Rockwell Automation DEVICENET 1771-SDN, Руководство пользователя, Страница: 22 / 139

Publication 1771-6.5.132 - June 2000
1-6 Before You Begin
Communicating with
Your Devices
The 1771-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 1771-SDN module and made available
to the processor. Data received from your PLC-5 processor, or output data,
is organized in the 1771-SDN module and sent on to your devices.
IMPORTANT
Throughout this document, input and output are defined
from the PLC-5 processor’s point of view. Output is data
sent from the PLC-5 processor
to a device. Input is data
collected by the PLC-5 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 1771-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 1771-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.
1771-SDN Scanner Module
DeviceNet Devices
Input Data Storage
Input Data From
DeviceNet Devices
E
Y
X
Y
Y
Data from a single device can
be mapped to separate
1771-SDN 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.
Output Data Storage
Output Data To
DeviceNet Devices
Byte
,
0
1
2
3
4
5
6
Z
Y
Y
Y
output from
the PLC-5
processor
B
A1
C
D
A2
E
E
input from the
devices to the
PLC-5 processor
B
C
D
X
Z
A1 A2
Y