[DoCAN] Vehicle Diagnostic Communication Part 2 [CAN 1]

[DoCAN] Vehicle Diagnostic Communication Part 2 [CAN 1] 車両診断通信
[DoCAN] Vehicle Diagnostic Communication Part 2 [CAN 1]

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The physical layer of ISO 11898, CAN, is described.
The description will be limited to the scope necessary for vehicle diagnostic communication.

Review of the standards that came out last time.

Remember last time I explained the layers of vehicle diagnostic communication and some of the standard numbers that came up there?
Let’s review just the UDS side just in case.

  • ISO11898-2 (CAN physical layer)
  • ISO11898-1 (CAN data link layer)
  • ISO 15765-2 (Network and transport layer for diagnostic communication)
  • ISO 14229-2 (Session layer of UDS)
  • ISO 14229-2 (Application layer of UDS)

Therefore, I will explain from the lower levels of the OSI model.

ISO11898-2 (CAN physical layer)

When using tools such as Vector’s CANoe, the physical and data link layers of CAN are often not a concern.
For this reason, many people do not have a good grasp of the physical and data link layers of CAN.
However, if you actually want to realize DoCAN, it is still inevitable to acquire knowledge of CAN.

If you are confident in your Japanese, you may want to read the following documents.

And Wikipedia has quite a bit more information.

The following are well-known books.
You may want to read this book as it describes general CAN connectors, pin assignments, and historical stories.

Embedded Networking with CAN and CANopen

General CAN in DoCAN

The information in Vector’s document, Wikipedia, and books is almost sufficient for information on CAN.
However, I will try to briefly explain the part that corresponds to the premise of DoCAN.

CAN itself has variations: high speed, low speed, and CAN-FD.
But in most cases, DoCAN makes the following assumptions

  • High-speed CAN
  • Baud rate is 500 kbps
  • CAN-FD is not applicable

Some of you may have wondered about the baud rate.
According to the CAN specifications, the baud rate can be up to 1 Mbps.
However, for some reason, the baud rate is limited to 500 kbps.

My guess is that there are several reasons for this.

  • The law specifies 250 Kbps or 500 Kbps, whichever is faster.
  • At 1 Mbps, the sampling point must be set farthest back to be stable.
    • The output and input sides together have a total delay of 240[ns], which is equivalent to 1/4 of 1 bit at 1Mbps.

In other words, 1 Mbps is a bit of a dangerous setting due to the physical specifications of the CAN transceiver IC.

The key word “sample point” is used here without explanation, but some people may not understand what it refers to.
I will explain “sample point” next time, since it refers to the data link layer of CAN.


  • The required standard numbers were reviewed.
  • CAN has become a relatively common specification, and you can find information there on the Internet and in books.
  • 1 Mbps is not often used in DoCAN.
    • For legal reasons
    • 1Mbps lacks stability in some aspects.

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