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STX 02 FD Data[1024] CRC CRC
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SOH 03 FC Data[128] CRC CRC
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SOH 04 FB Data[100] CPMEOF[28] CRC CRC
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EOT
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ACK
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G
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SOH 00 FF NUL[128] CRC CRC
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Chapter 6 XMODEM Protocol Enhancements
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X/YMODEM Protocol Reference 10-27-87 18
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7. XMODEM PROTOCOL OVERVIEW
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8/9/82 by Ward Christensen.
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I will maintain a master copy of this. Please pass on changes or
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suggestions via CBBS/Chicago at (312) 545-8086, CBBS/CPMUG (312) 849-1132
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or by voice at (312) 849-6279.
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7.1 Definitions
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<soh> 01H
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<eot> 04H
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<ack> 06H
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<nak> 15H
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<can> 18H
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<C> 43H
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7.2 Transmission Medium Level Protocol
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Asynchronous, 8 data bits, no parity, one stop bit.
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The protocol imposes no restrictions on the contents of the data being
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transmitted. No control characters are looked for in the 128-byte data
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messages. Absolutely any kind of data may be sent - binary, ASCII, etc.
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The protocol has not formally been adopted to a 7-bit environment for the
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transmission of ASCII-only (or unpacked-hex) data , although it could be
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simply by having both ends agree to AND the protocol-dependent data with
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7F hex before validating it. I specifically am referring to the checksum,
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and the block numbers and their ones- complement.
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Those wishing to maintain compatibility of the CP/M file structure, i.e.
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to allow modemming ASCII files to or from CP/M systems should follow this
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data format:
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+ ASCII tabs used (09H); tabs set every 8.
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+ Lines terminated by CR/LF (0DH 0AH)
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+ End-of-file indicated by ^Z, 1AH. (one or more)
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+ Data is variable length, i.e. should be considered a continuous
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stream of data bytes, broken into 128-byte chunks purely for the
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purpose of transmission.
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+ A CP/M "peculiarity": If the data ends exactly on a 128-byte
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boundary, i.e. CR in 127, and LF in 128, a subsequent sector
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containing the ^Z EOF character(s) is optional, but is preferred.
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Some utilities or user programs still do not handle EOF without ^Zs.
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Chapter 7 Xmodem Protocol Overview
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X/YMODEM Protocol Reference 10-27-87 19
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+ The last block sent is no different from others, i.e. there is no
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"short block".
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Figure 8. XMODEM Message Block Level Protocol
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Each block of the transfer looks like:
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