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Chapter 6 Rev 10-27-87 Typeset 10-27-87 11
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Chapter 6 ZMODEM Protocol 12
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7. ZZZZMMMMOOOODDDDEEEEMMMM BBBBAAAASSSSIIIICCCCSSSS
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7.1 PPPPaaaacccckkkkeeeettttiiiizzzzaaaattttiiiioooonnnn
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ZMODEM frames differ somewhat from XMODEM blocks. XMODEM blocks are not
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used for the following reasons:
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o+ Block numbers are limited to 256
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o+ No provision for variable length blocks
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o+ Line hits corrupt protocol signals, causing failed file transfers. In
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particular, modem errors sometimes generate false block numbers, false
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EOTs and false ACKs. False ACKs are the most troublesome as they cause
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the sender to lose synchronization with the receiver.
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State of the art programs such as Professional-YAM and ZCOMM overcome
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some of these weaknesses with clever proprietary code, but a stronger
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protocol is desired.
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o+ It is difficult to determine the beginning and ends of XMODEM blocks
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when line hits cause a loss of synchronization. This precludes rapid
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error recovery.
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7.2 LLLLiiiinnnnkkkk EEEEssssccccaaaappppeeee EEEEnnnnccccooooddddiiiinnnngggg
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ZMODEM achieves data transparency by extending the 8 bit character set
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(256 codes) with escape sequences based on the ZMODEM data link escape
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character ZDLE.[1]
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Link Escape coding permits variable length data subpackets without the
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overhead of a separate byte count. It allows the beginning of frames to
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be detected without special timing techniques, facilitating rapid error
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recovery.
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Link Escape coding does add some overhead. The worst case, a file
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consisting entirely of escaped characters, would incur a 50% overhead.
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The ZDLE character is special. ZDLE represents a control sequence of some
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sort. If a ZDLE character appears in binary data, it is prefixed with
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ZDLE, then sent as ZDLEE.
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The value for ZDLE is octal 030 (ASCII CAN). This particular value was
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chosen to allow a string of 5 consecutive CAN characters to abort a ZMODEM
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__________
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1. This and other constants are defined in the _z_m_o_d_e_m._h include file.
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Please note that constants with a leading 0 are octal constants in C.
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Chapter 7 Rev 10-27-87 Typeset 10-27-87 12
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Chapter 7 ZMODEM Protocol 13
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session, compatible with YMODEM session abort.
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Since CAN is not used in normal terminal operations, interactive
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applications and communications programs can monitor the data flow for
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ZDLE. The following characters can be scanned to detect the ZRQINIT
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header, the invitation to automatically download commands or files.
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Receipt of five successive CAN characters will abort a ZMODEM session.
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Eight CAN characters are sent.
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The receiving program decodes any sequence of ZDLE followed by a byte with
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bit 6 set and bit 5 reset (upper case letter, either parity) to the
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equivalent control character by inverting bit 6. This allows the
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transmitter to escape any control character that cannot be sent by the
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communications medium. In addition, the receiver recognizes escapes for
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0177 and 0377 should these characters need to be escaped.
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ZMODEM software escapes ZDLE, 020, 0220, 021, 0221, 023, and 0223. If
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preceded by 0100 or 0300 (@), 015 and 0215 are also escaped to protect the
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Telenet command escape CR-@-CR. The receiver ignores 021, 0221, 023, and
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0223 characters in the data stream.
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