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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. |
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