W8SDZ@SIMTEL20.ARPA (Keith Petersen) (01/13/87)
For some time we've hoped that the Modem7 batch protocol would be
defined in easily-understood terms (which might help in finding how to
fix the infinite loop that happens if there is an error when sending
the filename). I found this in a file called VAX-XMDM.ARC (available
in the SIMTEL20 PD:<MISC.VAXVMS> directory, an improved XMODEM (ver.
5.53) for VAX-VMS which does (optional) batch file transfer and
internal TEXT conversion.
--Keith
Batch protocol for MODEM7 and XMODEM multi-file transfer
Each must specify batch since different treatment of ACK
as first character of "block" is needed.
CAN character is frequently tested for and interpreted as cancel everything
1 sec timeout on characters usually
CHECKSUM used on filename sequence even if CRC to be used in file transfer
on error during filename sequence, both return to beginning
sender returns to NAK wait
and receiver returns to NAK send
Time sequence of transfer:
SENDER RECEIVER
waits for NAK with 80 sec timeout
clear checksum
NAK at 10 sec interval 'til ACK or CAN
ACK
waits for filename chars 1 sec timeout
on error goes to above NAK
Loop 11 times (though filename buffer
allows more)
(Strip parity bit)
send next filename character
(in CP/M FCB form, no dot,
blank filled, last 3 bytes
are type)
add to checksum
await ACK 1 second timeout
add to checksum
ACK
End Loop
EOF (CTRL-Z)
add to checksum
add to checksum
send Checksum byte
await ACK
(note: if receives noise here,
one end may proceed to
normal file transfer, and other to
restart filename and hang)
verifies checksum
OKNMCH (ACK) or BDNMCH ("u")
Normal file transfer starts
NAK or "C"
SOH start of block
.
.
.
EOT end of file
ACK
expects NAK to start next filename
NAK
ACK set up receiver for filename
EOT as first char of filename
terminates batch transfer
(possibly as ANY character
of filename)