens. When remote users
send your computer an XOFF
(<Ctrl-S>) and you stop transmitting,
the data in transit doesn't exceed the
size of their screen.
S-Registers I-3