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INTRODUCING LXM
BRIEF HISTORY OF XMODEM
During the glory days of CP/M, the computer industry was
plagued with a plethora of diskette formats. Since there was
no single standard around, particularly when the 5" format
was introduced, data interchange between computers was
difficult.
Several manufacturers attempted to provide a solution by
giving their users the ability to at least read multiple
diskette formats. Rather than promoting a standard format,
this approach encouraged manufacturers to adopt proprietary
formats as a marketing tool. Partially as a response to this
situation, and partially out of a need to provide data
transmission capabilities, Ward Christiansen developed the
original specification for the XModem file transfer
protocol.
The XModem protocol, elegant in its simplicity and
effectiveness, has transcended the decline of CP/M-based
systems, and the ascendency of MS-DOS to become a de facto
standard for data transfer between remote computers. This
fact is true, even in the light of the near-universal
acceptance of another de facto standard for diskette
formats, as popularized by the IBM-PC family. With the
advent of the new PS/2 with its incompatible micro-
diskettes, the XModem protocol's place, at least for the
foreseeable future, seems assured.
PROTOCOL FUNDAMENTALS
THE TRANSMISSION BLOCK
The fundamental building block of XModem is an 8-bit byte,
no parity please, arranged into transmission blocks of
exactly 132 characters, never more nor less. In one
extension to XModem, commonly called CRC-XModem, the
transmission blocks are exactly 133 characters. We will
discuss this extension , and a second known as YModem,
later.
LXM(tm) - LITECOMM (tm) XMODEM ENGINE for Microsoft
Datalight and Turbo C
As you might guess, not only is the length of a transmission
block fixed, but so too is the format. The XModem block
format looks like this:
<SOH> <REC> <~REC> <...128 Data Bytes...> <checksum>
where:
SOH - 0x01 Signals the start of a block
REC - Is the sequential block number reduced modulo 256
~REC - The ones-compliment of REC
checksum - an 8-bit sum of the preceding 128 data
bytes, formed by adding each byte to an
accumulator, then dropping all but the low order 8
bits of the result.
The intent of the additional characters is to facilitate
error checking and recovery, making the XModem protocol an
"error-free" means of data transfers under otherwise hostile
conditions.
The block number and its compliment are included to insure
that no blocks are accidentally lost during transmission,
and to prevent the accidental duplication of a data block
which might be caused by spurious line noise. The checksum
seeks to insure the validity of the actual data which might
become garbled by line noise. The fixed length block format
tends to simplify the logic required to implement the
protocol in the first place.
THE INTER-COMPUTER DIALOG
Xmodem is essentially uni-directional in nature. That is,
the actual flow of meaningful information occurs in one
direction at a time, from the sending computer to the
receiving computer. This is not meant to imply, however,
that only one computer sends and the computer receives. To
the contrary, there is a constant "dialog" going on between
the two computers during the course of the transmission. It
is this dialog that permits XModem to be true, "error-free"
transfer method.
CopyRight (c) 1987, 1988 Information Technology, Ltd.
LXM(tm) - LITECOMM (tm) XMODEM ENGINE for Microsoft
Datalight and Turbo C
Xmodem is a receiver-driven protocol. The receiving computer
initiates and almost exclusively controls the transmissions,
through a series of pre-planned responses. A typical, though
brief dialog might look like this:
RECEIVER SENDER MEANING
<NAK> --------------> READY TO RECEIVE
<------------- <BLOCK1> FIRST BLOCK SENT
<ACK> --------------> BLOCK RECEIVED OK
<------------- <BLOCK2> SECOND BLOCK
<NAK> --------------> ERROR, RESEND
<------------- <BLOCK2> SECOND BLOCK
<ACK> --------------> BLOCK RECEIVED OK
<------------- <EOT> END OF FILE
<ACK> --------------> E-O-F UNDERSTOOD
As you can see, it is through the responses to the sending
computer that the receiver controls the link.
Xmodem also makes provision for unusual circumstances by
providing both time-out and cancellation mechanisms. The
rules for time-outs, time periods which may elapse before a
disruption in transmission occurs are as follows:
1. Waiting for SOH - 10 seconds, Resend last
acknowledgement on time-out.
2. Waiting for other block characters - 1 second,
replace expected character with some pre-defined
value upon time-out.
3. Waiting for a reply to a block - 10 seconds,
Resend last block upon time-out.
CopyRight (c) 1987, 1988 Information Technology, Ltd.
LXM(tm) - LITECOMM (tm) XMODEM ENGINE for Microsoft
Datalight and Turbo C
The original protocol description also makes it possible for
either the sender or receiver to cancel the transmission.
This is of particular value when too many transmission
errors occur, or, in rare cases, when the physical
connection is broken. The threshold beyond which either side
may request cancellation, although specified as 10 attempts
in the original description of the protocol, has become
somewhat arbitrary over time. The cancellation code, <CAN>
or 0x10, may be sent by either the sender or receiver in
place of the <SOH> or normal acknowledgment character
respectively.
ENHANCEMENTS TO XMODEM
Over the years, several significant enhancements have been
made to the protocol as originally defined by Ward
Christiansen. The intent of these enhancements have
generally been to improve upon the error-handling capability
of the protocol, and to make the protocol more amenable to
some less time-efficient communications environments,
particularly those presented by for-pay services such as
CompuServe.
To make the protocol more error-free, the checksum employed
in the original design has been replaced by a 16 bit
calculation, called a Cyclical Redundancy Check or CRC.
Based upon a polynomial equation, the CRC method can be
mathematically demonstrated to be sensitive to all but about
.03 per cent of errors. This error detection rate, about
99.97 percent accurate, is far superior to the approximately
90 to 92 percent accuracy of the checksum approach. The
overhead of an additional 8 bits per message seems to make
this enhancement well worth while.
The second enhancement seeks to improve communication over
packet-switched networks, like those employed by CompuServe
and other national services. Basically this enhancement
introduces a relaxed time-out between characters in a block,
allowing additional time before the protocol recognizes a
time-out condition.
CopyRight (c) 1987, 1988 Information Technology, Ltd.
LXM(tm) - LITECOMM (tm) XMODEM ENGINE for Microsoft
Datalight and Turbo C
A third enhancement, variant of the original XModem protocol
seeks to improve overall efficiency by expanding the block
size from 128 characters to 1024 characters. This variant,
commonly call ymodem, reduces the number of times that an
acknowledgment sequence must be sent, thereby increasing the
efficiency of actual data transfer.
The LXM XModem engine supports all of the enhancements
mentioned above.
THE LXM ENGINE
OVERVIEW
One of the reasons for the popularity of the XModem protocol
over time is the simplicity of the implementation. Given a
reasonable communications package upon which to build,
developing a basic implementation of XModem is relatively
simple, although under certain operating systems, such as
the Unix family, the attempt of the OS to be all things to
all people may actually become an impediment.
The intent of the LXM engine is to 1) provide XModem
capability, 2) in a flexible, easy to understand package, 3)
built upon a sound foundation, the LiteComm communications
ToolBox. Because of the manner in which the LXM engine was
designed virtually any application can have access to
XModem's error-free protocol, without being relegated to
simply transferring files.
MAJOR ENGINE COMPONENTS
The LXM engine consists, primarily of two functions, lcxrrec
and lcxtrec, receive a record and transmit a record
respectively. Because these two functions 'understand' the
XModem protocol internally, they assume full responsibility
for all of the necessary housekeeping, relying upon the host
program for only the most basic of functions.
CopyRight (c) 1987, 1988 Information Technology, Ltd.
LXM(tm) - LITECOMM (tm) XMODEM ENGINE for Microsoft
Datalight and Turbo C
The lcxrrec function receives one or more records from a
companion system. The host program controls the checking
method, CRC or checksum, the time-out method, normal or
relaxed, handles, in what ever way appropriate, blocks of
data that have been received, and can optionally monitor the
flow of data from the companion system. Lcxrrec assumes the
responsibility for synchronizing with the companion system
and for correctly maintaining the flow of information.
Lcxtrec performs in a like fashion when the host program
wants to send one or more records. The host program has only
to present the record to be transmitted, and lcxtrec does
the rest. The lcxtrec module assumes responsibility for
establishing synchronization with the receiving computer,
and for recognizing the checking method, CRC or checksum,
that the receiver wants to use. Lcxtrec also terminates the
transmission, when told to do so by the host program, and
permits the host to optionally monitor the data flow.
CopyRight (c) 1987, 1988 Information Technology, Ltd.
LXM(tm) - LITECOMM (tm) XMODEM ENGINE for Microsoft
Datalight and Turbo C
NOTES AND WARNINGS
MODIFICATIONS
The LXM engine is closely integrated with the LiteComm
ToolBox. While there is every likelihood that the engine can
be modified to function with other similar packages,
Information Technology, Ltd., can only support the LXM
engine when used with either the LiteComm ToolBox.
PARITY AND DATA BITS
The XModem protocol is an 8-bit protocol. That is to say it
neither recognizes nor tolerates parity checking, i.e. 7
data bits plus a specified parity. Since the LXM engine
cannot determine the current settings for parity and number
of data bits, the responsibility for controlling these
settings rests with the host program. Before using either of
the key functions lcxrrec or lcxtrec, the host program must
insure that the settings are no parity, 8 data bits, using
the comm_setup function. Upon final termination of the
function, the host program must reset these values to their
original settings, if necessary.
INTERNAL TIMER FUNCTION
Both lcxrrec and lcxtrec employ a hardware-based timing
function that connects directly to the normal real-time
clock vector 0x1C; The Microsoft and Turbo C version of this
function also establishes an special routine, using the
atexit() function, to insure that this vector is re-
established at is original setting when program termination
occurs. This is not true of the Datalight version however.
The Datalight library does not, at present, have an like
atexit().
CopyRight (c) 1987, 1988 Information Technology, Ltd.
LXM(tm) - LITECOMM (tm) XMODEM ENGINE for Microsoft
Datalight and Turbo C
While every effort has been made, for Datalight users, to
insure that the vector is properly reset, this plan may be
thwarted by abnormal termination of the host program
resulting in a subsequent system crash. The safest method to
Datalight user's to employ would be to use the lc_clrclock()
function before host program termination. As an alternate
approach, Datalight users may want to investigate STEVE'S
LIBRARY designed for Datalight C users's. This excellent
library does have an equivalent function to the atexit().
BUFFER SIZE RESTRICTION
The LiteComm ToolBox permits you a great deal of freedom in
tailoring the communications handler to meet your specific
requirements. We must caution you, however, that when you
are using the LXM engine, the minimum buffer sizes required
by the comm_opn function are not adequate to support LXM,
particularly when transmitting records at either low baud
rates, or on the new generation of high-speed (above 6 MHz)
processors.
During the development of LXM, it was quickly discovered
that too small a transmit buffer quickly resulted in a
buffer overflow condition. We recommend that, when using the
LXM engine, the transmit buffer be set at a minimum 256
bytes to avoid the overflow condition. If you intended to
employ the YModem variant, then the minimum buffer sizes
should be set to at least 1030.
XON-XOFF CONTROL
If your host program employs the XON-XOFF functions in
LiteComm, you must be sure that the automatic XON-XOFF
handing is disabled before you attempt to transmit or
receive records using LXM. Failure to observe this caution
may result in a system hang, requiring that the system be
re-booted.
USER-ACCESSIBLE DATA
CopyRight (c) 1987, 1988 Information Technology, Ltd.
LXM(tm) - LITECOMM (tm) XMODEM ENGINE for Microsoft
Datalight and Turbo C
Within LXM, certain variables have been defined as being
globally available. The host program may examine the
contents of these variables at any time to determine the
current state of the LXM engine. The correct definitions of
these variable is contained in litexm.h. Except as noted
below, the host program must NOT alter the contents of these
variables.
abort_flag - This is the only variable of the group
that may be altered by the host program. The
flag is checked periodically by the engine
functions. If _abort_flag has a value of 1, the
function in progress will be cancelled
automatically by sending a CAN instruction to
the companion system. See the TTL.C sample
program for an example of how this flag may be
set.
crc - If this variable has a non-zero value, then
the LXM engine is currently using the CRC error-
checking method. A zero value indicates that the
original checksum method is being used.
checksum and crcaccum - These variables hold the
current/last calculated check value. In a
practical sense, they are of no value to the
host program.
rec - This variable contains the current record
(block) number being processed. In the event of
an uncorrectable error, rec would contain the
number of the expected block. In the case of a
successful transmission or reception, rec-1 is
the number of the block sent or received. The
value contained in this variable, reduced modulo
256, provides the block number required by the
XModem protocol and must NEVER be disturbed.
ymodem - This variable is set to TRUE if the
host application wants to transmit a block in
YModem mode (1024 characters). See the
discussion of lcxtrec, below.
PROGRAMMERS REFERENCE
CopyRight (c) 1987, 1988 Information Technology, Ltd.
LXM(tm) - LITECOMM (tm) XMODEM ENGINE for Microsoft
Datalight and Turbo C
ENGINE-RELATED FUNCTIONS
The following pages document the LXM engine functions as
currently implemented. They follow, in style and content,
the documentation for the LiteComm ToolBox itself.
Use the following pages as an addendum to your LiteComm
documentation.
CopyRight (c) 1987, 1988 Information Technology, Ltd.
LXM(tm) - LITECOMM (tm) XMODEM ENGINE for Microsoft
Datalight and Turbo C
lcxrrec
_______________________________________________________________
SUMMARY
#include <litexm.h>
#include <litecomm.h>
int lcxrrec(port,buff,rbsize,hmode,hdshk)
unsigned port;
unsigned char *buff;
int *rbsize;
int hmode;
unsigned char *hdshk;
DESCRIPTION
Receive a block of either 128 or 1024 bytes from the
companion system, using the checking method specified in
hdshk, and the time-out value specified in hmode. If
necessary, establish synchronization with the companion
system.
The port parameter is the same as used throughout the
LiteComm ToolBox.
Buff should be a pointer to an array of 128 bytes in when
using XModem mode, 1024 bytes for YModem mode. All record
characters are received into this area, and, if the area is
too small, LXM will overwrite adjacent data without warning.
Rbsize is a POINTER to an integer variable. Lcxrrec returns
the actual length of the received block to this variable.
This approach is needed since, using YModem, blocks may be
either 1024 bytes or 128 bytes at the discretion of the
SENDER. Lcxrrec automatically detects the block length and
returns it to the caller so that the caller can process the
block correctly.
The value of hmode determines whether normal or relaxed
time-out values are used. Please use the constants provided
in the litexm header file to insure a proper setting.
The value contained initially in hdshk must be either CRC or
NAK, defined in litexm.h. The former specifies the use of
CopyRight (c) 1987, 1988 Information Technology, Ltd.
LXM(tm) - LITECOMM (tm) XMODEM ENGINE for Microsoft
Datalight and Turbo C
the 16 bit CRC checking mode, while the latter specifies the
use of the original checksum method. Note - If you intend
to operate in YModem mode, hdshk must specify CRC.
Note that hdshk is a pointer to a character. Lcxrrec uses
this area to store its reply to the last received block.
Once you have set this value, and have started to receive,
DO NOT alter its value under any circumstances. Doing so may
cause unpredictable results.
EXAMPLE
See the accompanying program TTL for an example of lcxrrec
usage.
RETURN VALUES
Lcxrrec may return several values, as defined in the
litexm.h file. Each return value should cause the host
program to respond in specific ways.
SUCCESS - A record has been successfully received into the
buff area. Host program process the record and calls lcxrrec
again.
RETRIES - The maximum number of attempts to receive a single
record has been exceeded. Lcxrrec automatically cancels the
session. Host program should handle in an appropriate
manner, e.g. issue error message.
ERR (-1) - Lcxrrec has detected that the host program has
requested cancellation of transmissions through the
_abort_flag setting (see below) or a fatal record sequence
has occurred, e.g. a block number has been skipped. Lcxrrec
automatically cancels the session.
CAN - The sending program has requested cancellation. Host
program should handle in a fashion similar to RETRIES.
EOT - The sending program has no more data to transmit.
Lcxrrec acknowledges the EOT message automatically. Host
program handles like an end-of-file condition.
CopyRight (c) 1987, 1988 Information Technology, Ltd.
LXM(tm) - LITECOMM (tm) XMODEM ENGINE for Microsoft
Datalight and Turbo C
TOUT - Lcxrrec failed to establish synchronization with the
sending program while waiting to receive the SOH (or STX for
YModem) character at the start of the block. The session is
automatically cancelled.
DUPSEQ - The block just received is a duplicate of the
preceding block. The host program should ignore the data
contained within the block, the call lcxrrec to proceed with
data transfer.
CopyRight (c) 1987, 1988 Information Technology, Ltd.
LXM(tm) - LITECOMM (tm) XMODEM ENGINE for Microsoft
Datalight and Turbo C
lcxtrec
_______________________________________________________________
SUMMARY
#include <litexm.h>
#include <litecomm.h>
int lcxtrec(port, buff)
unsigned port;
unsigned char *buff;
DESCRIPTION
Transmit a block of 128 or 1024 bytes to the companion
system. The checking protocol, CRC or checksum, is detected
automatically when the receiving station issues its initial
handshaking sequence. If necessary, establish
synchronization with the companion system before
transmitting.
The port parameter is the same as used throughout the
LiteComm ToolBox.
Buff should be a pointer to a 128 or 1024 byte record to be
sent to the companion system. This is not a typical, null-
terminated string as usually found in C. All bytes, starting
at the pointer will be transmitted. It is the responsibility
of the host program to provide any padding that might be
required to satisfy the length requirement. Generally,
short records are padded with NULLS or ^Z (0x1A).
Lcxtrec determines the block size to be transmitted based
upon the current value of the global variable ymodem (see
the introduction). If ymodem has a value of TRUE, then buff
is assumed to point to a 1024 byte record. If ymodem is
FALSE, then a 128 byte record is assumed. This feature has
particular value when operating in YModem mode.
CopyRight (c) 1987, 1988 Information Technology, Ltd.
LXM(tm) - LITECOMM (tm) XMODEM ENGINE for Microsoft
Datalight and Turbo C
When using YModem protocol, block sizes may be either 128 or
1024 bytes, at the senders discretion. This serves two
purposes. In the case of a large number of errors, many
YModem implementations will automatically switch to the
smaller block size if too many errors occur using the larger
block size, in an effort to get the data through, then
revert back to the normal block of 1024 characters. Lcxtrec
implements this feature automatically.
Generally, at the end of a file, the final block to be sent
will not exactly match the transmission block size. Under
XModem, this means that as many as 127 wasted characters
might be transmitted. Under YModem, as many as 1023 wasted
characters can be transmitted in the final block. However,
the host program that calls lcxtrec can determine if the
block to be sent is less than 1024 bytes, set the ymodem
variable to FALSE, and send multiple 128 byte blocks. Using
this approach, the amount of wasted characters will be no
worse than the original XModem. The TTL sample program,
included in the package, shows one way of approaching this
problem.
EXAMPLE
See the accompanying program TTL for an example of lcxtrec
usage.
RETURN VALUES
Lcxtrec may return several values, as defined in the
litexm.h file. Each return value should cause the host
program to respond in specific ways.
SUCCESS - The record has been successfully sent from the
buff area. Host program either calls lcxtrec with the next
record to transmit, or lcxteot to indicate End of
Transmission to the companion system.
RETRIES - The maximum number of attempts to send a single
record has been exceeded. Lcxtrec automatically cancels the
session. Host program should handle in an appropriate
manner, e.g. issue an error message.
CopyRight (c) 1987, 1988 Information Technology, Ltd.
LXM(tm) - LITECOMM (tm) XMODEM ENGINE for Microsoft
Datalight and Turbo C
ERR (-1) - Lcxtrec has detected that the host program has
requested cancellation of transmissions through the
_abort_flag setting (see below).
CAN - The receiving program has requested cancellation.
Host program should handle in a fashion similar to RETRIES.
CopyRight (c) 1987, 1988 Information Technology, Ltd.
LXM(tm) - LITECOMM (tm) XMODEM ENGINE for Microsoft
Datalight and Turbo C
lcxteot
_______________________________________________________________
SUMMARY
#include <litexm.h>
#include <litecomm.h>
int lcxtrec(port)
unsigned port;
DESCRIPTION
Send and End of Transmission (End of File) to the receiving
system. This function must be called to successfully close
out the transmission session.
EXAMPLE
See the accompanying program TTL for an example of lcxtrec
usage.
RETURN VALUES
Lcxteot returns one of two values, as defined in the
litexm.h file.
SUCCESS - The receiving station has correctly acknowledged
the EOT. The host program terminates transmission mode
normally.
CAN - Either the receiving system has responded to the EOT
message with a CAN code, or has failed to respond correctly
and the lcxteot function has sent the CAN code to the
receiving system. Host program should handle in an
appropriate manner, e.g. issue an appropriate error message.
Regardless of the value returned, invoking lcxteot always
terminates the current transmission session.
CopyRight (c) 1987, 1988 Information Technology, Ltd.