Omron C200HW-CORT21-V1, Operation Manual

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Introduction
Section 6-1
86
6-1 Introduction
Error checking CAN
Error checking CANopen
The C200HW-CORT21-V1 uses several error detection and error handling
mechanisms.
CANopen is based on the serial bus protocol of CAN (see section 1-2-1). The
data link layer of the CAN protocol combines 5 error detection mechanisms
(CRC check, frame check, Ack check, bit check, bit stuffing check). This
combination results in a Hamming distance of 6. This means that at least 6
bits in the message frame must have been disturbed to possibly remain
undetected. The overall residual error probability is extremely low and this
makes CAN-based protocols very reliable and suitable for harsh
environments.
Besides the error detection implemented in the data link layer, CANopen
adds some higher layer error handling mechanisms:
•
Emergency message
- Used to notify the status of the Unit to
other nodes on the network.
•
Heartbeat or Node / Life
guarding protocol
- Used for mutual life checking.
See sections 1-3-4-2, 3-3-2-3, 3-3-2-4
for details.
•
Error register,
Predefined error field
- Object entries that indicate the actual
error(s) and record the occurred errors
since power-on or the last reset. Another
node (or the host PLC) can read these
entries to retrieve the error information.
The C200HW-CORT21-V1 also has some manufacturer specific status
indication mechanisms:
•
Fatal PLC error handling
- Mechanism to notify the network of the
current status of the PLC.
•
Status words
- IR n+3 ~ IR n+5 indicate the current
status of the Unit to the PLC.
See section 4-3-2 for details.
•
LEDs, 7-segment display
- Visual status indication of the Unit.
This chapter will give a detailed description of these higher layer error
handling and status mechanisms. The heartbeat and Node / Life guarding
protocol, and the status words are not described in this chapter because they
are already described in detail in the referred sections.
Flow diagrams To get a better understanding of how the error handling and status
mechanisms are related, this chapter concludes with some flow diagrams
that describe the basic functionality of the Unit.