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