Allen-Bradley DeviceNet 1784-PCIDS, Руководство пользователя, Страница: 19 / 101

Publication 1784-6.5.30 - February 2001
Before You Begin 1-5
Communicating with Your
Devices
Your scanner 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 scanner and made available to your host
application. Data received from your host application, or output data, is
organized in the scanner and sent on to your devices.
IMPORTANT
Throughout this document,
input
and
output
are
defined from the host application’s point of view.
Output is data sent from the host application
to
a
device. Input is data collected by the host application
from
a device.
All data is sent and received on a DeviceNet network
in byte lengths. A device may, for example, produce
only two bits of input information. But since the
minimum data size is one byte, the two bits of
information are embedded in a full byte of data sent
by the device. In this case, the other six bits are
insignificant.
A strobe message is a
multicast transfer of data
(which is 64 bits in length)
sent by the scanner 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 scanner 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.
1784-PCIDS Card
DeviceNet Devices
Input Data Storage
Input Data From
DeviceNet Devices
B
A1
C
D
A2
E
A1 A2
B
C
D
E
X
Y
Z
X
Y
Y
Data from a single device can
be mapped to separate scanner
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
word A, stored as separate
bytes A1 and A2
Output Data Storage
Output Data To
DeviceNet Devices
output from
the host application
input from the
devices to the
host application
Word
0
1
2
3
4
5
6
E
Z
Y
Y
Y
Y
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