MDR-2000 VJ User's Reference Manual
Communications
The packet consists
of
the data message encapsulated in the protocol. The data is preceded by two
SYN (ASCII 16H) characters, an 8-bit byte count, a 3 byte modulo 1000 sequence number, and the
Customer Number which is usually '00'. The data follows the byte count and it
is
followed by 2 8-
bit characters containing a 16-bit checksum
of
the data.
The first byte in the checksum
is
the low order digit and the second byte the high order digit. The
highorder bit (bit 7)
of
both checksum bytes
is
always 'O'. The checksum is an arithmetic sum
of
the data portion
of
the packet.
If
either byte calculates to a '
ODH'
then that byte
is
incremented
by
'1
'.
The last byte in the message is always a Carriage-Return which is used as a record terminator in
packet-switched networks.
In the case
of
blocks ofrecords transmitted
by
the 'XB' command, each record retains the same
format described above. This includes the two SYN characters and the terminating Carriage-Return
on
each record
in the block. It
is
the responsibility
of
the downstream processing facility to process
each record in the block.
All retransmission
is
handled by the host system, as it is responsible for testing the checksum and,
because the MDR-2000 never initiates communication, it is the host system's responsibility to
handle error conditions by requesting retransmission.
In
the case
of
CDR
data the data collection routine should check the first character for a '
X'
(Normal expanded record) or
'B'
(Bad record) and take appropriate action. When the record is
stored, a record terminator (Carriage-Return/Linefeed) may have to be appended depending on the
recording device and method.
A collection routine can determine
if
the information from the MDR-2000 is CDR or image data by
examining the ID Type following the Customer Number.
If
this is an
'I'
then the remainder
of
the
record constitutes the image data received from the PBX (or other device).
4-6
To Next Page
Loading...