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1-4
Before You Begin
Communicating with
Your Devices
A strobe message is a
multicast transfer of data
(which is 64 bits in length)
sent by the 1756-DNB
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 1756-DNB 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.
Publication 1756-6.5.19 - May 2000
The 1756-DNB 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 1756-DNB module and made available
to the controller. Data received from your Logix5550 controller, or output
data, is organized in the 1756-DNB module and sent on to your devices.
IMPORTANT
Different portions of data from
a single device can be mapped
to separate 1756-DNB module
memory locations. For example,
"On/Off" values can be mapped
to one location, diagnostic
values to another, etc. This is
known as "map segmenting."
This concept is illustrated by
,
byte A, stored separately as
segments A1 and A2.
Throughout this document, input and output are defined
from the Logix5550 controller's point of view. Output is
data sent from the Logix5550 controller to a device. Input
is data collected by the Logix5550 controller 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.
1756-DNB Module
Input Data Storage
Byte
A1
0
B
1
C
2
input from the
A2
3
devices to the
4
D
controller
E
5
E
6
Output Data Storage
X
output from
Y
the Logix 5550
Y
controller
Y
Y
Z
DeviceNet Devices
Input Data From
DeviceNet Devices
A2
A1
B
C
D
E
Output Data To
DeviceNet Devices
X
Y
Y
Z
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