digdon@ug.cs.dal.ca (Mike Digdon) (01/28/91)
I'm working with a term program (well, passing the time away by pretending to write one) and I would like it to have transfers. I have Punter, and I can get it to work and such, so that is no problem. My big problem is Xmodem. I have nothing on it. Does anyone have any kind of source for it, and maybe also the docs on how to execute it and set it up? Any help would be greatly appreciated. -- +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + Mike Digdon: digdon@ug.cs.dal.ca + My 64 can still get the + + mike@ac.dal.ca + job at hand completed!! + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
root@zswamp.fidonet.org (Geoffrey Welsh) (01/30/91)
In a letter to All, Mike Digdon (digdon@ug.cs.dal.ca ) wrote: >My big problem is Xmodem. I have nothing on it. >Does anyone have any kind of source for it, and maybe >also the docs on how to execute it and set it up? How about the specifications? The following is extracted from Chuck Forsberg's YMODEM spec, which I was going to copy here in its entirety until I noticed that it is some 66K in length and guessed that doing so would be a quick way to Usenet flame-Hell. BTW, if you (or anyone) want some fast CRC routines, drop me E-mail... my terms are very reasonable. 7. XMODEM PROTOCOL OVERVIEW 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. 7.1 Definitions <soh> 01H <eot> 04H <ack> 06H <nak> 15H <can> 18H <C> 43H 7.2 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: + ASCII tabs used (09H); tabs set every 8. + Lines terminated by CR/LF (0DH 0AH) + End-of-file indicated by ^Z, 1AH. (one or more) + 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. + 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. + The last block sent is no different from others, i.e. there is no "short block". Figure 8. 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. 7.3 File Level Protocol 7.3.1 Common_to_Both_Sender_and_Receiver 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 aborting transmission. The protocol may be considered "receiver driven", that is, the sender need not automatically re-transmit, although it does in the current implementations. 7.3.2 Receive_Program_Considerations 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 synchronization, such as the rare case of the sender getting a line-glitch that looked like an <ack>. Abort the transmission, sending a <can> 7.3.3 Sending_program_considerations 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 9. 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> ---> <--- <anything except ack> <eot> ---> <--- <ack> (finished) 7.4 Programming Tips + 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. + 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. + 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 + 00H. 8. XMODEM/CRC Overview Original 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) 885-1105. 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.[1] 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 10. 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. 8.1 CRC Calculation 8.1.1 Formal_Definition 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 block is the coefficient of X^0 in the message polynomial. Figure 11. Example of CRC Calculation written in C The following XMODEM crc routine is taken from "rbsb.c". Please refer to the source code for these programs (contained in RZSZ.ZOO) for usage. A fast table driven version is also included in this file. /* update CRC */ unsigned short updcrc(c, crc) register c; register unsigned crc; { register count; for (count=8; --count>=0;) { if (crc & 0x8000) { crc <<= 1; crc += (((c<<=1) & 0400) != 0); crc ^= 0x1021; } else { crc <<= 1; crc += (((c<<=1) & 0400) != 0); } } return crc; } 8.2 CRC File Level Protocol Changes 8.2.1 Common_to_Both_Sender_and_Receiver 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. 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. 8.2.2 Receive_Program_Considerations There are at least 4 things that can go wrong with the mode setting handshake. 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. 8.2.3 Sending_Program_Considerations 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. 8.3 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. Figure 12. Data Flow: Receiver has CRC Option, Sender Doesn't SENDER RECEIVER <--- <C> times out after 3 seconds, <--- <C> times out after 3 seconds, <--- <C> times out after 3 seconds, <--- <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 13. 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> -- UUCP: watmath!xenitec!zswamp!root | 602-66 Mooregate Crescent Internet: root@zswamp.fidonet.org | Kitchener, Ontario FidoNet: SYSOP, 1:221/171 | N2M 5E6 CANADA Data: (519) 742-8939 | (519) 741-9553 MC Hammer, n. Device used to ensure firm seating of MicroChannel boards Try our new Molson 'C' compiler... it specializes in 'case' statements!