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Added 'Linux, Telnet, Zmodem and binary files' section to README

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mecparts
2020-12-02 22:31:17 -07:00
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README.md
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@@ -64,6 +64,43 @@ the first complete unit and while I don't expect the hardware to change
tweaking and adding to that for some time to come. However, it's
completely usable at the moment.
### Linux, Telnet, Zmodem and downloading binary files
Have you used the modem to 'dial' into a Linux box? And have you done a
`sz binary_file` on the Linux box? And at a completely reproducible
point in the file, has the connection dropped? But other binary files
work just fine? Then read on.
This drove me slightly batty for months. I finally narrowed it down to
trying to send blocks of binary data with a large number of FF bytes. I
eventually created a test file consisting of 2K of FF and used that to
test with. I could download it through the modem with Xmodem just fine.
Ymodem also worked if I kept the block size down to 128 bytes - but the
connection would drop instantly if I tried sending 1K blocks. Same thing
with Zmodem.
In fact, if I just tried `cat binary_file`, the connection would
drop. Which eventually got me thinking. Sitting at the console on my
main Linux box, I telnet'd to the same box and logged in. No WiFi modem
involved anywhere, just a telnet session on the console to the same box.
I then did a `cat binary_file`. The telnet connection dropped, and I
was back in my original session.
It's the Linux telnet daemon. Not the modem at all.
To prove it to myself, I hooked up WiFi modems to two systems on their
serial ports and had one dial into the other. I could send the all FF
binary file back and forth with Zmodem and Ymodem, no trouble at all.
But you really, really need to download that binary file through the
modem from a telnet connection to a Linux box? You're not going to be
able to use Zmodem. Ymodem will work (the sy command defaults to 128
byte blocks), as will Xmodem. But not Zmodem.
Oddly enough, the telnet daemon has no trouble *receiving* the all FF
binary file. Only sending it. Your guess as to why is probably better
than mine.
## The Hardware
![Interior](images/Interior.jpg "Interior")