User guide

2.7 Communication Objects
The data-byte units transported through a CAN network are called communication
objects (COBs).
SimplIQ servo drive uses the following COB types:
COB Type Description
Service data object (SDO) SDO messages are used to manipulate OD objects
according to their IDs. The server receives the SDO,
which specifies in its message which object is to be
dealt with.
SDO messages can be chained to form a “domain
transfer,” which is useful for sending large data items
such as long strings. Domain transfers are time-
consuming, because the CAN bus is half-duplex. Each
time a data segment is downloaded, a full-sized data
segment is uploaded for verification, and vice versa.
Process data object (PDO) PDO messages are used to manipulate OD objects
without explicit reference to the object identifier, which
is possible if there is an a-priori convention concerning
the OD item referenced. Such conventions are called
“PDO mappings”; these are actually OD objects
themselves, and may be defined and manipulated using
an SDO.
Emergency (EMCY) Emergency messages are used by the servo drives to
warn of an exception. The EMCY is the only COB type
that a servo drive may transmit without first being
explicitly asked. EMCY objects are similar to servo
drive “interrupts”: they eliminate the need to poll the
servo drive continuously for the emergency status.
Network Management (NMT) NMT objects are used by CAN clients to initialize a
servo drive as a server.
Layer Setting Service (LSS) This service is used to assign IDs and baud rates to
newly-installed products.
Table 2-1: Communication Objects Used by SimplIQ Servo Drives
The type of COB transmitted is indicated in the arbitration field of the message, and
thereby determines its priority. The relation between bits 8 to 11 of the arbitration field
(COB-ID) and the COB type is presented in the following table:
CANopen DS 301 Implementation Guide CANopen Basics
MAN-CAN301IG (Ver. 2.1)
2-3