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Chapter 7 ZMODEM Protocol 15
Hex encoding protects the reverse channel from random control characters.
The hex header receiving routine ignores parity.
Use of Kermit style encoding for control and paritied characters was
considered and rejected because of increased possibility of interacting
with some timesharing systems' line edit functions. Use of HEX headers
from the receiving program allows control characters to be used to
interrupt the sender when errors are detected. A HEX header may be used
in place of a binary header wherever convenient. If a data packet follows
a HEX header, it is protected with CRC-16.
A hex header begins with the sequence ZPAD, ZPAD, ZDLE, ZHEX. The _z_g_e_t_h_d_r
routine synchronizes with the ZPAD-ZDLE sequence. The extra ZPAD
character allows the sending program to detect an asynchronous header
(indicating an error condition) and then call _z_g_e_t_h_d_r to receive the
header.
The type byte, the four position/flag bytes, and the 16 bit CRC thereof
are sent in hex using the character set 01234567890abcdef. Upper case hex
digits are not allowed; they false trigger XMODEM and YMODEM programs.
Since this form of hex encoding detects many patterns of errors,
especially missing characters, a hex header with 32 bit CRC has not been
defined.
A carriage return and line feed are sent with HEX headers. The receive
routine expects to see at least one of these characters, two if the first
is CR. The CR/LF aids debugging from printouts, and helps overcome
certain operating system related problems.
An XON character is appended to all HEX packets except ZACK and ZFIN. The
XON releases the sender from spurious XOFF flow control characters
generated by line noise, a common occurrence. XON is not sent after ZACK
headers to protect flow control in streaming situations. XON is not sent
after a ZFIN header to allow clean session cleanup.
0 or more data subpackets will follow depending on the frame type.
The function _z_s_h_h_d_r sends a hex header.
FFFFiiiigggguuuurrrreeee 4444.... HEX Header
* * ZDLE B TYPE F3/P0 F2/P1 F1/P2 F0/P3 CRC-1 CRC-2 CR LF XON
(TYPE, F3...F0, CRC-1, and CRC-2 are each sent as two hex digits.)
Chapter 7 Rev 10-27-87 Typeset 10-27-87 15
Chapter 7 ZMODEM Protocol 16
7.4 BBBBiiiinnnnaaaarrrryyyy DDDDaaaattttaaaa SSSSuuuubbbbppppaaaacccckkkkeeeettttssss
Binary data subpackets immediately follow the associated binary header
packet. A binary data packet contains 0 to 1024 bytes of data.
Recommended length values are 256 bytes below 2400 bps, 512 at 2400 bps,
and 1024 above 4800 bps or when the data link is known to be relatively
error free.[4]
No padding is used with binary data subpackets. The data bytes are ZDLE
encoded and transmitted. A ZDLE and frameend are then sent, followed by
two or four ZDLE encoded CRC bytes. The CRC accumulates the data bytes
and frameend.
The function _z_s_d_a_t_a sends a data subpacket. The function _z_r_d_a_t_a receives
a data subpacket.
7.5 AAAASSSSCCCCIIIIIIII EEEEnnnnccccooooddddeeeedddd DDDDaaaattttaaaa SSSSuuuubbbbppppaaaacccckkkkeeeetttt
The format of ASCII Encoded data subpackets is not currently specified.
These could be used for server commands, or main transfers in 7 bit
environments.