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From: Chuck Forsberg WA7KGX <caf%omen.uucp@BRL.ARPA>
Newsgroups: net.micro,net.micro.pc
Subject: YMODEM (=XMODEM+1) Doc
Message-ID: <173@omen.UUCP>
Date: 1 Jun 85 19:30:58 GMT
Xref: seismo net.micro:11239 net.micro.pc:4440
To:       info-micro@brl-vgr.ARPA

By popular request, I am posting this document describing the XMODEM
protocol, plus the XMODEM/CRC and YMODEM (Batch transmission, etc.)
protocol extensions.

Ward Christensen's article on this subject will appear late this year
in Byte Magazine.

				  - 1 -

		 A compendium of documents describing the
			    XMODEM and YMODEM
			 File Transfer Protocols

			 Edited	by Chuck Forsberg

		 Please	distribute as widely as	possible.

		       Questions to Chuck Forsberg

			   Omen	Technology Inc
			17505-V	Sauvie Island Road
			  Portland Oregon 97231
			   Voice: 503-621-3406
	    Modem (Telegodzilla): 503-621-3746 Speed 1200,300
			  Compuserve: 70715,131
		    UUCP: ...!tektronix!reed!omen!caf


1.  ROSETTA STONE.....................................................	 2

2.  TOWER OF BABEL ??.................................................	 3
    2.1	 Some Messages from the	Pioneer...............................	 3

3.  XMODEM PROTOCOL ENHANCEMENTS......................................	 6
    3.1	 CAN-CAN Abort................................................	 6
    3.2	 CRC-16	Option................................................	 6
    3.3	 1024 Byte Packet Option......................................	 7

4.  YMODEM Batch File Transmission....................................	 8

6.  XMODEM PROTOCOL OVERVIEW..........................................	13
    6.1	 Definitions..................................................	13
    6.2	 Transmission Medium Level Protocol...........................	13
    6.3	 File Level Protocol..........................................	14
    6.4	 Programming Tips.............................................	15

7.  XMODEM/CRC Overview...............................................	17
    7.1	 CRC Calculation..............................................	17
    7.2	 CRC File Level	Protocol Changes..............................	18
    7.3	 Data Flow Examples with CRC Option...........................	19

8.  MORE INFORMATION..................................................	21

9.  YMODEM Programs...................................................	22

				  - i -


 Figure	1.  1024 byte Packets.........................................	 7

 Figure	2.  Mixed 1024 and 128 byte Packets...........................	 7

 Figure	3.  Batch Transmission Session................................	11

 Figure	4.  Filename packet transmitted	by sb.........................	11

 Figure	5.  Header Information used by YMODEM Implementations.........	11

 Figure	6.  XMODEM Message Block Level Protocol.......................	14

 Figure	7.  Data flow including	Error Recovery........................	15

 Figure	8.  Message Block Level	Protocol, CRC mode....................	17

 Figure	9.  Example of CRC Calculation written in C...................	18

Figure 10.  Data Flow: Receiver	has CRC	Option,	Sender Doesn't........	19

Figure 11.  Receiver and Sender	Both have CRC Option..................	20

				  - ii -

				  - 2 -
X/YMODEM Protocol Reference	 5-30-85				 2


Here are some definitions which	reflect	the current vernacular in the
computer media.	 The attempt here is identify the file transfer	protocol
rather than specific programs.

XMODEM	refers to the original 1979 file transfer etiquette introduced by
	Ward Christensen's 1979	MODEM2 program.	 It's also called the
	MODEM or MODEM2	protocol.  Some	who are	unaware	of MODEM7's batch
	file mode call it MODEM7.  Other aliases include "CP/M Users's
	Group" and "TERM II FTP	3".  This protocol is supported	by every
	serious	communications program because of its universality,
	simplicity, and	reasonable performance.

XMODEM/CRC replaces XMODEM's 1 byte checksum with CRC-16, giving modern
	error detection	protection.

YMODEM	refers to the XMODEM/CRC protocol with the throughput and batch
	transmission enhancements described below.

								 Chapter 1

				  - 3 -
X/YMODEM Protocol Reference	 5-30-85				 3


In the interest	of fostering compatibility among communications	programs,
part of	of the Professional-YAM	manual is reproduced here to minimize the
Electronic Tower of Babel.

The YMODEM Protocol is supported by the	public domain programs YAM (CP/M),
YAM(CP/M-86), YAM(CCPM-86),  MODEM76.ASM (CP/M), rb/sb (Unix, VMS,
Berkeley Unix, Venix, Xenix, Coherent, IDRIS, Regulus) as well as
Professional-YAM.  These programs have been in use since 1981.

The protocols described	below are enhancements to Ward Christensen's
XMODEM protocol, and are public	domain.

The 1k packet length capability	described below	may be used in conjunction
with the Batch Protocol, or with single	file transfers identical to the
XMODEM/CRC protocol except for the minimal changes to support 1k packets.

To complete this tome, Ward Christensen's original protocol document and
John Byrns's CRC-16 document are included for reference.

References to the MODEM	or MODEM7 protocol have	been changed to	XMODEM to
accommodate the	vernacular.

Watch for an article describing	the YMODEM protocol in a more coherent
fashion	later this year.  This article will include some interesting
history	on the development of microcomputer file transfers.

2.1  Some Messages from	the Pioneer

#: 130940 S0/Communications 25-Apr-85  18:38:47
Sb: my protocol
Fm: Ward Christensen 76703,302 (EDITED)
To: all

Be aware the article1 DID quote	me correctly in	terms of the phrases like
"not robust", etc.

It was a quick hack I threw together, very unplanned (like everything I
do), to	satisfy	a personal need	to communicate with "some other" people.

ONLY the fact that it was done in 8/77,	and that I put it in the public
domain immediately, made it become the standard	that it	is.


 1. Infoworld April 29 p. 16

Chapter	2

				  - 4 -
X/YMODEM Protocol Reference	 5-30-85				 4

I think	its time for me	to

(1) document it; (people call me and say "my product is	going to include
it - what can I	'reference'", or "I'm writing a	paper on it, what do I put
in the bibliography") and

(2) propose an "incremental extension" to it, which might take "exactly"
the form of Chuck Forsberg's YAM protocol.  He wrote YAM in C for CP/M and
put it in the public domain, and wrote a batch protocol	for Unix called	rb
and sb (receive	batch, send batch), which was basically	XMODEM with
   (a) a record	0 containing filename date time	and size
   (b) a 1K block size option
   (c) CRC-16.

He did some clever programming to detect false ACK or EOT, but basically
left them the same.

People who suggest I make SIGNIFICANT changes to the protocol, such as
"full duplex", "multiple outstanding blocks", "multiple	destinations", etc
etc don't understand that the incredible simplicity of the protocol is one
of the reasons it survived to this day in as many machines and programs	as
it may be found	in!

Consider the PC-NET group back in '77 or so - documenting to beat the band
- THEY had a protocol, but it was "extremely complex", because it tried	to
be "all	things to all people" -	i.e. send binary files on a 7-bit system,
etc.  I	was not	that "benevolent". I (emphasize	> I < )	had an 8-bit UART,
so "my protocol	was an 8-bit protocol",	and I would just say "sorry" to
people who were	held back by 7-bit limitations.
Block size: Chuck Forsberg created an extension	of my protocol,	called
YAM, which is also supported via his public domain programs for	UNIX
called rb and sb - receive batch and send batch.  They cleverly	send a
"block 0" which	contains the filename, date, time, and size.
Unfortunately, its UNIX	style, and is a	bit weird2 - octal numbers, etc.
BUT, it	is a nice way to overcome the kludgy "echo the chars of	the name"
introduced with	MODEM7.	 Further, chuck	uses CRC-16 and	optional 1K
blocks.	 Thus the record 0, 1K,	and CRC, make it a "pretty slick new
protocol" which	is not significantly different from my own.

Also, there is a catchy	name - YMODEM.	That means to some that	it is the
"next thing after XMODEM", and to others that it is the	Y(am)MODEM
protocol.  I don't want	to emphasize that too much - out of fear that
other mfgrs might think	it is a	"competitive" protocol,	rather than an


 2. The	Unix style stuff (time,	file mode) is optional.	 The pathname and
    file length	may be sent alone if desired.

								 Chapter 2

				  - 5 -
X/YMODEM Protocol Reference	 5-30-85				 5

"unaffiliated" protocol.  Chuck	is currently selling a much-enhanced
version	of his CP/M-80 C program YAM, calling it Professional Yam, and its
for the	PC - I'm using it right	now.  VERY slick!  32K capture buffer,
script,	scrolling, previously captured text search, plus built-in commands
for just about everything - directory (sorted every which way),	XMODEM,
YMODEM,	KERMIT,	and ASCII file upload/download,	etc.  You can program it
to "behave" with most any system - for example when trying a number for
CIS it detects the "busy" string back from the modem and substitutes a
diff phone # into the dialing string and branches back to try it.

Chapter	2				      XMODEM Protocol Enhancements

				  - 6 -
X/YMODEM Protocol Reference	 5-30-85				 6


Professional-YAM uses several compatible extensions and	logic enhancements
to the widely used Ward	Christensen XMODEM protocol.

This chapter discusses the protocol extensions to Ward Christensen's 1982
XMODEM protocol	description document.

YAM does not ask the operator whether he wishes	to keep	retrying after 10
attempts have failed to	correctly transfer a packet.  Virtually	all
correctable errors are corrected within	the first few retransmissions.	If
the line is so bad that	ten attempts are insufficient, there is	a
significant danger of undetected errors.  In that case,	it's better to
redial for a better connection.

3.1  CAN-CAN Abort

YAM recognizes a sequence of two consecutive CAN (Hex 18) characters
without	modem errors (overrun, framing,	etc.) as a transfer abort
command.1 The check for	two consecutive	CAN characters virtually
eliminates the possibility of a	line hit aborting the transfer.	 YAM sends
five CAN characters when it aborts a XMODEM protocol file transfer,
followed by five backspaces to delete the CAN characters from the remote's
keyboard input buffer (in case the remote had already aborted the

3.2  CRC-16 Option

The XMODEM protocol uses an optional two character CRC-16 instead of the
one character arithmetic checksum used by the original protocol	and by
most commercial	implementations.  CRC-16 guarantees detection of all
single and double bit errors,  all errors with an odd number of	error
bits, all burst	errors of length 16 or less, 99.9969% of all 17-bit error
bursts,	and 99.9984 per	cent of	all possible longer error bursts.  By
contrast, a double bit error, or a burst error of 9 bits or more can sneak
past the XMODEM	protocol arithmetic checksum.

The XMODEM/CRC protocol	is similar to the XMODEM protocol, except that the
receiver specifies CRC-16 by sending C (Hex 43)	instead	of NAK when
requesting the FIRST packet.  A	two byte CRC is	sent in	place of the one
byte arithmetic	checksum.  YAM's c option to the _r command enables CRC-16
in single file reception, corresponding	to the original	implementation in


 1. This is recognized when YAM	is waiting for the beginning of	a packet
    or for an acknowledge to one that has been sent.

XMODEM Protocol	Enhancements					 Chapter 3

				  - 7 -
X/YMODEM Protocol Reference	 5-30-85				 7

the MODEM7 series programs.  Many commercial communications programs and
bulletin board systems still do	not support CRC-16, especially those
written	in Basic or Pascal.

XMODEM protocol	with CRC is accurate provided both sender and receiver
both report a successful transmission.	The protocol is	robust in the
presence of characters lost by buffer overloading on timesharing systems.

Professional-YAM adds several proprietary logic	enhancements to	XMODEM's
error detection	and recovery.  These compatible	enhancements eliminate
most of	the bad	file transfers other programs make when	using the XMODEM
protocol under less than ideal conditions.

3.3  1024 Byte Packet Option

If YAM is sending with the k option, the transmitted packet contains 1024
bytes of data.	An STX (02) replaces the SOH (01) at the beginning of the
transmitted block to notify the	receiver of the	longer packet length.  The
receiver should	be able	to accept any mixture of 128 and 1024 byte
packets.  The packet number is incremented by one for each packet.2

If 1024	byte packets are being used, it	is possible for	a 128 byte file	to
"grow" to 1024 bytes on	CP/M.  This does not waste disk	space on CP/M
because	the allocation granularity is 1k.  When	1024 byte packets are used
with batch transmission, the file length transmitted in	the file name
packet allows the receiver to discard the padding.

CRC-16 should be used with the k option	to preserve data integrity over
phone lines.  The k option may be used with batch file transmission, or
with single file transmission.


 2. The	displayed sector number	should be the number of	bytes divided by
    128.  This maintains a familiar unit of measurement	for the	progress
    report and avoids ambiguity	caused by rollover of the sector number.

Chapter	3				      XMODEM Protocol Enhancements

				  - 8 -
X/YMODEM Protocol Reference	 5-30-85				 8

	  Figure 1.  1024 byte Packets

						  "s -k"
	  " open	x.x minutes"
	  STX 01 FE Data[1024] CRC CRC
	  STX 02 FD Data[1024] CRC CRC
	  STX 03 FC Data[1000] CPMEOF[24] CRC CRC

	  Figure 2.  Mixed 1024	and 128	byte Packets

						  "s -k"
	  " open	x.x minutes"
	  STX 01 FE Data[1024] CRC CRC
	  STX 02 FD Data[1024] CRC CRC
	  SOH 03 FC Data[128] CRC CRC
	  SOH 04 FB Data[100] CPMEOF[28] CRC CRC

4.  YMODEM Batch File Transmission

The YMODEM Batch protocol is an	extension to the XMODEM/CRC protocol that
allows 0 or more files to be transmitted with a	single command.	 (Zero
files may be sent if none of the requested files is accessible.) The
design approach	of the YAM Batch protocol is to	use the	normal routines
for sending and	receiving XMODEM packets in a layered fashion similar to
packet switching methods.

Why was	it necessary to	design a new batch protocol when one already
existed	in MODEM7?1 The	batch file mode	used by	MODEM7 is unsuitable


 1. The	MODEM7 batch protocol transmitted CP/M FCB bytes f1...f8 and
    t1...t3 one	character at a time.  The receiver echoed these	bytes as
    received, one at a time.

XMODEM Protocol	Enhancements					 Chapter 4

				  - 9 -
X/YMODEM Protocol Reference	 5-30-85				 9

because	it does	not permit full	pathnames, file	length,	file date, or
other attribute	information to be transmitted.	Such a restrictive design,
hastily	implemented with only CP/M in mind, would not have permitted
extensions to current areas of personal	computing such as Unix,	DOS, and
object oriented	systems.

As in the case of single a file	transfer, the receiver initiates batch
file transmission by sending a "C" character (for CRC-16).

The sender opens the first file	and sends packet number	0 with the
following information.2

Only the pathname (file	name) part is required for batch transfers.

To maintain upwards compatibility, all unused bytes in packet 0	must be
set to null.

Pathname The pathname (conventionally, the file	name) is sent as a null
     terminated	ASCII string.  This is the filename format used	by the
     handle oriented MSDOS(TM) functions and C library fopen functions.
     An	assembly language example follows:
			      DB      '',0
     No	spaces are included in the pathname.  Normally only the	file name
     stem (no directory	prefix)	is transmitted unless the sender has
     selected YAM's f option to	send the full pathname.	 The source drive
     (A:, B:, etc.) is not sent.

     Filename Considerations:

	o+ File names should be translated to lower case	unless the sending
	  system supports upper/lower case file	names.	This is	a
	  convenience for users	of systems (such as Unix) which	store
	  filenames in upper and lower case.

	o+ The receiver should accommodate file names in	lower and upper

	o+ The rb(1) program on Unix systems normally translates	the
	  filename to lower case unless	one or more letters in the
	  filename are already in lower	case.

	o+ When transmitting files between different operating systems,
	  file names must be acceptable	to both	the sender and receiving
	  operating systems.
     If	directories are	included, they are delimited by	/; i.e.,


 2. Only the data part of the packet is	described here.

Chapter	4				      XMODEM Protocol Enhancements

				  - 10 -
X/YMODEM Protocol Reference	 5-30-85				10

     "subdir/foo" is acceptable, "subdir\foo" is not.

Length The file	length and each	of the succeeding fields are optional.3
     The length	field is stored	in the packet as a decimal string.  The
     file length does not include any CPMEOF (^Z) characters the
     communications program might use to pad the last packet.

     If	the file being transmitted is growing during transmission, the
     length field should be set	to at least the	final expected file
     length, or	not sent.

     The receiver stores the specified number of characters, discarding
     any padding added by the sender to	fill up	the last packet.

Mod Date A single space	separates the modification date	from the file

     The mod date is optional, and the filename	and length may be sent
     without requiring the mod date to be sent.

     The mod date is sent as an	octal number giving the	time the contents
     of	the file were last changed measured in seconds from Jan	1 1970
     Universal Coordinated Time	(GMT).	A date of 0 implies the
     modification date is unknown and should be	left as	the date the file
     is	received.

     This standard format was chosen to	eliminate ambiguities arising from
     transfers between different time zones.

     Two Microsoft blunders complicate the use of modification dates in
     file transfers with MSDOS(TM) systems.  The first is the lack of
     timezone standardization in MS-DOS.  A file's creation time can not
     be	known unless the timezone of the system	that happened to write the
     file4 is known.  Unix solved this problem (for planet Earth, anyway)
     by	stamping files with Universal Time (GMT).  Microsoft would have	to
     include the timezone of origin in the directory entries, but does
     not.  YAM gets around this	problem	by using the z parameter which is
     set to the	number of minutes local	time lags GMT.	For files known	to
     originate from a different	timezone, the -zT option may be	used use T
     as	the timezone for an individual file transfer.

     The second	problem	is the lack of a separate file creation	date in
     DOS.  Since some backup schemes used with DOS rely	on the file


 3. Fields may not be skipped.

 4. Not	necessarily that of the	transmitting system!

XMODEM Protocol	Enhancements					 Chapter 4

				  - 11 -
X/YMODEM Protocol Reference	 5-30-85				11

     creation date to select files to be copied	to the archive,	back-
     dating the	file modification date could interfere with the	safety of
     the transferred files.  For this reason, Professional-YAM does not
     modify the	date of	received files with the	header information unless
     the d parameter is	non zero.

Mode A single space separates the file mode from the modification date.
     The file mode is stored as	an octal string.  Unless the file
     originated	from a Unix system, the	file mode is set to 0.	rb(1)
     checks the	file mode for the 0x8000 bit which indicates a Unix type
     regular file.  Files with the 0x8000 bit set are assumed to have been
     sent from another Unix (or	similar) system	which uses the same file
     conventions.  Such	files are not translated in any	way.

Serial Number A	single space separates the serial number from the file
     mode.  The	serial number of the transmitting program is stored as an
     octal string.  Programs which do not have a serial	number should omit
     this field, or set	it to 0.  The receiver's use of	this field is

The rest of the	packet is set to nulls.	 This is essential to preserve
upward compatibility.5 After the filename packet has been received, it is
ACK'ed if the write open is successful.	 The receiver then initiates
transfer of the	file contents according	to the standard	XMODEM/CRC
protocol.  If the file cannot be opened	for writing, the receiver cancels
the transfer with CAN characters as described above.

After the file contents	have been transmitted, the receiver again asks for
the next pathname.  Transmission of a null pathname terminates batch file
transmission.  Note that transmission of no files is not necessarily an
error.	This is	possible if none of the	files requested	of the sender
could be opened	for reading.

In batch transmission, the receiver automatically requests CRC-16.

The Unix programs sb(1)	and rb(1) included in the source code file
RBSB.QQQ ( should answer questions about YAM's batch protocol.


 5. If,	perchance, this	information extends beyond 128 bytes (possible
    with Unix 4.2 BSD extended file names), the	packet should be sent as a
    1k packet as described above.

Chapter	4					  Xmodem Protocol Overview

				  - 12 -
X/YMODEM Protocol Reference	 5-30-85				12

	    Figure 3.  Batch Transmission Session

						    "sb	foo.*"
	    "sending in	batch mode etc."
	    SOH	00 FF foo.c NUL[123] CRC CRC
	    SOH	01 FE Data[128]	CRC CRC
	    SOH	02 FD Data[1024] CRC CRC
	    SOH	03 FC Data[128]	CRC CRC
	    SOH	04 FB Data[100]	CPMEOF[28] CRC CRC
	    SOH	00 FF NUL[128] CRC CRC

       Figure 4.  Filename packet transmitted by sb

		    -rw-r--r--	6347 Jun 17 1984 20:34 bbcsched.txt

       00 0100FF62 62637363 6865642E 74787400	|...bbcsched.txt.|
       10 36333437 20333331 34373432 35313320	|6347 3314742513 |
       20 31303036 34340000 00000000 00000000	|100644..........|
       30 00000000 00000000 00000000 00000000
       80 000000CA 56

    Figure 5.  Header Information used by YMODEM Implementations
    Program	    Name    Length  Date    Mode    S/N	    1k Blocks
    Unix rb/sb	    yes	    yes	    yes	    yes	    no	    yes
    VMS	rb/sb	    yes	    yes	    no	    no	    no	    yes
    Pro-YAM	    yes	    yes	    yes	    no	    yes	    yes
    cp/M YAM	    yes	    no	    no	    no	    no	    yes
    MODEM76	    yes	    no	    no	    no	    no	    no

Xmodem Protocol	Overview					 Chapter 5

				  - 13 -
X/YMODEM Protocol Reference	 5-30-85				13


8/9/82 by Ward Christensen.

I will maintain	a master copy of this.	Please pass on changes or
suggestions via	CBBS/Chicago at	(312) 545-8086,	CBBS/CPMUG (312) 849-1132
or by voice at (312) 849-6279.

6.1  Definitions

  <soh>	01H
  <eot>	04H
  <ack>	06H
  <nak>	15H
  <can>	18H
  <C>	43H

6.2  Transmission Medium Level Protocol

Asynchronous, 8	data bits, no parity, one stop bit.

The protocol imposes no	restrictions on	the contents of	the data being
transmitted.  No control characters are	looked for in the 128-byte data
messages.  Absolutely any kind of data may be sent - binary, ASCII, etc.
The protocol has not formally been adopted to a	7-bit environment for the
transmission of	ASCII-only (or unpacked-hex) data , although it	could be
simply by having both ends agree to AND	the protocol-dependent data with
7F hex before validating it.  I	specifically am	referring to the checksum,
and the	block numbers and their	ones- complement.

Those wishing to maintain compatibility	of the CP/M file structure, i.e.
to allow modemming ASCII files to or from CP/M systems should follow this
data format:

   o+ ASCII tabs	used (09H); tabs set every 8.

   o+ Lines terminated by CR/LF (0DH 0AH)

   o+ End-of-file indicated by ^Z, 1AH.	(one or	more)

   o+ Data is variable length, i.e. should be considered	a continuous
     stream of data bytes, broken into 128-byte	chunks purely for the
     purpose of	transmission.

   o+ A CP/M "peculiarity": If the data ends exactly on a 128-byte
     boundary, i.e. CR in 127, and LF in 128, a	subsequent sector
     containing	the ^Z EOF character(s)	is optional, but is preferred.
     Some utilities or user programs still do not handle EOF without ^Zs.

Chapter	6					  Xmodem Protocol Overview

				  - 14 -
X/YMODEM Protocol Reference	 5-30-85				14

   o+ The last block sent is no different from others, i.e.  there is no
     "short block".

	      Figure 6.	 XMODEM	Message	Block Level Protocol

      Each block of the	transfer looks like:
     <SOH><blk #><255-blk #><--128 data	bytes--><cksum>
	 in which:
     <SOH>	 = 01 hex
     <blk #>	 = binary number, starts at 01 increments by 1,	and
		   wraps 0FFH to 00H (not to 01)
     <255-blk #> = blk # after going thru 8080 "CMA" instr, i.e.
		   each	bit complemented in the	8-bit block number.
		   Formally, this is the "ones complement".
     <cksum>	 = the sum of the data bytes only.  Toss any carry.

6.3  File Level	Protocol

6.3.1  _C_o_m_m_o_n__t_o__B_o_t_h__S_e_n_d_e_r__a_n_d__R_e_c_e_i_v_e_r
All errors are retried 10 times.  For versions running with an operator
(i.e. NOT with XMODEM),	a message is typed after 10 errors asking the
operator whether to "retry or quit".

Some versions of the protocol use <can>, ASCII ^X, to cancel transmission.
This was never adopted as a standard, as having	a single "abort" character
makes the transmission susceptible to false termination	due to an <ack>
<nak> or <soh> being corrupted into a <can> and	cancelling transmission.

The protocol may be considered "receiver driven", that is, the sender need
not automatically re-transmit, although	it does	in the current

6.3.2  _R_e_c_e_i_v_e__P_r_o_g_r_a_m__C_o_n_s_i_d_e_r_a_t_i_o_n_s
The receiver has a 10-second timeout.  It sends	a <nak>	every time it
times out.  The	receiver's first timeout, which	sends a	<nak>, signals the
transmitter to start.  Optionally, the receiver	could send a <nak>
immediately, in	case the sender	was ready.  This would save the	initial	10
second timeout.	 However, the receiver MUST continue to	timeout	every 10
seconds	in case	the sender wasn't ready.

Once into a receiving a	block, the receiver goes into a	one-second timeout
for each character and the checksum.  If the receiver wishes to	<nak> a
block for any reason (invalid header, timeout receiving	data), it must
wait for the line to clear.  See "programming tips" for	ideas

Synchronizing:	If a valid block number	is received, it	will be: 1) the
expected one, in which case everything is fine;	or 2) a	repeat of the
previously received block.  This should	be considered OK, and only
indicates that the receivers <ack> got glitched, and the sender	re-
transmitted; 3)	any other block	number indicates a fatal loss of

Xmodem Protocol	Overview					 Chapter 6

				  - 15 -
X/YMODEM Protocol Reference	 5-30-85				15

synchronization, such as the rare case of the sender getting a line-glitch
that looked like an <ack>.  Abort the transmission, sending a <can>

6.3.3  _S_e_n_d_i_n_g__p_r_o_g_r_a_m__c_o_n_s_i_d_e_r_a_t_i_o_n_s
While waiting for transmission to begin, the sender has	only a single very
long timeout, say one minute.  In the current protocol,	the sender has a
10 second timeout before retrying.  I suggest NOT doing	this, and letting
the protocol be	completely receiver-driven.  This will be compatible with
existing programs.

When the sender	has no more data, it sends an <eot>, and awaits	an <ack>,
resending the <eot> if it doesn't get one.  Again, the protocol	could be
receiver-driven, with the sender only having the high-level 1-minute
timeout	to abort.

Here is	a sample of the	data flow, sending a 3-block message.  It includes
the two	most common line hits -	a garbaged block, and an <ack> reply
getting	garbaged.  <xx>	represents the checksum	byte.

       Figure 7.  Data flow including Error Recovery

       SENDER				       RECEIVER
				       times out after 10 seconds,
			       <---	       <nak>
       <soh> 01	FE -data- <xx> --->
			       <---	       <ack>
       <soh> 02	FD -data- xx   --->    (data gets line hit)
			       <---	       <nak>
       <soh> 02	FD -data- xx   --->
			       <---	       <ack>
       <soh> 03	FC -data- xx   --->
	  (ack gets garbaged)  <---	       <ack>
       <soh> 03	FC -data- xx   --->	       <ack>
       <eot>		       --->
			       <---	       <ack>

6.4  Programming Tips

   o+ The character-receive subroutine should be	called with a parameter
     specifying	the number of seconds to wait.	The receiver should first
     call it with a time of 10,	then <nak> and try again, 10 times.

     After receiving the <soh>,	the receiver should call the character
     receive subroutine	with a 1-second	timeout, for the remainder of the
     message and the <cksum>.  Since they are sent as a	continuous stream,
     timing out	of this	implies	a serious like glitch that caused, say,
     127 characters to be seen instead of 128.

Chapter	6					  Xmodem Protocol Overview

				  - 16 -
X/YMODEM Protocol Reference	 5-30-85				16

   o+ When the receiver wishes to <nak>,	it should call a "PURGE"
     subroutine, to wait for the line to clear.	 Recall	the sender tosses
     any characters in its UART	buffer immediately upon	completing sending
     a block, to ensure	no glitches were mis- interpreted.

     The most common technique is for "PURGE" to call the character
     receive subroutine, specifying a 1-second timeout,1 and looping back
     to	PURGE until a timeout occurs.  The <nak> is then sent, ensuring
     the other end will	see it.

   o+ You may wish to add code recommended by John Mahr to your character
     receive routine - to set an error flag if the UART	shows framing
     error, or overrun.	 This will help	catch a	few more glitches - the
     most common of which is a hit in the high bits of the byte	in two
     consecutive bytes.	 The <cksum> comes out OK since	counting in 1-byte
     produces the same result of adding	80H + 80H as with adding 00H +


 1. These times	should be adjusted for use with	timesharing systems.

Xmodem Protocol	Overview					 Chapter 6

				  - 17 -
X/YMODEM Protocol Reference	 5-30-85				17

7.  XMODEM/CRC Overview

1/13/85	by John	Byrns -- CRC option.

Please pass on any reports of errors in	this document or suggestions for
improvement to me via Ward's/CBBS at (312) 849-1132, or	by voice at (312)

The CRC	used in	the Modem Protocol is an alternate form	of block check
which provides more robust error detection than	the original checksum.
Andrew S. Tanenbaum says in his	book, Computer Networks, that the CRC-
CCITT used by the Modem	Protocol will detect all single	and double bit
errors,	all errors with	an odd number of bits, all burst errors	of length
16 or less, 99.997% of 17-bit error bursts, and	99.998%	of 18-bit and
longer bursts.

The changes to the Modem Protocol to replace the checksum with the CRC are
straight forward. If that were all that	we did we would	not be able to
communicate between a program using the	old checksum protocol and one
using the new CRC protocol. An initial handshake was added to solve this
problem. The handshake allows a	receiving program with CRC capability to
determine whether the sending program supports the CRC option, and to
switch it to CRC mode if it does. This handshake is designed so	that it
will work properly with	programs which implement only the original
protocol. A description	of this	handshake is presented in section 10.

  Figure 8.  Message Block Level Protocol, CRC mode

  Each block of	the transfer in	CRC mode looks like:

  <SOH><blk #><255-blk #><--128	data bytes--><CRC hi><CRC lo>
      in which:
  <SOH>		    = 01 hex
  <blk #>     =	binary number, starts at 01 increments by 1, and
		wraps 0FFH to 00H (not to 01)
  <255-blk #> =	ones complement	of blk #.
  <CRC hi>    =	byte containing	the 8 hi order coefficients of the CRC.
  <CRC lo>    =	byte containing	the 8 lo order coefficients of the CRC.
		See the	next section for CRC calculation.

7.1  CRC Calculation

7.1.1  _F_o_r_m_a_l__D_e_f_i_n_i_t_i_o_n
To calculate the 16 bit	CRC the	message	bits are considered to be the
coefficients of	a polynomial. This message polynomial is first multiplied
by X^16	and then divided by the	generator polynomial (X^16 + X^12 + X^5	+
1) using modulo	two arithmetic.	The remainder left after the division is
the desired CRC. Since a message block in the Modem Protocol is	128 bytes
or 1024	bits, the message polynomial will be of	order X^1023. The hi order
bit of the first byte of the message block is the coefficient of X^1023	in
the message polynomial.	 The lo	order bit of the last byte of the message

Chapter	7					  Xmodem Protocol Overview

				  - 18 -
X/YMODEM Protocol Reference	 5-30-85				18

block is the coefficient of X^0	in the message polynomial.

   Figure 9.  Example of CRC Calculation written in C

    * This function calculates the CRC used by the XMODEM/CRC Protocol
    * The first	argument is a pointer to the message block.
    * The second argument is the number	of bytes in the	message	block.
    * The function returns an integer which contains the CRC.
    * The low order 16 bits are	the coefficients of the	CRC.

   int calcrc(ptr, count)
   char	*ptr;
   int count;
       int crc,	i;

       crc = 0;
       while(--count >=	0) {
	   crc = crc ^ (int)*ptr++ << 8;
	   for(i = 0; i	< 8; ++i)
	       if(crc &	0x8000)
		   crc = crc <<	1 ^ 0x1021;
		   crc = crc <<	1;
       return (crc & 0xFFFF);

7.2  CRC File Level Protocol Changes

7.2.1  _C_o_m_m_o_n__t_o__B_o_t_h__S_e_n_d_e_r__a_n_d__R_e_c_e_i_v_e_r
The only change	to the File Level Protocol for the CRC option is the
initial	handshake which	is used	to determine if	both the sending and the
receiving programs support the CRC mode. All Modem Programs should support
the checksum mode for compatibility with older versions.  A receiving
program	that wishes to receive in CRC mode implements the mode setting
handshake by sending a <C> in place of the initial <nak>.  If the sending
program	supports CRC mode it will recognize the	<C> and	will set itself
into CRC mode, and respond by sending the first	block as if a <nak> had
been received. If the sending program does not support CRC mode	it will
not respond to the <C> at all. After the receiver has sent the <C> it will
wait up	to 3 seconds for the <soh> that	starts the first block.	If it
receives a <soh> within	3 seconds it will assume the sender supports CRC
mode and will proceed with the file exchange in	CRC mode. If no	<soh> is
received within	3 seconds the receiver will switch to checksum mode, send
a <nak>, and proceed in	checksum mode. If the receiver wishes to use
checksum mode it should	send an	initial	<nak> and the sending program
should respond to the <nak> as defined in the original Modem Protocol.

Xmodem Protocol	Overview					 Chapter 7

				  - 19 -
X/YMODEM Protocol Reference	 5-30-85				19

After the mode has been	set by the initial <C> or <nak>	the protocol
follows	the original Modem Protocol and	is identical whether the checksum
or CRC is being	used.

7.2.2  _R_e_c_e_i_v_e__P_r_o_g_r_a_m__C_o_n_s_i_d_e_r_a_t_i_o_n_s
There are at least 4 things that can go	wrong with the mode setting

  1.  the initial <C> can be garbled or	lost.

  2.  the initial <soh>	can be garbled.

  3.  the initial <C> can be changed to	a <nak>.

  4.  the initial <nak>	from a receiver	which wants to receive in checksum
      can be changed to	a <C>.

The first problem can be solved	if the receiver	sends a	second <C> after
it times out the first time. This process can be repeated several times.
It must	not be repeated	too many times before sending a	<nak> and
switching to checksum mode or a	sending	program	without	CRC support may
time out and abort. Repeating the <C> will also	fix the	second problem if
the sending program cooperates by responding as	if a <nak> were	received
instead	of ignoring the	extra <C>.

It is possible to fix problems 3 and 4 but probably not	worth the trouble
since they will	occur very infrequently. They could be fixed by	switching
modes in either	the sending or the receiving program after a large number
of successive <nak>s. This solution would risk other problems however.

7.2.3  _S_e_n_d_i_n_g__P_r_o_g_r_a_m__C_o_n_s_i_d_e_r_a_t_i_o_n_s
The sending program should start in the	checksum mode. This will insure
compatibility with checksum only receiving programs. Anytime a <C> is
received before	the first <nak>	or <ack> the sending program should set
itself into CRC	mode and respond as if a <nak> were received. The sender
should respond to additional <C>s as if	they were <nak>s until the first
<ack> is received. This	will assist the	receiving program in determining
the correct mode when the <soh>	is lost	or garbled. After the first <ack>
is received the	sending	program	should ignore <C>s.

7.3  Data Flow Examples	with CRC Option

Here is	a data flow example for	the case where the receiver requests
transmission in	the CRC	mode but the sender does not support the CRC
option.	This example also includes various transmission	errors.	 <xx>
represents the checksum	byte.

Chapter	7					  Xmodem Protocol Overview

				  - 20 -
X/YMODEM Protocol Reference	 5-30-85				20

      Figure 10.  Data Flow: Receiver has CRC Option, Sender Doesn't

      SENDER				      RECEIVER
			      <---	      <C>
				      times out	after 3	seconds,
			      <---	      <nak>
      <soh> 01 FE -data- <xx> --->
			      <---	      <ack>
      <soh> 02 FD -data- <xx> --->    (data gets line hit)
			      <---	      <nak>
      <soh> 02 FD -data- <xx> --->
			      <---	      <ack>
      <soh> 03 FC -data- <xx> --->
	 (ack gets garbaged)  <---	      <ack>
				      times out	after 10 seconds,
			      <---	      <nak>
      <soh> 03 FC -data- <xx> --->
			      <---	      <ack>
      <eot>		      --->
			      <---	      <ack>

Here is	a data flow example for	the case where the receiver requests
transmission in	the CRC	mode and the sender supports the CRC option.  This
example	also includes various transmission errors.  <xxxx> represents the
2 CRC bytes.

	   Figure 11.  Receiver	and Sender Both	have CRC Option

     SENDER				       RECEIVER
			       <---	       <C>
     <soh> 01 FE -data-	<xxxx> --->
			       <---	       <ack>
     <soh> 02 FD -data-	<xxxx> --->    (data gets line hit)
			       <---	       <nak>
     <soh> 02 FD -data-	<xxxx> --->
			       <---	       <ack>
     <soh> 03 FC -data-	<xxxx> --->
	(ack gets garbaged)    <---	       <ack>
				       times out after 10 seconds,
			       <---	       <nak>
     <soh> 03 FC -data-	<xxxx> --->
			       <---	       <ack>
     <eot>		       --->
			       <---	       <ack>

Xmodem Protocol	Overview					 Chapter 7

				  - 21 -
X/YMODEM Protocol Reference	 5-30-85				21


More information may be	obtained by calling Telegodzilla at 503-621-3746.
Hit RETURNs for	baud rate detection.

A version this file with boldface, underlining,	and superscripts for
printing on Epson or Gemini printers is	available on Telegodzilla as

UUCP sites can obtain this file	with
	     uucp omen!/usr/spool/uucppublic/ymodem.doc	/tmp

The following L.sys line calls Telegodzilla (Pro-YAM in	host operation).
Telegodzilla waits for carriage	returns	to determine the incoming speed.
If none	is detected, 1200 bps is assumed and a greeting	is displayed.

In response to "Name Please:" uucico gives the Pro-YAM "link" command as a
user name.  The	password (Giznoid) controls access to the Xenix	system
connected to the IBM PC's other	serial port.  Communications between Pro-
YAM and	Xenix use 9600 bps; YAM	converts this to the caller's speed.

Finally, the calling uucico logs in as uucp.

		     omen Any ACU 1200 1-503-621-3746
	      ase:--ase: link ord: Giznoid ogin:--ogin:	uucp

Chapter	8					  Xmodem Protocol Overview

				  - 22 -
X/YMODEM Protocol Reference	 5-30-85				22

9.  YMODEM Programs

A demonstration	version	of Professional-YAM is available as YAMDEMO.LQR	(A
SQueezed Novosielski library).	This may also be used to exercize YMODEM

Unix programs supporting the YMODEM protocol are available on Telegodzilla
in the "upgrade" subdirectory as RBSM.SHQ (a SQueezed shell archive).
Most Unix like systems are supported, including	V7, Sys	III, 4.2 BSD, SYS
V, Idris, Coherent, and	Regulus.

A version for Vax-VMS is available in VRBSB.SHQ.

A CP/M assembly	version	is available as	MODEM76.AQM and	MODEM7.LIB.

Technical questions about the Professional-YAM communications program and
requests for evaluation	copies for magazine reviews may	be directed to:
     Chuck Forsberg
     Omen Technology Inc
     17505-V Sauvie Island Road
     Portland Oregon 97231
     Voice: 503-621-3406
     Modem: 503-621-3746 Speed:	1200,300
     Usenet: ...tektronix!reed!omen!caf
     Compuserve: 70715,131
     Source: TCE022

        Chuck Forsberg WA7KGX       ...!tektronix!reed!omen!caf
Omen Technology Inc     17505-V NW Sauvie Island Road Portland OR 97231
Voice: 503-621-3406     Modem: 503-621-3746 (Hit CR's for speed detect)
Home of Professional-YAM, the most powerful COMM program for the IBM PC



file: /Techref/ymodem.htm, 45KB, , updated: 2007/5/17 07:19, local time: 2024/7/18 04:14, owner: JMN-EFP-786,

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