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8. PPPPRRRROOOOTTTTOOOOCCCCOOOOLLLL TTTTRRRRAAAANNNNSSSSAAAACCCCTTTTIIIIOOOONNNN OOOOVVVVEEEERRRRVVVVIIIIEEEEWWWW
As with the XMODEM recommendation, ZMODEM timing is receiver driven. The
transmitter should not time out at all, except to abort the program if no
headers are received for an extended period of time, say one minute.[1]
8.1 SSSSeeeessssssssiiiioooonnnn SSSSttttaaaarrrrttttuuuupppp
To start a ZMODEM file transfer session, the sending program is called
with the names of the desired file(s) and option(s).
The sending program may send the string "rz\r" to invoke the receiving
program from a possible command mode. The "rz" followed by carriage
return activates a ZMODEM receive program or command if it were not
already active.
The sender may then display a message intended for human consumption, such
__________
4. Strategies for adjusting the subpacket length for optimal results
based on real time error rates are still evolving. Shorter subpackets
speed error detection but increase protocol overhead slightly.
1. Special considerations apply when sending commands.
Chapter 8 Rev 10-27-87 Typeset 10-27-87 16
Chapter 8 ZMODEM Protocol 17
as a list of the files requested, etc.
Then the sender may send a ZZZZRRRRQQQQIIIINNNNIIIITTTT header. The ZZZZRRRRQQQQIIIINNNNIIIITTTT header causes a
previously started receive program to send its ZZZZRRRRIIIINNNNIIIITTTT header without
delay.
In an interactive or conversational mode, the receiving application may
monitor the data stream for ZDLE. The following characters may be scanned
for BBBB00000000 indicating a ZRQINIT header, a command to download a command or
data.
The sending program awaits a command from the receiving program to start
file transfers. If a "C", "G", or NAK is received, an XMODEM or YMODEM
file transfer is indicated, and file transfer(s) use the YMODEM protocol.
Note: With ZMODEM and YMODEM, the sending program provides the file name,
but not with XMODEM.
In case of garbled data, the sending program can repeat the invitation to
receive a number of times until a session starts.
When the ZMODEM receive program starts, it immediately sends a ZZZZRRRRIIIINNNNIIIITTTT
header to initiate ZMODEM file transfers, or a ZZZZCCCCHHHHAAAALLLLLLLLEEEENNNNGGGGEEEE header to verify
the sending program. The receive program resends its header at _r_e_s_p_o_n_s_e
_t_i_m_e (default 10 second) intervals for a suitable period of time (40
seconds total) before falling back to YMODEM protocol.
If the receiving program receives a ZZZZRRRRQQQQIIIINNNNIIIITTTT header, it resends the ZZZZRRRRIIIINNNNIIIITTTT
header. If the sending program receives the ZZZZCCCCHHHHAAAALLLLLLLLEEEENNNNGGGGEEEE header, it places
the data in ZP0...ZP3 in an answering ZZZZAAAACCCCKKKK header.
If the receiving program receives a ZZZZRRRRIIIINNNNIIIITTTT header, it is an echo
indicating that the sending program is not operational.
Eventually the sending program correctly receives the ZZZZRRRRIIIINNNNIIIITTTT header.
The sender may then send an optional ZZZZSSSSIIIINNNNIIIITTTT frame to define the receiving
program's AAAAttttttttnnnn sequence, or to specify complete control character
escaping.[2]
If the ZSINIT header specifies ESCCTL or ESC8, a HEX header is used, and
the receiver activates the specified ESC modes before reading the
following data subpacket.
The receiver sends a ZZZZAAAACCCCKKKK header in response, optionally containing the
__________
2. If the receiver specifies the same or higher level of escaping, the
ZSINIT frame need not be sent unless an Attn sequence is needed.
Chapter 8 Rev 10-27-87 Typeset 10-27-87 17