MODEM64: DOCUMENTATION
BY CHRIS LAMPTON, 75275,1373
01/29/84


    MODEM64 IS A SIMPLE TERMINAL
EMULATOR WITH XMODEM DOWNLOAD FACILI-
TIES, DESIGNED TO RUN UNDER COMMODORE
64 CP/M. IT WILL NOT RUN UNDER OTHER
VERSIONS OF CP/M; IN FACT, IT WILL ONLY
RUN UNDER THE CP/M PACKAGE OFFERED BY
COMMODORE BUSINESS MACHINES.
    WHEN FIRST RUN, MODEM64 IS IN
NORMAL TERMINAL MODE. ASCII DATA RE-
CEIVED THROUGH THE RS-232 PORT IS DIS-
PLAYED ON THE SCREEN. DATA TYPED ON THE
KEYBOARD IS TRANSMITTED. PARITY IS
IGNORED. MOST STANDARD ASCII CONTROL
CODES ARE IMPLEMENTED. TO EXIT MODEM64
PRESS FUNCTION KEY F2 (SHIFT-F1).
XMODEM DOWNLOADS ARE INITIATED BY
PRESSING FUNCTION KEY F8 (SHIFT-F7).


USING MODEM64 WITH AN RCP/M
(REMOTE CP/M DATABASE)


    THE STANDARD RCP/M COMMAND FOR
XMODEM TRANSMISSION IS:

XMODEM S FILENAME.EXT

    IF THE SPECIFIED FILE IS AVAILABLE,
THE RCP/M WILL RESPOND WITH THE NUMBER
OF 128 BYTE BLOCKS IN THE FILE AND THE
APPROXIMATE DOWNLOAD TIME. IF YOU
SHOULD DECIDE AT THIS POINT THAT YOU
WISH TO ABORT THE DOWNLOAD, PRESS
CTRL-'X' TO SEND AN ASCII CANCEL
SIGNAL TO THE SENDING COMPUTER.
TO BEGIN THE DOWNLOAD, PRESS F8,
AND MODEM64 WILL AUTOMATICALLY
REQUEST THE RCP/M TO START TRANS-
MITTING. THE MESSAGE 'XMODEM TRANSMIS-
SION INITIATED' WILL APPEAR ON THE
SCREEN. IF FOR ANY REASON THE TRANS-
MISSION IS NOT RECEIVED, MODEM64 WILL
CONTINUE BROADCASTING THE REQUEST FOR
APPROXIMATELY 100 SECONDS BEFORE
ABORTING THE DOWNLOAD AND RETURNING TO
TERMINAL MODE. SHOULD THIS OCCUR, THE
MESSAGE 'BLOCK NOT RECEIVED, XMODEM
TRANSMISSION ABORTED' WILL BE DIS-
PLAYED. THE DOWNLOAD MAY BE MANUALLY
ABORTED BY PRESSING THE RUN-STOP KEY.
    ONCE TRANSMISSION HAS BEGUN, THE
DOWNLOADED BLOCKS WILL BE AUTOMATICALLY
SAVED TO THE DISK IN CP/M FORMAT. AS
EACH BLOCK IS RECEIVED, THE MESSAGE
'BLOCK #NN RECEIVED' WILL BE DISPLAYED,
WHERE NN IS THE BLOCK NUMBER IN HEXA-
DECIMAL. IF MORE THAN 255 (FF HEX)
BLOCKS ARE TRANSMITTED, THE BLOCK
NUMBER WILL ROLL OVER TO 00. CHECKSUMS
ARE USED TO VERIFY THE ACCURACY OF THE
TRANSMITTED DATA. AT THE END OF TRANS-
MISSION, MODEM64 WILL INFORM THE
SENDING COMPUTER OF THE SUCCESSFUL
RECEIPT OF THE FILE AND WILL DISPLAY
THE MESSAGE 'XMODEM TRANSMISSION COM-
PLETED' BEFORE RETURNING TO TERMINAL
MODE.
    FILES WILL BE SAVED TO THE DISK
UNDER THE FILENAME 'NEWFILE,' WITH THE
EXTENSION '.XMD'. FILES DOWNLOADED
DURING A SINGLE SESSION WILL BE SEQUEN-
TIALLY NUMBERED, WITH THE FIRST FILE
GIVEN THE NAME 'NEWFILE1.XMD,' THE
SECOND FILE THE NAME 'NEWFILE2.XMD,'
AND SO FORTH. LETTERS OF THE ALPHABET
WILL BE USED ONCE THE TEN NUMERALS ARE
EXHAUSTED. GIVEN THE OBVIOUS LIMITATION
ON FILE NAMES, IT IS RECOMMENDED THAT
YOU NOT DOWNLOAD MORE THAN 35 FILES IN
ONE SESSION, OR YOU MAY FIND UNTYPABLE
ASCII CHARACTERS IN THE NAMES.
    IT IS ALSO RECOMMENDED THAT YOU
RENAME ALL DOWNLOADED FILES IMMEDIATELY
AFTER THE SESSION, USING THE CP/M REN
COMMAND, BECAUSE EXISTING NEWFILES ON
THE DISK WILL BE DELETED BY MODEM64
DURING LATER SESSIONS TO MAKE ROOM FOR
FRESHLY DOWNLOADED FILES WITH THE SAME
SEQUENCE NUMBERS.
    IN THE EVENT THAT YOU ATTEMPT TO
DOWNLOAD TO A FULL DISK (OR A FULL DISK
DIRECTORY), AN ERROR MESSAGE WILL BE
PRINTED AND THE DOWNLOAD ABORTED. ANY
DATA DOWNLOADED TO THE CURRENT FILE
BEFORE THE DISK LIMIT WAS REACHED WILL
BE PRESERVED.
    THIS IMPLEMENTATION OF XMODEM IS
NOT BULLET PROOF. IT IS POSSIBLE FOR
THE SENDING COMPUTER AND THE RECEIVING
COMPUTER TO FALL OUT OF SYNC AND NOT
RECOVER, THOUGH THIS IS NOT A VERY
LIKELY EVENT. SHOULD IT OCCUR, THE
DOWNLOAD WILL MORE THAN LIKELY ABORT
BY ITSELF. HOWEVER, SHOULD YOU NOTICE
AN UNUSUALLY LONG PAUSE BETWEEN BLOCKS
-- SAY 20 SECONDS OR MORE -- YOU SHOULD
ABORT MANUALLY WITH THE RUN-STOP KEY.
THE SENDING COMPUTER MAY CONTINUE
BROADCASTING DATA, BUT WILL NOTICE
WITHIN A FEW SECONDS THAT NO ACKNOW-
LEDGING SIGNAL IS BEING RECEIVED AND
WILL CANCEL THE DOWNLOAD. YOU MAY THEN
INITIATE THE DOWNLOAD AGAIN. THIS HAS
HAPPENED TO ME ONCE IN ROUGHLY 40
DOWNLOADS. SECURITY WILL BE TIGHTENED
IN SUBSEQUENT VERSIONS.
    SUGGESTIONS FOR FUTURE IMPROVEMENTS
AND REPORTS OF CURRENT BUGS SHOULD
BE CONVEYED TO THE AUTHOR VIA COMPU-
SERVE INFORMATION SERVICE EMAIL OR THE
CIS COMMODORE-64 SIG.