Rockwell Automation 1760-DNET Pico DeviceNet Communications Module User Manual - Manual (Page 34)

Publication 1760-UM003A-EN-P - September 2005
4-10 DeviceNet Functions
• Time
• Image data
• Function blocks (counters, timers, analog value comparators,...).
General Method of Operation
The general method of operation with the 1760-DNET should be
presented in the following. The acyclic data transfer is realised with
the aid of explicit messages. The function blocks of the Pico basic unit
can be addressed via the service code = 32
hex
. The assigned attribute
ID is here used to distinguish between different parameters and
functions.
Digression:
DeviceNet based on the standard CAN protocol and therefore uses an
11 bit message identifier. As a result 2
11
= 2048 messages (000
hex
-
7FF
hex
) are distinguishable. Six bits are sufficient for identification of a
device as a DeviceNet network is limited to a maximum of 64 stations.
These are referred to as the MAC-ID (device or node address).
Four message groups of differing sizes are available to suit the
utilization model.
In DeviceNet language terms the CAN identifier is referred to as the
Connection ID. This is comprised of the identifier for the message
group (Message ID) and the MAC ID of the device:
• The source and target addresses are possible as the MAC ID; the
definition is dependant on the message group and message ID.
• The significance of the message is defined in the system with the
message ID.
Four message groups are available in the DeviceNet world. The
1760-DNET uses message group 2. This group uses 512 CAN
identifiers (400
hex
- 5FF
hex
). Most of the message IDs defined for this
group are optional and defined for use of the ‘Predefined Master/Slave
Connection Sets’. A message ID is used for network management. The
priority is primarily determined by the device address and then by the
message ID. If the bit position is examined in detail, you will find that
a CAN controller with an 8 bit mask is capable of filtering out its
group 2 messages.
Service Code Object Address
Class ID Instance ID
32
hex
64
hex
01
hex