home
***
CD-ROM
|
disk
|
FTP
|
other
***
search
/
Collection of Tools & Utilities
/
Collection of Tools and Utilities.iso
/
comm
/
hst_set5.zip
/
HST_SET.TXT
< prev
Wrap
Text File
|
1991-03-16
|
25KB
|
605 lines
HST (DS) Setup Notes
----------------------
John Souvestre & Jim King
March 15, 1991
These notes describe how to set up the U. S. Robotics Courier
HST (14.4K bps), or HST Dual Standard, modem and typical,
IBM-type, PC communications software, such as:
FOSSIL BBS Driver Terminal
-------- ----------- ------ ------------
BNU BinkleyTerm DSZ ProComm Plus
OpusComm D'Bridge QModem
X00 FrontDoor Telix
Maximus
Opus
RBBS
The first section has some miscellaneous information about
running a high speed modem. The next section covers the setup
for the HST itself. The remaining four sections cover typical
FOSSIL, BBS, Driver, and Terminal program setups.
This note is not a tutorial on modems in general, or on the HST.
It is a "setup philosophy" for the HST. We highly recommend
that you read the HST manual, even if you don't understand all
of it.
If you have any questions, suggestions, or setup information for
other programs, please contact us via The Southern Star BBS,
FidoNet 1:396/1, 504-885-5928. On this BBS you will also find
a current version of this file under the name HST_SET.ZIP.
================================================================
General Notes on High Speed Modems
------------------------------------
The RS-232C cable connecting the modem to the computer's serial
port should have at least the following pins connected: 1, 2,
3, 4, 5, 6, 7, 8, and 20. Get an Ohm Meter, Multimeter, or
other form of continuity tester, and check your cable. Some
connectors have pins which are looped back, and which are not
connected to the connector at the other end of the cable.
To get the "most" out of your HST you will have to set the
computer-to-modem speed higher than the modem-to-modem speed.
This is called "locking" the serial port, since it won't be
operating at the same speed as the modem itself is. Why is this
necessary?
For a few reasons: Most computer serial ports don't operate at
14400 bps, the basic speed of the HST. Thus the computer has to
operate at a higher speed. Because of this, however, the
computer and the modem have to be able to "handshake" to prevent
overrun, and loss of data. This is called "flow control".
Hardware handshaking, using the RTS and CTS lines (in the cable
mentioned above) does the job. Avoid software handshaking, as
it is protocol sensitive.
Another reason is that the HST does not need to send the start
and stops bit which make up part of the the asynchronous data
from/to the computer. This means that for every 10 bits of
computer-to-modem data, there is only 8 bits of modem-to-modem
data.
Given that each of these represents one character, the cps
(characters per second) rating of both links is the same. When
talking about bps (bits per second) however, this means that the
computer-to-modem link needs to be 25% faster to keep up with
the modem-to-modem link, even though both are at the same cps
rate! Thus it is possible to get more than 100% throughput with
an HST.
Now let's talk about data compression. MNP compression adds
about 5% overhead to the data you are sending. So, if the data
is already compressed, with PKZip for example, then you will
probably just end up losing the 5%. If you are sending ASCII
text however, the HST's compression will buy you about 50-100%
improvement.
The HST allows you to set up compression in one of 3 ways:
Always on, always off, and do what the other end wants. In
general, the calling modem should be set to optimize the type of
transfers it expects to be doing. The answering modem should
allow the caller to choose whether he wants compression or not.
If you have a V.42 model, then you have an additional mode. It
allows you to use V.42 compression (which will not add any
overhead for an already compressed file), but not MNP
compression.
Transfers using compression, at 14400 bps, should generally lock
the serial port at 38400 bps. 19200 bps probably won't be fast
enough to allow the modem to run at full speed, depending on the
type of file being compressed.
In general, unless you expect to be doing lots of transfers
using compression, it is our opinion that locking the serial
port at 19200 bps is the better way to go. It is less demanding
of the computer, thus reducing the chances of losing data in
marginal cases.
When transferring previously compressed data, where the modem is
not doing the compression, the maximum rate you will be able to
achieve is approximately 1710 cps (178% of 9600 bps) on the
computer-to-modem link. [Note: 1710 cps is 14400 bps, plus 25%
due to start/stop bit stripping, minus 5% for modem-to-modem
overhead.] If you are running an older 9600 bps HST, the
maximum rate is about 1140 cps (119% of 9600 bps).
When testing to determine your transfer rate, test with a known
good system! If the other system isn't working properly, then
you are just wasting your time. Be sure to use a previously
compressed file which takes at least a few minutes to transfer.
Test both transmitting and receiving. Don't trust your
communications program's timer - some have been know to be off
by as much as 20%. An efficient protocol will have an overhead
of 2 - 3%. Some programs count this overhead in the total
(total characters transferred), while others don't ("useful"
characters transferred).
For the rest of this note we will assume that you want to lock
the serial port at 19200 bps. If you want to lock at 38400 bps
instead, just change the "19200"s to "38400"s.
Some communications programs and most FOSSILs allow you to
specify the size of the receive and/or transmit buffers. The
best size will depend on many factors, and may require a bit of
testing on your part. For starters however, we recommend that
you make the both the transmit buffer and the receive buffer
1K. If you are running under a multi-tasking operating system
then you might want to start with both, or just the receive
buffer, set to 2K, instead.
One thing that you will notice when running a high speed modem
is that all file protocols are not created equal! Protocols
which require an "Ack" for every block sent do not perform well.
Thus you will want to avoid XModem, for example. YModem does
better because it uses a larger block size.
Streaming protocols, such as ZModem, work fine. However, avoid
protocols like YModem-G, which use error detection, but no error
recovery, simply aborting the transfer upon an error. Although
the modem-to-modem transfer is guaranteed by a 16 bit CRC used
by the modems themselves, errors can still occur between the
computer and the modem at either end. In some situations it is
not uncommon to lose an occasional character (see below). It is
not worth this chance for a speed advantage of less than 1%.
We highly recommend replacing the serial interface chip (UART),
a 8250 or 16450, with a 16550A. This chip contains a 16
character FIFO, as opposed to a double buffer. If your
communications program makes use of the FIFO, it will allow you
longer interrupt latency and/or less interrupt overhead.
Various manufacturers make versions of the 16550A. National
Semiconductor was the first. Their full part number for the
40-pin DIP is NS16550AN or NS16550AFN. Make sure that the part
you get is a 16550A, and not the older 16550. Avoid the Western
Digital version (WD16550...).
In all the cases that follow we recommend installing a 16550A.
In some of the cases alternatives are also presented.
Some computers (particularly 4.77 MHz, 8088 machines) are not
fast enough to support 19200 or 38400 bps. If you have this
type of computer, you can use 9600 bps instead, but this will
not give you the HST's full speed.
Certain disk controllers, with