Allen-Bradley SLC 500 DeviceNet, User Manual, Page: 21 / 139

Publication 1747-6.5.5 - August 2000
Before You Begin 1-5
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.
A strobe message is a
multicast transfer of data
(which is 64 bits in length)
sent by the 1747-SDN
module that solicits a
response from each strobed
slave device. There is one bit
for each of the possible 64
node addresses. The devices
respond with their data,
which can be as much as 8
bytes.
A poll message is a
point-to-point transfer of
data (0-255 bytes) sent by
the 1747-SDN module that
solicits a response from a
single device. The device
responds with its input data
(0-255 bytes).
A change of state message
is a transfer of data sent
whenever a data change
occurs. A user-configurable
heartbeat rate can also be
set to allow devices to
indicate proper operation
during intervals between
data changes. This does not
solicit response data, but
may receive an acknowledge
message.
A cyclic message is sent only
at a user-configurable rate,
such as every 10 ms.