XMODEM47.DOC
             (revised 10/20/81)

XMODEM47 is based on the last known reliable working 
copy of xmodem, XMODEM43.  XMODEM47 incorporates John
Mahr's modifications for CRC (cyclic redundancy check)
verification of the transmitted data.  Versions 44 and
45 were known to have incorrect coding modifications
made to them, and XMODEM46 was an experimental copy
which never should have been released.  Its operation
for all conditionals and options was never verified.

Below is some modification history of XMODEM.

******************************************************


[John Mahr's XMODMX41.ASM as modified by Keith Peter-
 sen.  The modifications, according to Keith, are
 relatively minor.  But the difference between this
 and XMODEM41.ASM is fundamental and parallels the
 difference between MODEM527 and its successor,
 MODEM206 -- XMODEM42 and MODEM206 both use the new
 extended version of the Christensen protocol, and
 have now been worked into relatively final form.]  

This program is an extension of Keith Petersen's
XMODEM program.  It adds checking for framing errors,
overruns, and parity errors.  These checks will only
be made if a PMMI modem is being used.  These checks
were added because sometimes the checksum does not
catch all transmission errors.  Hopefully, by checking
for framing errors in particular, all transmission
errors will be caught.  This program does not get rid
of the checksum, but rather adds the framing error,
overrun, and parity error check to that function.  The
parity error check is only meaningful if parity is being
used.  If parity is not being used, the check will still
be made, but no parity errors will ever be found.

Whenever a framing error and/or overrun is detected on a
file receive, the following takes place:
     1. Purge the line (receive rest of sector and throw
	it away).
     2. If error limit has not been exceeded, send a
	NAK so that the sender will resend the sector.
     3. If error limit has been exceeded, abort the
	transmission.

The capability to display the status of the PMMI on a front
panel has also been added.  The default for the front panel
is port FFh.  If you do not have a front panel, then set
the FRNTPNL EQU to FALSE.  If you have a front panel at a
port other than FFh, then change the PANEL EQU to your port
address.  The status is only displayed when data is being
by the PMMI modem.  See your PMMI manual for the meaning of
the PMMI status bits.
	- bit 0 is transmit buffer empty
	- bit 1 is input data is available
	- bit 3 is a parity error has been detected
	- bit 4 is an overrun has occurred
	- bit 5 is a framing error has occurred


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