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MODEM.DOC
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1984-04-29
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54 lines
DOCUMENTATION OF THE CP/M MODEM PROGRAM
by Ward Christensen
MODEM is a program for transferring files between CP/M systems.
Files are sent a sector at a time. The program prints mes-
sages showing how things are going. A hexadecimal sector
number is printed, in a message: "WAITING FOR SECTOR NN" on
one system, and "SENDING SECTOR NN" on the other system.
Users of the program should list it and study it if they
are interested in learning more about it. At a minimum, it
must be edited and modified for the particular serial
interface used for the modem. As it stands, it is for my
MITS dual serial I/O board, ports 4 and 5.
To use the program, one user types:
MODEM SEND filename.filetype
The other user types:
MODEM RECEIVE filename.filetype
(SEND and RECEIVE may be abbreviated S and R)
The program has an error counter. Each block is tried up
to 10 times. If 10 occur, it asks if you want to retry
or quit. If you are near the beginning of a file, you
might want to quit, and re-dial the phone to get a better
connection. If you are further along in the transmission,
you would likely want to retry. Note that the program does
not close the file if you say quit.
There is one known design error in this program: After the
file is sent, the sending program sends a single EOT character.
The receiving program, upon receiving an EOT, takes closes
the file, and returns to CP/M. This is normally not a problem,
but means that if the receiving program gets a glitch when
it is looking for the start of a block, it will shut down
early. Also if the valid EOT from the sender is glitched,
the receiving program will wait, eventually timing out.
I have nearly completed a version of the program which does
better handshaking at EOF, which closes the file if you
quit, and which allows a 'second logical channel' allowing
keyboard-to-keyboard communications in parallel with the
file transfer. Thus, you could type "I'll send you spat.com
when this is done, OK" and this message will be sent after
the next data block is sent or received. The other user
will see the message and can reply.
Ward Christensen
c/o CACHE
Box 52
South Holland, Il. 60473