DSZ ZMODEM problem

Sat Mar 23 07:32:58 AEST 1991

jim at crom2.uucp (James P. H. Fuller) wrote:
> 
>     I'd like to hear about this also.  I have Forsberg's rz and sz compiled
> for ISC Unix rel.2.2  (SysVr3.2) and whenever people call from DOS machines
> and try to download stuff using dsz they get lots of timeouts and error re-
> covery and a very low transfer rate.  My modem is a T2500 with the registers
> optimized (as best I can) for unix/uucp.  Also my rz/sz is fairly old, since
> it's what came as a freebie on the disk Omen Tech sent me when I registered
> dsz-for-DOS a couple of years ago.  Would a newer rz/sz help, or is it my
> modem settings or something else?  My serial card has only a 16450 UART, but
> all the errors happen even at 2400bps, so it doesn't seem that the lack of
> a buffer is the problem.

Ahh.  I run a number of UNIX and MINIX (yes!) machines over here (work and
home), and I also observed funny behaviour with rz/sz ...

I use Telebit Trailblazer modems here, and I noticed that when _downloading_
from the UNIX systems (Interactive IX/386, BTW), all went well: just a 
continuous byte stream coming in.

However, when _uploading_ a file from any MINIX system, I found that
Zmodem was waiting for ACKs every 1024 bytes.  When using 19200bps-or-
above links, this means a drop in performance of about 50% :-(

I then installed the 3.06 version of rz/sz (I was using something like
1.18), but that didn't do any good.  Then I noticed a little remark
in the sources about non-blocking I/O on UNIX-ish systems...

It turned out, that when I recompiled the programs with my version of
non-blocking I/O (I implemented the TIOCICNT call in MINIX's ioctl(2)),
all went perfect....

So: when rz/sz is _slow_, you may want to recompile with non-blocking
    I/O enabled.

    when rz/sz is _bad_, you may suffer from a bad handshake with the
    modem(s).  Many UNIX systems do not properly handshake with the
    modem(s) (with RTS/CTS and/or XON/XOFF), which, with high-speed
    modems, or modems with an interspeeder, causes data loss...

Hope this helps,

Fred van Kempen
<waltje at uwalt.nl.mugnet.org>