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fatal with ZMODEM, but it does slow error recovery.
To manage the window size, the sending program uses ZCRCQ data
subpackets to trigger ZACK headers from the receiver. The returning
ZACK headers inform the sender of the receiver's progress. When the
window size (current transmitter file offset - last reported receiver
file offset) exceeds a specified value, the sender waits for a
ZACK[5] packet with a receiver file offset that reduces the window
size.
Unix _s_z versions beginning with May 9 1987 control the window size
with the "-w N" option, where N is the maximum window size. Pro-YAM,
ZCOMM and DSZ versions beginning with May 9 1987 control the window
size with "zmodem pwN". This is compatible with previous versions of
these programs.[6]
__________
4. If sampling is possible.
5. ZRPOS and other error packets are handled normally.
6. When used with modems or networks that simultaneously assert flow
Chapter 9 Rev 10-27-87 Typeset 10-27-87 22
Chapter 9 ZMODEM Protocol 23
9.2 FFFFuuuullllllll SSSSttttrrrreeeeaaaammmmiiiinnnngggg wwwwiiiitttthhhh RRRReeeevvvveeeerrrrsssseeee IIIInnnntttteeeerrrrrrrruuuupppptttt
The above method cannot be used if the reverse data stream cannot be
sampled without entering an I/O wait. An alternate method is to
instruct the receiver to interrupt the sending program when an error
is detected.
The receiver can interrupt the sender with a control character, break
signal, or combination thereof, as specified in the AAAAttttttttnnnn sequence.
After executing the AAAAttttttttnnnn sequence, the receiver sends a hex ZZZZRRRRPPPPOOOOSSSS
header to force the sender to resend the lost data.
When the sending program responds to this interrupt, it reads a HEX
header (normally ZRPOS) from the receiver and takes the action
described in the previous section. The Unix sssszzzz....cccc program uses a
setjmp/longjmp call to catch the interrupt generated by the AAAAttttttttnnnn
sequence. Catching the interrupt activates the getinsync() function
to read the receiver's error header and take appropriate action.
When compiled for standard SYSTEM III/V Unix, sssszzzz....cccc uses an AAAAttttttttnnnn
sequence of Ctrl-C followed by a 1 second pause to interrupt the
sender, then give the sender (Unix) time to prepare for the
receiver's error header.
9.3 FFFFuuuullllllll SSSSttttrrrreeeeaaaammmmiiiinnnngggg wwwwiiiitttthhhh SSSSlllliiiiddddiiiinnnngggg WWWWiiiinnnnddddoooowwww
If none of the above methods is applicable, hope is not yet lost. If
the sender can buffer responses from the receiver, the sender can use
ZCRCQ data subpackets to get ACKs from the receiver without
interrupting the transmission of data. After a sufficient number of
ZCRCQ data subpackets have been sent, the sender can read one of the
headers that should have arrived in its receive interrupt buffer.
A problem with this method is the possibility of wasting an excessive
amount of time responding to the receiver's error header. It may be
possible to program the receiver's AAAAttttttttnnnn sequence to flush the
sender's interrupt buffer before sending the ZRPOS header.
__________________________________________________________________________
control with XON and XOFF characters aaaannnndddd pass XON characters that
violate flow control, the receiving program should have a revision
date of May 9 or later.
Chapter 9 Rev 10-27-87 Typeset 10-27-87 23