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The ZMODEM routines in zm.c accept an option to escape all control
characters, to allow operation with less transparent networks. This
option can be given to either the sending or receiving program.
7.3 HHHHeeeeaaaaddddeeeerrrr
All ZMODEM frames begin with a header which may be sent in binary or HEX
form. ZMODEM uses a single routine to recognize binary and hex headers.
Either form of the header contains the same raw information:
o+ A type byte[2] [3]
o+ Four bytes of data indicating flags and/or numeric quantities depending
on the frame type
__________
2. The frame types are cardinal numbers beginning with 0 to minimize
state transition table memory requirements.
3. Future extensions to ZMODEM may use the high order bits of the type
byte to indicate thread selection.
Chapter 7 Rev 10-27-87 Typeset 10-27-87 13
Chapter 7 ZMODEM Protocol 14
FFFFiiiigggguuuurrrreeee 1111.... Order of Bytes in Header
TYPE: frame type
F0: Flags least significant byte
P0: file Position least significant
P3: file Position most significant
TYPE F3 F2 F1 F0
-------------------
TYPE P0 P1 P2 P3
7.3.1 11116666 BBBBiiiitttt CCCCRRRRCCCC BBBBiiiinnnnaaaarrrryyyy HHHHeeeeaaaaddddeeeerrrr
A binary header is sent by the sending program to the receiving program.
ZDLE encoding accommodates XON/XOFF flow control.
A binary header begins with the sequence ZPAD, ZDLE, ZBIN.
The frame type byte is ZDLE encoded.
The four position/flags bytes are ZDLE encoded.
A two byte CRC of the frame type and position/flag bytes is ZDLE encoded.
0 or more binary data subpackets with 16 bit CRC will follow depending on
the frame type.
The function _z_s_b_h_d_r transmits a binary header. The function _z_g_e_t_h_d_r
receives a binary or hex header.
FFFFiiiigggguuuurrrreeee 2222.... 16 Bit CRC Binary Header
* ZDLE A TYPE F3/P0 F2/P1 F1/P2 F0/P3 CRC-1 CRC-2
7.3.2 33332222 BBBBiiiitttt CCCCRRRRCCCC BBBBiiiinnnnaaaarrrryyyy HHHHeeeeaaaaddddeeeerrrr
A "32 bit CRC" Binary header is similar to a Binary Header, except the
ZZZZBBBBIIIINNNN (A) character is replaced by a ZZZZBBBBIIIINNNN33332222 (C) character, and four
characters of CRC are sent. 0 or more binary data subpackets with 32 bit
CRC will follow depending on the frame type.
The common variable _T_x_f_c_s_3_2 may be set TRUE for 32 bit CRC iff the
receiver indicates the capability with the CCCCAAAANNNNFFFFCCCC33332222 bit. The zgethdr,
zsdata and zrdata functions automatically adjust to the type of Frame
Check Sequence being used.
FFFFiiiigggguuuurrrreeee 3333.... 32 Bit CRC Binary Header
* ZDLE C TYPE F3/P0 F2/P1 F1/P2 F0/P3 CRC-1 CRC-2 CRC-3 CRC-4
7.3.3 HHHHEEEEXXXX HHHHeeeeaaaaddddeeeerrrr
The receiver sends responses in hex headers. The sender also uses hex
headers when they are not followed by binary data subpackets.
Chapter 7 Rev 10-27-87 Typeset 10-27-87 14