IMC CANSAS - Page 216

216 Measurement Technique
imc CANSAS Users Manual - Doc. Version 1.9 - 05.12.2014© 2014 imc Meßsysteme GmbH
Other factors influencing the delay time:
Other message structures. E.g. Channel 1 and Channel 16 in one message for a module (like the
SCxx modules) using a multiplexer. Since Channel 1 is sampled at the beginning and Channel 16
really at almost the end of a sampling interval, the message can only be outputted at the end of the
sampling interval, thus delaying Channel 1 significantly, and Channel 16 almost not at all. For quick
response, only adjacent channels (e.g. 1 through 4) should be placed in a message.
Virtual channels are always calculated after a delay. For swift response, no virtual channels should
be packaged in a message.
Synchronicity: In synchronized mode, old messages are outputted on the CAN-Bus following
disturbances on the bus so that they can be collected in chronologically correct order without data
loss by the data acquisition device. For swift response, the synchronized mode should not be used if
bus disturbances are to be expected.
Choice of CAN identifier: Messages for which the user wishes to have rapid responses should have
lower CAN-identifiers (and thus higher priority) than other existing CAN-messages.
Choice of sampling interval. Observe the tables in chapter 6 and their explanations.
The specifications in the tables in chapter 6 are valid for:
o For SCxx modules: Use the same sampling time for all channels.
o Only position channels with the same sampling rate in a single message. Only then are the
specifications in the table valid.
With some module types, (such as SCxx modules), the filters can be deactivated. If the filters are
deactivated, the delay times are correspondingly shorter. This is because the low-pass filters used
cause delay. Only use of the filters ensures the validity of the spec sheet data concerning signal
noise, suppression of common mode frequencies and synchronization of sampled values. If the
filters are deactivated, the delay time is reduced, but in that case it's the user's responsibility to
measure signals which are good enough not to need filtering and also that the synchronization of
multiple channels doesn't play any role.