Publication 1747-UM655B-EN-P - June 2007
16 Before You Begin
Communicating with Your
Devices
The 1747-SDN module communicates with a device via strobe, poll,
change of state, and/or cyclic messages. It uses these messages to
solicit data from or deliver data to each device. Data received from the
devices, or input data, is organized by the 1747-SDN module and
made available to the processor. Data received from your SLC 500
processor, or output data, is organized in the 1747-SDN module and
sent on to your devices.
IMPORTANT
Throughout this document, input and output are defined from
the SLC 500 processor’s point of view. Output is data sent from
the SLC 500 processor to a device. Input is data collected by the
SLC 500 processor from a device.
All data sent and received on a DeviceNet network is in byte
lengths. A device may, for example, produce only two bits of
input information. Nevertheless, since the minimum data size
on a DeviceNet network is one byte, two bits of information are
included in the byte of data produced by the device. In this
example (only two bits of input information), the upper six bits
are insignificant.
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