Very Computer

Not sure this is the right group for this, but it was the closest I could find.
I am having problems sending files from a UNIX mainframe to my PC using zmodem
(I have used it extensively in PC to PC transfers and found it fast, reliable
and efficient). When I initiate the transfer from the mainframe (sz foo.bar)
and start the download on the PC end, the transfer begins but invariably starts
to generate errors (typically ZERPOS header, but also CRC ERROR and various
others). The errors start at a very consistent byte count (about 5K as I recall)
and then repeat frequently (about every 4 to 10K, though the interval is not
perfectly regular). Although zmodem will retransmit until the file gets through,
the transfer is much too slow, making it utterly impractical for large files.
The problem seems to be independent of phone lines, modem, baud rate etc. and
transfers from the PC to the mainframe work fine. I suspect that the problem
might have something to do with timesharing on the mainframe and vaguely recall
seeing a message somewhere to this effect, along with a suggested cure which I
do not recall. In case it matters, the mainframe is an HP 9000/850 running HP-UX
3.1 (very similar to system V). Any ideas for a cure would be appreciated! I
know there could be many other factors at issue and am mostly wondering if this

I believe the problem to be that the current versions (up to about 9 months
old) disable the handshaking on the modem port.  I see this problem
commonly on systems where the modem does a baudrate conversion.  Example,
a telebit with the port baud locked at 19.2 and the dialin port allowed to
adjust to the caller's rate.

Knowing whether or not the modem on the other end is of this type is
crucial information.

In the interim, a quick fix is to use the -w option to limit the window
size to less than the capacity of the modem buffer.  That way the computer-
modem link stops and waits for ACKs.  You won't see any degradation in
performance on the 2400 baud line because of this, as the ack will be
returned before the buffer completely clears.

In the meanwhile, I'm working on an implementation of zmodem  for a
commercial application in my work and will be looking at this problem
in detail.

John

--
John De Armond, WD4OQC  | We can no more blame our loss of freedom on congress-
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