Date   : Sat, 14 Jan 1984 00:56:03 EST
From   : Keith Petersen <w8sdz@brl>
Subject: MDM717.MSG

TOPIC: MDM717.ASM MODEM PROGRAM

FROM : IRV HOFF W6FFC

DATE : 11 JAN 84


       NOTE:   This program when assembled is 69 sectors
               long.  Use this figure when merging the
               appropriate overlay file for your computer
               via DDT, etc.  (Most of the overlays were
               written when MDM7xx.COM was only 66 sectors
               and the example included in each says to
               store 66 sectors.)  For MDM717 use:

                       B>SAVE 69 MDM717.COM

       NOTE:   If using the phone number overlay to change
               the phone library numbers, be sure to use:

                       ORG     0D00H


     Most users will not need the lengthy (152k) source code at all.
     Just get MDM717.COM and then check one of the associated over-
     lay programs to obtain the overlay for your particular computer.
     Merge that with MDM717.COM according to the instructions near
     the start of the overlay file, using DDT.COM, etc.  (See above
     note relative to saving 69 sectors.  STAT.COM would then show
     138 records for 18k.)


CURRENT CHANGES FOR MDM717

     MDM717 allows characters with parity bit set to be properly handled
during propagation overruns after an X-off.  This occurs during a "save
to disk" after the disk buffer fills.  (This problem was noticed on Com-
puserve which sends some characters with the parity bit set.)

     The disk buffer size was restored to 16k.  This is the length of
"one file extent".  Even slow floppy disks can store 16k in a reason-
able amount of time.  This should remain 16k for distribution copies of
the source code although it can be easily changed to suit the individual
user's own preference.  (It could even be lengthened to 32k if you like
fewer disk operations.  This would make the printer buffer proportion-
ally smaller but most printers are so fast the buffer is rarely filled
in any case.)

     Fixed a stack problem introduced in v716 in the "V" flag routine to
allow the user to show ASCII characters on the CRT during a file trans-
fer.

     Fixed the "L" Logon feature so it should be consistent.  At times
it would run away without waiting for the echo characters, thus not cor-
-rectly displaying the Logon message.

     Restored the ACKNAK feature developed for the exclusive use of the
ARPANET networking group.  When set normal ("YES") it resends a disk re-
cord after any NON-ACK character is received.  This has been the normal
configuration for all RCPM systems using the XMODEM file transfer pro-
gram.  When set "NO" for ARPANET use, it resends a record only after a
NAK has been received.  Other characters are ignored.  Some systems will
resend a NAK after a 10-second TIMEOUT.  This slows things considerably,
which allows the main frame time to recover if busy.  This tends to run
the phone bill higher for RCPM use, but is necessary for ARPANET to pre-
vent aborting the file transfer too quickly if the main frame is busy.
If a normal TIMEOUT sequence does attempt to abort the transfer with the
ACKNAK equate set to NO, it will ask if you want to try again or abort.
(RCPM systems would have already timed completely out with 10 consecutive
errors, making the question worthless and misleading.  ARPANET does not
have a similar feature, and the user can manually force the transfer to
continue.)

                               - Irv Hoff