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4.3.3 CAN Protocol.
Two separate protocols are available from the setup – either multiple or single message. For the
former, the microDAQ is allocated a fixed message for each group of 4 pressure channels, ie for a
64 channel scanner, 16 discrete CAN message identifiers are required. Alternatively for a more
economical use of identifiers within a system, a single message id may be used, with all channels
sent over this message sequentially. Channel numbers are positively identified with a channel
identifier byte within the message.
For the multiple message option, data are sent on 8 byte messages, 4 channels per message – the
pressure channels associated with a particular message number being fixed. The 2 most
significant digits of the message identifier are user selectable via the setup program – so for
example the identifier for channels 1-4 for user selected 0x22n would be 0x220, and the identifiers
for channels 5-8, 9-12 etc. follow on incrementally from this first identifier as shown in Figure 4.3. It
is the user’s responsibility to avoid overlap of message identifiers in a multiple microDAQ
installation.
Data are two bytes unsigned, scaled to +/- full scale, ie 0x0000 might represent –5psi, 0xffff +5psi.
The data are user selectable as 16 bit big or little ended. An example of the packing is illustrated in
Figure 4.3. Use of the 16 bit little ended option is recommended as it requires least processor
overhead within the microDAQ unit.
The single message identifier option is included to reduce the number of message id’s required
within a system. Data are packed in 7 byte messages as 3 channels per message with one
channel identifier byte. This identifier byte is incremented for each message in the sequence,
starting at 0x00 for channels 1,2 3, then 0x01 for channels 4,5,6 etc. Odd leftover channels (eg 16
channels/3 = 5 full messages plus a message with a single channel and two ‘leftovers’) are set to
0x0000 and should be discarded. Figure 4.4 shows the structure of a message for the single
message identifier protocol. A selectable inter message delay of between 1ms and 200ms is
available to match message generation timing to the user’s system. Note that it is the user’s
responsibility to select appropriate delays with respect to channel rate and CAN bus baudrate.
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General