TriContinent C24000, Product Manual

Page: 108 / 144

OEM communication protocol
Reference information
108
Pump address
The pump address is specific, selected on each individual pump, as
described in the instructions for that pump.
Sequence number/repeat flag
The sequence number is a single byte that conveys both a sequence
number (legal values: 0 through 7) and a bit flag indicating that the
command block is being repeated due to a communications breakdown.
The sequence number is used as an identity stamp for each command
block. Since it is only necessary that every message carry a different
sequence number from the previous message (except when repeated),
the sequence number may be toggled between two different values
(e.g., 1 and 2) as each command block is constructed. During normal
communication exchanges, the sequence number is ignored. If, however,
the repeat flag is set, the pump compares the sequence number with
that of the previously received command block to determine if the com-
mand should be executed or merely acknowledged without executing.
Note: If the operator chooses not to use the error detection option, the
sequence number can be set to a fixed value of 1 (31h).
The following two scenarios clarify this error detection mechanism.
Scenario 1
1.
The computer sends a command block stamped with sequence number 1
to the pump.
2.
The pump receives the command, sends an acknowledgment to the PC,
and executes it.
3.
Transmission of the acknowledgment message is imperfect; the PC does
not receive it.
4.
The PC waits 100 ms for the acknowledgment, then retransmits the com-
mand block with the sequence number left at 1 and the repeat bit set to
indicate a retransmission.
5. The pump receives the transmission, identified as such by the repeat bit.
6.
The pump checks the sequence number against that of the previously
received command block. Noting a match, the pump sends an acknowl-
edgment to the PC, but does not execute the command (since it has
already been executed as indicated by the repeat bit).
7.
The PC receives the acknowledgment and continues with normal com-
munications.
8.
The next command block is stamped with sequence number 2 to indicate
a new command.
Scenario 2
1.
The computer sends a command block stamped with sequence number 1
to the pump.
2.
The pump never receives the command due to a communication error
and thus does not send an acknowledgment to the PC.