Kvaser CAN Protocol Course: The CAN Physical Layer (Part 4)

Summary

The CAN bus system utilizes Non-Return to Zero signaling with bit-stuffing and offers two signaling states: dominant (0) and recessive (1). It supports different physical layers, such as high-speed CAN (ISO 11898-2) and low-speed CAN (ISO 11898-3). Key components include transceivers like the 82C250 and TJA1054. Maximum bus speeds vary based on the layer used, with the standard being 1 Mbps, and cable lengths are restricted by the speed of light. Proper bus termination with a 120 Ohm resistor is crucial, and the ISO 11898 standard specifies cable impedance and connector types. Some physical layers, like low-speed CAN, may have different requirements.

Takeaways

• 😀 The CAN bus uses Non-Return to Zero (NRZ) signaling with bit-stuffing and has two states: dominant (logically 0) and recessive (logically 1).
• 😀 The CAN bus operates using various physical layers, each defining the electrical levels and signaling schemes on the bus.
• 😀 The most common physical layer for CAN is defined by ISO 11898-2, which uses a two-wire balanced signaling scheme, also known as high-speed CAN.
• 😀 Low-speed CAN, defined by ISO 11898-3, uses a similar two-wire balanced signaling but supports fault tolerance in case of wire damage.
• 😀 Single-wire CAN, as defined by SAE J2411, uses one wire plus ground and is commonly used in automotive applications like GM-LAN.
• 😀 Some physical layers may not interoperate well, with certain combinations working only under good conditions.
• 😀 CAN transceivers are manufactured by companies like Philips, Bosch, Infineon, and Siliconix, with popular models such as the 82C250 for high-speed CAN and the TJA1054 for low-speed CAN.
• 😀 The maximum speed for a CAN bus is 1 Mbps, but some controllers can support higher speeds for specialized applications.
• 😀 Bus speeds and cable lengths are interconnected, with maximum lengths of 40 meters at 1 Mbps, 100 meters at 500 kbps, and up to 6 km at 10 kbps.
• 😀 Proper bus termination is essential for an ISO 11898 CAN bus, requiring 120 Ohm resistors at each end to avoid signal reflections and ensure correct DC levels.

Q & A

What are the two signaling states used in the CAN bus?

The two signaling states used in the CAN bus are dominant (logically 0) and recessive (logically 1).

What is the role of the physical layer in a CAN bus system?

The physical layer defines the electrical levels, signaling scheme, cable impedance, and other factors that ensure proper data transmission over the bus.

How do different physical layers impact CAN bus communication?

Different physical layers determine the electrical characteristics and signaling methods on the bus. Some layers, like ISO 11898-2 (high-speed CAN), use two-wire balanced signaling, while others, like SAE J2411, use a single-wire configuration.

Can different physical layers on a CAN bus interoperate?

Typically, different physical layers cannot interoperate due to differing signaling standards. While some combinations might work under specific conditions, most do not.

What is the maximum speed of a CAN bus?

The maximum speed of a CAN bus, according to the standard, is one megabit per second (1 Mbps). Some controllers can handle higher speeds for special applications.

What are the maximum cable lengths for different CAN bus speeds?

At 1 Mbps, the maximum cable length is 40 meters. At 500 kbps, it's 100 meters; at 250 kbps, it's 200 meters; at 125 kbps, it's 500 meters; and at 10 kbps, it's 6 kilometers.

Why is bus termination required for an ISO 11898 CAN bus?

Bus termination using a 120-ohm resistor is required to prevent signal reflections and to maintain correct DC levels. This ensures stable communication over the bus.

What types of connectors are used for CAN buses?

Common CAN bus connectors include the 9-pin DSUB, 5-pin Mini-C and Micro-C, and the 6-pin Deutch connector, each typically associated with specific higher layer protocols like DeviceNet and SDS.

How does the CAN bus handle different physical layer requirements?

While ISO 11898 prescribes a twisted-pair cable with an impedance of 120 ohms, variations in cable impedance are allowed within a range of 108 to 132 ohms. The requirements may vary based on the physical layer being used.

What is the impact of using optocouplers in CAN bus communication?

Using optocouplers for galvanic isolation can reduce the maximum bus length, especially at higher speeds. It’s crucial to select fast optocouplers and consider their delay characteristics.