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Modem Protocols
Modem protocols- That's going way back.
IIRC, Xmodem was developed by the legendary Ward Christiansen, an early
developer of modem type programs for CP/M computers.
Having two computers talk together could be painful in the old days. (still
can be today.) Non-standard implementations of the RS-232 so-called
"standard". Getting the cable right, and the baud rate, and hardware
handshaking (if any) right and software handshaking(X-ON/X-OFF). And some
won't work reliably above a certain baud rate on a given machine.
Handling of control characters, (which can include multi-character
sequences), video terminal codes, needed delays at end of lines while the
screen scrolled and could not receive characters.
While you may establish limited serial connection on an Apple II with an
IN#2, PR#2, it is limited. And implemented inconsistently among members of
the Apple II family. A socalled "Modem" program, (referring to the device to
allow communication over telephone) which may be used without a modem, is
needed for high speed file transfer.
Many such programs talk directly to the hardware (usually the 6551 ACIA) in
a super-serial card. Many of these programs may not work with a IIgs, which
uses a different serial chip.
Early protocols were for sending data in 128 byte chunks, doing error check,
and retrying if bad.
Differences in Xmodem, Ymodem, Zmodem, dealt with thinks like size of
chunk-128 bytes, 1024 bytes, 4096 bytes?,
type of checksum, 8 bit CRC, 16bit checksum or CRC, and then batch
protocols, which allowed you to send multiple files with filenames in the
data stream. With early protocols, you had to type in the filename at both
the transmitting and receiving ends, and press <RETURN> nearly
simultaneously, or a certain order first and second.
Some files transmitted with these protocols may not work properly if they
need their filesizes to be exact (they may be padded to nearest 128 or
multiple bytes). Also in ProDOS, you may not get the right filetype.
Having the file in a shrinkit archive usually takes care of that problem.
There were many different implementations and variants of these protocols.
Especially when using different programs on each end (and different
operating systems) you may find that even though you picked the same
protocol on each end, it still may not work. I used to have to determine by
trial and error which combination of programs and protocol settings would
work (assuming you got the cable right, and the baud rate, and hardware
handshaking (if any) right. And some won't work reliably above a certain
baud rate on a given machine.
More than you wanted to know, right?
Joel
"Laine" <laine_h@yahoo.com> wrote in message
9debd907.0212030724.178eddb0@posting.google.com">news:9debd907.0212030724.178eddb0@posting.google.com...
> > > > What's the difference between all those
> > > > protocols anyway?
>
>
>
> http://www.geocities.com/SiliconValley/Peaks/4392/ftp.html