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1989-04-17
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------------------------------------------------------------------------
Searchlight BBS
Guide To High Speed Operations
Using the US Robotics Courier HST
------------------------------------------------------------------------
The following document discusses in detail the setup and use of the
US Robotics Courier HST modem with Searchlight BBS, including the use of
"high speed locked DTE" mode as implemented in Searchlight versions 1.73
and later. While the specific command strings and switch settings given
here are for the US Robotics modem, the concepts discussed should apply
equally to other high speed modems, such as the Hayes 9600.
There are two distinct modes of operation supported by the US
Robotics high speed modems, called "variable DTE" mode and "locked DTE"
mode. In variable DTE mode (DTE refers to the baud rate used for
communication between your computer and the modem), the USR operates
exactly like most standard 1200/2400 baud modems, except of course with
the addition of a 9600 baud speed. Most software, including all versions
of Searchlight, support the USR in this mode. Locked DTE mode is a
special mode which can allow the HST, under good line conditions, to
provide data speeds of up to 14.4k baud (when connected to another HST)
and to provide somewhat better throughput at all speeds, due to data
buffering and compression. However, the locked DTE mode is more
difficult to set up and has some drawbacks. We strongly suggest that you
read your US Robotics manual and this document carefully, and fully
understand how locked DTE mode works, before you attempt to use it on
your BBS.
-- (I) Dip Switch Settings ---------------------------------------------
The following dip switch settings are recommended for all modes of
operation:
Switch Setting Function
------ ------- --------
1 Up Enables normal DTR operations.
2 Up Enables verbal result codes.
3 Down Displays result codes.
4 Up Modem echos commands.
5 Up Enables auto-answering.
6 Up Enables true normal Carrier-Detect function.
7 Up Enables result codes in all modes.
8 Down Normal AT command set recognition.
9 Up Escape code recognition.
10 Up Loads configuration from NRAM at power-up.
-- (II) Variable DTE Operations ----------------------------------------
The following communications parameters (as set via Menu #3 in the
Searchlight CONFIG program) are recommended for normal, variable DTE
rate operations:
2. Support 300 Baud .............. Yes
3. Support 1200 Baud ............. Yes
4. Support 2400 Baud ............. Yes
5. Support 4800 Baud ............. Yes
6. Support 9600 Baud ............. Yes
7. Support 19200 Baud ............ No
8. Remote Init String ............ ATM0H0X1S0=1&B0&H0!
9. Local Init String ............. ATH1!
10. Baud Detect Method ............ Modem Msg
11. Locked Baud Rate .............. None
12. Hardware Flow Control ......... Off
13. Hardware Modem Breaks ......... Off
14. Output Buffering Factor ....... (any value)
Note the following:
Support for 300 baud may be disabled if you wish.
The init strings given assume that the factory defaults are in place
for the remaining parameters. If you are unsure, you may want to reset
your modem to its factory defaults (using the AT&F command) before you
begin. If your modem will be used primarily with Searchlight BBS, you
may want to write the above initialization parameters to the modem's
nonvolatile memory (use the AT&W command).
For additional parameters, refer to your HST manual.
The "Modem Msg" baud detect method will work correctly as long as any
"X" modem command, except X0, is in force. You can use the "Key Hit"
method if you wish. Please notice that because the HST modem tries to
establish a number of things each time it connects to another modem,
such as the MNP level, it takes longer for the HST to connect after
answering the telephone than a standard 1200/2400 baud modem. This is
normal.
The output buffering factor may be set to any appropriate value for
your system, as described in the Searchlight documentation. As always,
we recommend setting the buffering factor to zero if you experience any
unusual system lockups or modem problems when running Searchlight.
In this mode, the HST modem will behave much like a standard Hayes-
compatible 300/1200/2400 baud modem. When you start Searchlight BBS, it
opens the communications port at 9600 baud, issues the remote init
string to the modem, and waits for a connection. When the modem receives
a call and establishes a connection, it will issue a "CONNECT xxxx"
message, indicating the speed of the connection, and immediately change
to that speed. Searchlight, after reading the CONNECT message, will also
switch speeds, and the call will proceed at the specified baud rate of
300, 1200, 2400 or 9600 baud. After the call terminates, the modem is
reset to 9600 baud for the next call.
The advantages of using variable DTE mode are simplicity of setup and
compatibility with other software, such as external transfer protocols
or DOOR programs, which may not support locked DTE operations. Since the
HST does not use extensive modem buffering and flow control in this
mode, it will not interfere with Searchlight's flow control operations,
as it can in locked DTE mode.
-- (III) Locked DTE Operations -----------------------------------------
Locked DTE means that the baud rate at which Searchlight BBS opens
and sends data to the communications port is "locked" at a fixed, high
rate of speed, such as 19,200 baud, rather than being variable depending
on the speed of the connection. During a locked DTE call, Searchlight
will ALWAYS communicate with the modem at the "locked" baud rate, even
though the HST may be communicating with the remote modem at any baud
rate from 300 to 14.4k baud. In order for Searchlight to be able to send
data to the HST modem at speeds faster than the modem is transmitting,
the HST provides a method of "flow control". Flow control means that
when the HST has all the data that it can handle at one time, it can ask
Searchlight to stop transmitting data for a short period of time until
it is ready to begin accepting data again.
To use locked DTE, set your CONFIG parameters as follows:
2. Support 300 Baud .............. Yes
3. Support 1200 Baud ............. Yes
4. Support 2400 Baud ............. Yes
5. Support 4800 Baud ............. Yes
6. Support 9600 Baud ............. Yes
7. Support 19200 Baud ............ No
8. Remote Init String ............ ATM0H0X1S0=1&B1&H1&K0&Y0S15=8S38=2!
9. Local Init String ............. ATH1!
10. Baud Detect Method ............ Modem Msg
11. Locked Baud Rate .............. 19200 or 38400
12. Hardware Flow Control ......... On
13. Hardware Modem Breaks ......... Off [SEE NOTE BELOW]
14. Output Buffering Factor ....... 0
Please note the following:
In NO CASE should field 7 (Support 19200 Baud) be set to "Yes", since
this rate of speed is NOT supported by the modem. All calls which are
completed by the HST at high speed (9600 baud or 14.4k baud) will be
reported by the HST, and recorded by Searchlight, as 9600 baud calls.
The &B1 parameter is the switch that sets the HST for locked baud
rate operations.
Field 11 should be set to 19,200 or 38,400 for best performance, but
only if your communications ports can handle those speeds. See note
below.
Hardware (CTS) flow control MUST be enabled for reliable high speed
operations. WARNING: Your serial port and modem cable MUST support this
feature. If they do not, your entire system can EASILY hang while
Searchlight sits waiting for a clear-to-send from the modem! The only
way out if this happens is to reboot and try it again.
The &K0 modem command DISABLES modem data compression. I recommend
this setting because data compression works only on textual data, such
as when reading messages interactively, and tends to degrade the
transmission of archived or binary files. Usually, file transfer speed
is considered most important. If you don't do many file transfers (or if
most of your files are text files), use &K1 to enable compression.
Field 13, Hardware Modem Breaks, enables Searchlight to use "hard"
RS232 breaks to control the modem's output buffers. I suggest leaving
this Off for now, because it appears to lock-up some systems during non-
MNP calls.
Output buffering (field 14) should be set to zero when using locked
DTE, since the modem provides its own buffering in locked modes. See the
discussion below for more information.
Modem register S15 is used to configure the modem's buffer size to
128 bytes during a non-MNP 1200 or 2400 baud call. This is the
recommended setting as it will reduce hot key delays. Register S38 sets
a hangup time delay for the modem after DTR is dropped. If your remote
users report that your logoff screens or closing messages are not being
fully received before the system hangs up, set S38 to a larger value.
NOTE: The S15 and S38 registers are not available on some older model
HST's.
There are four main reasons why the HST modem can provide better
perfomance in Locked DTE mode:
(1) Buffering. The HST provides up to 3.5k of RAM output buffering
while in locked mode, enabling the BBS to perform other
operations while data is being transmitted. HOWEVER, Searchlight
BBS also performs output buffering (when a nonzero Output
Buffering Factor is specified). Therefore, this advantage is
not as important when using Searchlight BBS as it is for those
using other bulletin board software.
(2) High Speed Connections. The HST is capable of communicating at
baud rates of up to 14,400 baud with compatible HST modems.
Because the communications ports on the IBM-PC can only be
operated at fixed speeds of 9600 or 19,200 baud, using a locked
baud rate of 19,200 is the ONLY way in which to allow the HST
to make use of its special, high-speed connect rates.
(3) Online Fallback. If the HST is operating at high speed but
detects that the telephone connection is unusually "dirty" or
unreliable, it can automatically "step-down" the transmission
speed to the next lower baud rate (it will also "step-up" the
baud rate if the line becomes clean again). This feature is
only possible in locked DTE mode.
(4) Data Compression. The HST can perform MNP data compression on
most textual data transmitted through it. However, in order for
data compression to have any effect on the total speed of the
transmission, locked DTE rate must be used.
Before you decide to use a locked DTE baud rate on your system, however,
please note well the following comments and remarks:
(1) The single most important concept to realize when dealing with
the HST in "locked" mode is that the HST is a BUFFERED modem. Buffering
means that it can accept data from the computer, store it in a RAM
buffer, and transmit it to the remote user at a later time. While this
has the effect of increasing throughput, it has the side effect of
REDUCING, in some cases severely, the response time of Searchlight's own
flow control commands- such as ^C, ^S, spacebar, and "hot" keys. For
example, suppose a user requests to read a certain message. Searchlight
will bring up the message and, almost instantly, "dump" the message into
the modem's buffer at a high rate of speed. At this point, Searchlight,
assuming the message has been sent, draws its usual options menu at the
bottom of the screen, and waits for a response. Meanwhile, the modem is
still transmitting the text in its buffer to the remote user. If the
remote user decides to press a hot key, or pause or abort the message
during this time, his request will NOT be honored- because Searchlight
thinks the message has already been sent! Fortunately, the HST has
certain commands which can be used to reduce the likelyood of this
happening. However, you will always notice a delay in the response time
to hot keys when using locked baud rates; the delay is most noticeable
at lower baud rates, and when using MNP modems.
(2) Many computers DO NOT support high speed (19,200 or 38,400 baud)
operations! In particular, many older PC's and XT's cannot support these
speeds. If your computer cannot support high speed, it may lock up or
fail to operate correctly if high speed operations are attempted. You
may have to purchase a replacement RS232 card capable of supporting high
speed before you can use high-speed locked DTE on your BBS (try to
obtain a board which uses the 164500 UART chip).
(3) The data compression and error checking performed by the HST in
high speed modes can speed up normal, interactive communications a great
deal, but it can also slow down file transfers, particularly when you
are transferring compressed files, binary files, or when using a
protocol that employs small data packets (such as Xmodem or Kermit). You
must determine which aspect of your BBS system is most important to
yourself and your users. Use &K0 (disable data compression) in your
modem init string if you want high-speed binary file transfers; use &K1
if you want compression of textual data. To minimize file transfer
times, use a long-block protocol (such as Zmodem or Xmodem/1K) during an
error checked (/ARQ) connection, and disable error checking if you need
to use Xmodem or Kermit frequently.
(4) Any external protocols you use, as well as any DOOR programs you
use which perform their own modem i/o, MUST be able to recognise the
"locked" baud rate and to perform the proper flow control handshaking
with the modem. In particular, DOOR programs written for PCBoard
version 12.0 systems will probably NOT run if the baud rate is locked at
19,200 or above. DOOR programs which use the PCBOARD.SYS files or the %B
command-line parameter will be passed the correct (ie. locked) baud
rate, but MUST perform CTS handshaking with the modem. DOOR programs
which use Searchlight's modem support are not affected.
(5) VERY IMPORTANT: The speed at which your BBS system operates is
dependent on MANY factors besides the modem! The speed of your disk
drives, your CPU, modem ports, operating system, etc. can all affect the
total speed of your system. Even with high-speed modems attached, your
BBS system may not be able to achieve the highest data speeds possible,
due to these factors. In many cases, the benefits of running in locked
mode aren't significant compared to straight 9600bps communications.
Experiment to see what works best for you!
------------------------------------------------------------------------
Frank LaRosa 4/15/89
