7
2.8
CANopen Messaging Protocol
Overview
Since the EM133 transfer buffers only support 32 bytes or less transfer blocks, all of the
device registers cannot possibly be sent over CANopen every scan. However, a messaging
mechanism allows a master to access all the device registers by placing a command
request pointing to the requested items in the output buffer and by receiving a
corresponding response in the input buffer. The CANopen master can access these output
and input buffers by writing to and reading from CANopen manufacturer-specific objects
using the PDO (Process Data Objects) communication mechanism with 11-bit CAN-ID (see
“Master Request Block” and “Device Response Block” below).
Since CANopen supports cyclic data transfer between a master and a slave that is suitable
for time critical data, a master command is sent constantly from a CANopen master to the
EM133. The device always handles read requests and the response block is normally
updated every 2ms (Inhibit time = 0). however, a write request is handled only once, that
is, the device ignores duplicate write commands. This means that a write command sent
multiple times would be handled only once. To send a write command once again, toggle
the synchronization bit in the control word (see “Master Request Block” below) so the
command frame would look different.
Data Types
Data transferred through the CANopen input and output buffers is represented in big
endian format: the bytes are ordered from least significant at the highest address to most
significant at the lowest address. Data is always transferred in integer format. Negative
numbers are represented in 2-complement code.
A CANopen master can request data either in 16-bit word format (unsigned UINT16 or
signed INT16 integers), or in 32-bit double word format (unsigned UINT32 or signed
INT32 long integers). 32-bit integers are sent in two adjacent words – most significant at
the lowest address. Up to 14 words can be read or written by a single master request –
14 data items using 16-bit format or 7 data items using 32-bit format.
The value range allowed for 16-bit data is 0 to 65535 for unsigned numbers and –32768
to 32767 for signed numbers. If the requested data exceeds a 16-bit word range, it is
truncated to the maximum allowable negative or positive number and the “over-range
error” exception code is set in the control word. When over-range occurs, an unsigned
value is reported as 65535, a positive signed value as 32767 and a negative signed value
as -32768. To avoid over-range errors with 16-bit integers, linear scaling may be applied
to analog data registers (see “16-bit Scaled Data”). Scaling cannot be used with counters
and bit-mapped binary registers. Whether scaling is to be used or not is specified in the
CANopen master request.
Data Addressing
Device data registers are addressed using 16-bit point identifiers (ID) that are given
throughout this guide in four-digit hexadecimal notation.
In the master request and the device response messages, data addresses are defined by
two parameters: a start point ID that opens a data block being read or written, and a
word count that specifies the number of consecutive words in the block.
Master Request Block
The request block size can vary from 4 to 32 bytes. Request formats for reading data
from and writing data to the device are shown in Tables 2-1 and 2-2. The first two words
contain a control word and a point ID (word size) for the first data item being accessed.
The requested data block size is specified in 16-bit words. In the event of 32-bit data, the
number of words must be even and would be twice the number of items being read or
written. Uneven word count will cause an “Invalid address” exception.
