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815 lines
SPORTSTER 14,400
CHAPTER 4. INTERFACE CONTROLS
Use the commands explained in this
chapter to select modem settings at the
serial port and phone line interfaces.
The operations affected include flow and
error control, data rate shifting, and
data compression.
TERMINOLOGY
ARQ (automatic repeat request, or
retransmission) is the term used by
USRobotics in error control commands and
response codes. An ARQ connection
indicates a call under error control.
The serial port rate is the rate at
which the Sportster 14,400 communicates
with its attached terminal (at the
serial port interface).
Terminal refers to the machine
(terminal or personal computer) to
which the Sportster 14,400 is attached.
The connection rate is the rate at
which the Sportster 14,400 communicates
with remote modems over a phone link.
MODEM INTERFACES
Figure 4.1 shows which commands control
modem operations at the serial port and
phone line interfaces.
Serial Port Interface Phone Line
Interface
Serial port rate: &Bn CCITT/Bell
answer sequence: Bn
Flow control Connection rate: &Nn
Transmit Data: &Hn Error control
(ARQ): &Mn
Received Data: &Rn, &In Data
Compression: &Kn
DSR override: &Sn
Figure 4.1_Interface Control Commands
Detailed command descriptions are in
this chapter. If you're familiar with
modem operations, you may want to
review Chapter 3, Quick Start, and then
use this chapter for reference.
CHANGING SETTINGS
When you change a setting, the modem
retains it until you do one of the
following.
∙ Change the setting again.
∙ Issue the ATZ command to reset the
modem. If DIP switch 7 is UP, the
modem resets to the defaults stored
in nonvolatile memory (NVRAM). If
DIP switch 7 is DOWN, the modem
resets to the factory settings
permanently stored in read-only
memory (ROM).
∙ Issue the AT&F command to reset the
modem to its factory settings.
∙ Turn the modem off and power it on
again. The modem loads either the
nonvolatile memory defaults (DIP
switch 7 UP) or the factory
settings (DIP switch 7 DOWN).
THROUGHPUT GUIDELINES
The following guidelines should help you
to make the most of your modem's
advanced performance features. In many
instances, experimentation and
experience will indicate what works best
for your applications.
Throughput is the volume of user
information transferred per second,
without Start and Stop bits and other
overhead information. You'll obtain
optimal throughput under the following
conditions.
1.Your communications software supports
a fixed serial port rate higher than
the connection rate, for example,
setting your software to lock into
the 38.4K bps rate, and setting the
modem to &B1. Check the quick
configuration guide that came with
the modem or your software manual,
and see Data Rate Commands in this
chapter. See Recommended Settings
and Alternative Settings in Chapter 3
for quick-start guidelines.
2.The call is under data compression.
See Data Compression (&Kn) in this
chapter.
3.You're transmitting text files.
Throughput is higher for text files
than other types of files, such as
.EXE or .COM binary files. For a
comparative table, see the end of
Appendix A.
4.File transfer may be slowed down by a
file-transfer protocol. Many non-
text files require a file-transfer
protocol, but throughput results
vary. Certain public domain file-
transfer protocols, for example, have
the following effects.
KermitWith the basic Kermit,
throughput is severely reduced
due to short block lengths
(under 128 bytes) and
acknowledgment turnaround
time. Later enhancements to
Kermit permit larger data
block lengths.
XmodemThroughput may be reduced if
your version uses short block
lengths, for example 128
bytes. Some versions use
blocks of 1 Kbyte, which is
much better, although overhead
(error-control protocol
information) still affects
overall throughput.
YmodemThis protocol is similar to
Xmodem with 1-Kbyte block
lengths, and allows multiple
files to be sent in one
transfer.
The above protocols further reduce
throughput during error-control (ARQ)
connections. The accuracy of the
data is checked twice, by the file-
transfer protocol and the modem. To
avoid redundancy, disable modem error
control by setting the modem to &M0.
For better throughput, we recommend
Ymodem-G, with the modem left at its
error-control default, &M4. Ymodem-G
assumes the modems are handling error
control: overhead is minimal, with
throughput almost equal to that
obtained with no file-transfer
protocol. However, keep in mind that
Ymodem-G is only useful if the modems
are using error control. In
addition, follow this recommendation
only if your machine and software
support hardware flow control.
On the other hand, the most current
version of Zmodem can yield even
greater efficiency. Leave the modem
at its error-control default (&M4)
and data compression default, &K1.
Zmodem performs the same kind of com
pression as V.42 bis; it turns off
its compression if files are already
compressed.
WARNING: If you are using an X-, Y-
or Zmodem-type protocol, do not use
the modem's software flow control.
See the more detailed warning in this
chapter under Warning on Software
Flow Control.
CCITT/BELL SETTINGS (BN)
B0 The CCITT answer sequence. This
setting is the default and is
required for the modem to answer
V.32 and V.32 bis calls. This
setting is also used to answer all
calls from overseas, but does not
affect U.S./ Canada calls. It
takes slightly longer to connect at
2400 bps and below, however.
B1 The Bell answer tone, commonly used
in the U.S. and Canada for
300/1200/2400 bps connections. If
you're only using the modem to call
or answer 2400-bps or slower
modems, the Sportster 14,400 will
make connec-tions more quickly at
this setting. Keep in mind,
however, that a modem set to B1
can't answer V.32 or V.32 bis
calls.
ERROR CONTROL (&MN)
The Sportster 14,400 first attempts a
connection using V.42 (LAPM) error
control and, if that doesn't succeed, it
attempts an MNP connection. If that
doesn't succeed, the modem tries to
connect without error control. (The
process takes much less time than it
takes to read this paragraph.)
Error control is possible at rates of
1200 bps and above. For background
information, see Appendix A. The
following options are available.
&M0Normal mode, no error control.
Because of the nature of phone-line
channels, this is never recommended
for calls above 2400 bps unless
you're using an error-correcting
file-transfer protocol.
&M1Reserved.
&M2Reserved.
&M3Reserved.
&M4Normal/ARQ mode. This setting is
the default. If the remote modem
doesn't recognize the Sportster
14,400's error-control signals_V.42
or MNP_the modem operates in Normal
mode, as though it were set to &M0.
&M5ARQ mode. If the remote modem
doesn't recognize the error-control
request_V.42 or MNP_the Sportster
14,400 hangs up.
Auto Answer with Error Control
When set to &M4 or &M5 and a call comes
in, the modem goes off hook and responds
to received error-control signals. If
the Sportster 14,400 doesn't receive
those signals and is set to Normal/ARQ
mode (&M4), it answers the call in Nor
mal mode (&M0). If it doesn't receive
the signals and is set to ARQ mode
(&M5), it hangs up.
ONLINE FALLBACK AND FALL FORWARD
When online at 14.4K bps under error
control, V.32 bis modems monitor the
condition of the phone line. If they
sense disturbances that threaten data in
tegrity, they fall back and retrain
(resynchronize) at the less vulnerable
speed of 12,000 bps.
If conditions remain poor, the modems
continue to fall back to the next lower
speed. As line improvements occur,
they fall forward to the next higher
speed, up to the link rate of the call.
Without online fallback, disturbances
on the line could cause the modems to
hang up from a high-speed connection.
The error-control protocols have a
retry timeout (a maximum of 12 re
transmissions) after which the modems
hang up. Falling back to keeps the
modems online to complete the call.
See Online Fallback/Fall Forward in
Appendix A for more information.
FLOW CONTROL OVERVIEW
This feature controls the flow of data
into and out of the modem's Transmit and
Received Data buffers. Due to varia
tions in terminal and communications
software features, the Sportster 14,400
is shipped with all flow control options
disabled.
NOTE: Flow control of data from the
terminal to the modem is required under
the circumstances described under
Transmit Data Flow Control (&Hn), later
in this chapter. Flow control of data
received over the phone link is not as
critical unless, for example, you are
writing incoming files to disk on a
very slow computer.
Transmit Data Buffer Sizes
The size of the Transmit Data buffer
depends on whether the connection is
under error control or not, as follows.
∙ ARQ connections: 3.25 Kbytes.
∙ Non-ARQ connections: 1.5 Kbytes.
This allows the use of error-
control file-transfer protocols
such as Xmodem and Ymodem without
flow control.
Received Data Buffer Size
The size of this buffer remains
constant at 2 Kbytes.
Hardware/Software Flow Control Options
The modem uses either hardware or
software flow control to manage the
amount of data stored in the buffers, to
prevent buffer overflow.
For the reasons described in the next
section, we recommend hardware flow
control. This type of flow control is
performed between the modem and terminal
with RS-232 signaling.
Software flow control uses the standard
ASCII Transmit OFF (XOFF) character,
<Ctrl>-S, and the Transmit ON (XON)
character, <Ctrl>-Q.
ASCII definitions are as follows.
XON <Ctrl>-Q ASCII 17 Decimal, 11
Hex
XOFF <Ctrl>-S ASCII 19 Decimal, 13
Hex
The ASCII definitions of these
characters may be user-defined; see
Registers S22 and S23 in Appendix B.
That appendix also includes an ASCII
chart.
NOTE: Both your software and terminal
must support the type of flow control
you select. Many terminals, and
computers that are 100% IBM PC-
compatible, support hardware flow
control. Apple computers, however, do
not. If you're using software that is
included in the quick configuration
guide, refer to that document to see
what your software supports.
Otherwise, see your software
documentation.
Warning on Software Flow Control
In ordinary operations, the only
characters the modem recognizes during a
call are the three pluses (+++) of the
escape code that returns it to command
mode. But when software flow control is
enabled, the modem or terminal
(receiving device) also looks for <Ctrl>-
S or <Ctrl>-Q characters. If these
characters occur in a file or as part of
a protocol, the device reads them as
XON/ XOFF characters and acts on them.
For example, XON/XOFF characters occur
in binary files, and are used by Xmodem-
type protocols. They may also come from
the remote system: an XON from the
remote system, after your modem has sent
an XOFF, can result in buffer overflow.
We recommend the following settings.
1.If possible, always use hardware flow
control. Software flow control may
prove satisfactory if you're only
transferring text files; however, you
may lose data if XON/ XOFF characters
occur in the data stream from other
sources.
2.If your equipment doesn't support
hardware flow control and you are
performing Xmodem-type protocol or bi
nary file transfers, set the modem to
the following factory defaults.
a.&H0, to disable flow control
entirely.
b.&B0, so that the serial port rate
switches to match the connection
rate.
c.&N0, to enable the variable
connection rate.
We also recommend that you select a
protocol that performs error
correction, and turn off the modem's
error control, that is, set it to
&M0.
TRANSMIT DATA FLOW CONTROL (&HN)
This type of flow control regulates
data your terminal transmits to the
modem for transmission over the phone
link. The modem monitors its Transmit
Data buffer as data comes in. If the
buffer approaches 90% capacity, the
modem signals the terminal to stop
transmitting. When the modem has sent
enough data over the link to half-empty
the buffer, it signals the terminal to
resume transmitting.
When it is Required
Transmit Data flow control should be
enabled in the following situations.
∙ You're using error control (any
allowable rate above 300 bps), with
or without data compression. If
data blocks must be sent again due
to errors, a continuous stream of
data from the terminal could
overflow the modem's buffer.
∙ The serial port rate is higher than
the connection rate, for example,
the serial port rate is 38.4K bps
and the connection rate is 14.4K
bps. Another example is a serial
port rate of 19.2K bps and a
connection rate of 2400 bps.
NOTE: You may have to set your software
as well to either hardware or software
flow control. Some programs also
require that you turn off the type you
are not using.
&H0Transmit Data flow control disabled.
Default.
&H1Hardware flow control. Recommended
setting. Requires that your
terminal and software support Clear
to Send (CTS). The modem drops the
Clear to Send (CTS) signal to the
terminal when the modem's buffer
nears 90% capacity, and starts
sending CTS again when the buffer
is about half full.
&H2Software flow control. Requires
that your software support XON/XOFF
signaling. The modem sends an XOFF
to the terminal when its buffer
nears 90% capacity, and sends an
XON when the buffer is about half
full.
&H3Use both hardware and software flow
control. Select this option if
you're not sure what your terminal
supports.
NOTE: If possible, always use hardware
flow control. For a detailed
explanation, see the Warning on
Software Flow Control, earlier in this
chapter.
RECEIVED DATA FLOW CONTROL
Separate commands, &Rn (hardware) and
&In (software), control the flow of
Received Data passed by the Sportster
14,400 to your terminal. Because
computers handle incoming data at a much
faster rate than the modem receives it
over the phone line, most users won't
need this type of flow control.
Both your terminal and software must
support the type of Received Data flow
control you select. Review the quick
configuration guide and, if necessary,
your software and computer manuals. We
recommend hardware flow control, if
possible. See the Flow Control Overview
and Warning on Software Flow Control
earlier in this chapter.
Hardware Control (&Rn)
When your terminal drops its Request to
Send (RTS) signal, the modem stops
passing along received data. The
terminal sends RTS again when it is
ready to receive more data.
Your terminal and software must support
RTS. You cannot use this type of flow
control, however, if your software
requires a constant RTS signal.
NOTE: Use this type of flow control
only for ARQ (error-control) calls,
because the V.42 and MNP protocols
control the flow of data across the
phone link. During non-ARQ connections,
however, there is no way to signal the
remote modem to stop sending data. If
your modem stops passing data to your
terminal and the remote modem keeps
sending across the link, the Received
Data buffer will overflow.
&R0Reserved.
&R1The modem ignores Request to Send
(RTS). Default. This setting is
required if your software does not
support RTS.
&R2Hardware flow control of received
data enabled. The modem passes
received data to your terminal only
on receipt of the RTS signal.
Software Control (&In)
When you send the modem a <Ctrl>-S
(XOFF) command from the keyboard, the
modem stops passing received data to
your terminal. (Hold down the Ctrl key
and press the letter. Don't type the
angle brackets.) When you send a <Ctrl>-
Q (XON) command, the modem resumes.
NOTE: Because of the risk of data loss,
all of the options that follow, except
&I0, are only recommended if your data
does not include XON/XOFF control
characters. For a more detailed ex
planation, see Warning on Software Flow
Control earlier in this chapter.
&I0Disables XON/XOFF flow control of
received data. Default.
Recommended for non-ARQ (Normal
mode) calls.
&I1The Sportster 14,400 acts on your
typed XON/XOFF commands, and passes
them to the remote computer. This
is not recommended for non-ARQ
calls. Use in ARQ mode only, but
keep in mind that the XON/ XOFF
characters sent to the remote
computer may interfere with XON/
XOFF signaling between the remote
computer and remote modem. &I2 is
the preferred option for ARQ calls.
&I2The Sportster 14,400 acts on your
XON/XOFF commands, but removes them
from the data stream instead of
passing them to the remote
computer. This is the recommended
setting for ARQ mode, only. It
ensures that the remote computer
does not confuse your XON/XOFF char
acters with those from its attached
modem.
If the call is not in ARQ mode,
there is no flow control on the
phone link. If you send an XOFF to
your modem and it stops passing
data, it has no way to tell the
remote computer and modem to stop
sending for a while, and your
modem's buffer may overflow.
DATA COMPRESSION (&KN)
When data compression is enabled, the
transmitting modem detects redundant
units of data and recodes them into
shorter units of fewer bits. The
receiving modem decompresses the redun
dant data units before passing them to
the receiving computer.
Compression does not occur unless the
modems are able to establish an error-
control connection. In addition, your
modem should be set for Transmit Data
flow control (&H1, 2 or 3), as described
earlier in this chapter.
If the modem makes a V.42 connection, it
negotiates V.42 bis compression. If
V.42 bis is not feasible, the connection
remains under error control, but without
data compression.
If the Sportster 14,400 makes an MNP
connection, it negotiates for MNP Level
5 (MNP5) data compression. If the
remote modem does not have MNP5
capability, the connection remains under
MNP error control, but without
compression.
&K0Data compression disabled.
&K1Auto enable/disable. This is the
default. The modem enables
compression if the serial port rate
is fixed, &B1, and disables
compression if the serial port rate
follows the connection rate, &B0.
The reason is that compression of
fers no throughput advantage when
the serial port and connection
rates are equal: compression may
even degrade throughput.
&K2Data compression enabled. Use this
setting to keep the modem from
disabling compression.
&K3Selective data compression. The
modem negotiates only for V.42 bis
compression, and disables MNP Level
5 (MNP5) compression. See the
following note.
NOTE: MNP5 compression is not useful
when transferring files that are already
compressed, such as the .ZIP files
downloaded from many Bulletin Boards,
and 8-bit binary files, which appear to
the modem to be already compressed.
MNP5 tends to add data to these files,
and throughput over the phone link
degrades.
V.42 bis compression dynamically detects
when data is already compressed and
turns off until it detects that compres
sion will work to advantage. The
special &K3 setting enables the best
throughput for already-compressed files
and 8-bit binary files.
See Throughput Guidelines and Data
Compression in Appendix A for more
information, including throughput to
expect for different kinds of files.
DATA RATE COMMANDS
The &Bn and &Nn commands allow you to
select variable or fixed data rates at
the serial port and phone line (modem-to-
modem) interfaces.
Throughout this manual we recommend
that, if your software allows, you
select a fixed rate at the serial port
interface (&B1) so that the serial port
rate can remain higher than the rate on
the phone link. You'll get greater
throughput regardless of the connection
rate. Just remember to also enable
Transmit Data flow control, preferably
hardware (&H1).
For the phone line interface we
recommend variable rates (&N0). This
enables the modem to switch its
connection rate to match the rate of a
remote modem, no matter what the remote
modem's rate is. If the connection
rate is fixed, for example at 9600 bps
(&N6), the modem only connects with
modems operating at that rate. Of
course, if your application requires
connections with modems at only one
rate, you may wish to set the modem to
a fixed connection rate.
Software Requirements
Both variable and fixed serial port
rates require communications software
support. Most communications programs
support variable rates. Not all
software supports a fixed serial port
rate independent of the connection
rate, although this support is in
creasing in program updates. Check the
quick configuration guide and/or your
software manual, especially if your
software is a later version than the
version in the quick configuration
guide.
Serial Port Rate Select (&Bn)
Use this command to select variable or
fixed rates at the serial port
interface. Initially, the modem's rate
depends on your software setting. The
modem detects this rate from the rate at
which it receives the AT command. After
that, the modem's serial port rate
depends on the modem's &B setting, as
follows.
&B0Variable serial port rates.
Default. The modem switches its
serial port rate to follow the
connection rate it establishes with
a remote modem. The software also
switches its serial port rate to
match the connection rate. If your
software has an Auto Baud Detect
feature, turn it on.
&B1Fixed serial port rate. The modem
detects its serial port rate from
your last AT command, and maintains
that rate regardless of the
connection rate. For the greatest
throughput, set your software to
57.6K or 38.4K bps first, and then
set the modem to &B1.
The serial port rate must be equal
to or higher than the connection
rate. In addition, this setting
requires Transmit Data flow con
trol, preferably hardware control
(&H1), so that the modem's buffer
doesn't overflow.
Some programs require that you turn
off Auto Baud Detect to fix the
serial port rate. Others use the
term Serial Port Lock (yes or no).
&B2Fixed for ARQ calls/Variable for non-
ARQ calls, Answer mode only. When
the modem goes off hook and answers
in ARQ mode, it shifts its serial
port rate up to the rate you've
written to NVRAM, for example,
38.4K bps. If the connection is
not under error control, the modem
behaves as if it were set to &B0
when it answers, and switches its
serial port rate to match the
connection rate of each call.
This option is designed for
installations such as Bulletin
Boards that receive calls from a
wide variety of modems, ranging
from the very slow to those with
the Sportster 14,400's advanced
design.
Connection Rate Select (&Nn)
Use this command to select variable or
fixed rates at the phone line interface.
Variable rates allow the modem to con
nect with a variety of remote modems,
while fixed rates limit calls to a
single connection rate.
&N0Variable rates. Default. The
Sportster 14,400 negotiates with
the remote modem for the highest
possible connection rate, depending
on the capabilities of the remote
modem. This is the recommended
setting.
&N Fixed rate. The Sportster 14,400
only connects if the
1O6remote modem is operating at the
same rate. If not, the Sportster
14,400 hangs up.
The fixed rate options are as
follows.
&N1 300 bps &N2
1200 bps
&N3 2400 bps &N4
4800 bps
&N5 7200 bps &N6 9600 bps
&N7 12K bps &N8 14.4K bps
By fixing the connection rate, you
can filter out calls at other than
a specific rate, for security or
other reasons.
DATA SET READY (&SN)
The modem sends your terminal a Data Set
Ready (DSR) signal. (Data Set is
industry jargon for modem.) Few, if
any, commercial communications programs
require the modem to control DSR, &S1.
Leave the modem set for DSR overridden,
&S0, unless you know that your
installation requires a different
setting.
&S0DSR is always ON (override).
Default.
&S1In Originate mode, the modem sends
DSR after dialing, when it detects
the remote modem's answer tone. In
Answer mode, the modem sends DSR
after it sends an answer tone.
BREAK HANDLING (&YN)
This command allows you to send a break
to abort data transfer without
disconnecting from the phone line
interface. The following options are
available.
&Y0Destructive, no Break transmitted:
the modem clears the data from its
transmit buffer (all data is lost)
but does not transmit the Break to
the remote modem.
&Y1Destructive, expedited: the modem
clears the buffer and immediately
sends a Break to the remote modem.
Default.
&Y2Nondestructive, expedited: the
modem retains data in the transmit
buffer, but immediately sends a
Break to the remote modem.
&Y3Nondestructive, unexpedited (send
Break in sequence): the modem
transmits any buffer data received
before the Break, sends the Break,
and then sends any subsequent input
from the terminal.
NOTE: If the call is under MNP5 data
compression, destructive Breaks cause
both modems to reset their data compres
sion tables. When transmission resumes,
the modems build new tables, and the
result is lower than normal throughput.