Software Communication 3-5
Pump Address
The pump address is a hexadecimal number specific for each pump.
Sequence Number/Repeat Flag
The sequence number is a single byte that conveys both a sequence number (legal
values: 1 to 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
carries 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 command should be executed or
merely acknowledged without executing.
NOTE If the operator chooses not to use this option, the sequence number can be set
to a fixed value of 1 (31h).
The following two scenarios should clarify this error detection mechanism.
Scenario 1.
1 The computer sends a command block stamped with sequence #1 to the pump.
2 The pump receives the command, sends an acknowledgement to the PC, and
executes it.
3 Transmission of the acknowledgement message is imperfect; the PC does not
receive it.
4 The PC waits 100 ms for the acknowledgement, then retransmits the command
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 acknowledgement to the
PC, but it does not execute the command (since it has already been executed).
7 The PC receives the acknowledgement and continues with normal
communications.
8 The next command block is stamped with sequence #2 to indicate a new
command.
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