Let’s dive deep into what makes field buses unique compared to each other. Building on our previous blog post, What Makes Field Bus Communication Unique, where we compared field bus protocols to digital ones, consider this your guided tour—no backpacks required!
Both RS-485 and CAN share many features. Their differential signaling enables longer distances, faster data rates, and lower power usage. CAN and RS-485 use termination resistors on A and B pins—120 ohms for CAN and 100-120 ohms for RS-485—to main signal integrity and reduce reflections.
RS-485
Introduced in 1983, RS-485, defined in EIA/TIA-485, remains widely used due to its robustness and ease of implementation. RS-485 is a multidrop protocol, allowing multiple devices to share a bus, though only one driver may transmit at a time to prevent collisions. These transceivers can operate in both half-duplex mode, where communication occurs in one direction at a time, and full-duplex mode, where data flows simultaneously in both directions, allowing flexibility for different applications.
To ensure stable operation, EIA/TIA-485 specifies a common-mode voltage range of -7V to +12V, improving resilience against electrical noise and ground potential differences. RS-485 receivers can detect voltage differences as small as 200mV, allowing precise data interpretation based on the polarity of the A and B pins, where A > B results in a logic high, and A < B results in a logic low.
The standard also defines maximum data rates for different distances, balancing speed and range. 10 Mbps supports up to 100 meters, while 100 kbps reaches 1.2 kilometers, ensuring stable long-distance communication.
Controller Area Network (CAN)
The Controller Area Network (CAN) protocol was first introduced by Bosch in 1986 to enable efficient communication between electronic control units in vehicles. In 1993, the International Organization for Standardization (ISO) standardized CAN as ISO 11898, further solidifying its role in automotive and industrial applications. ISO 11898 defines the common-mode voltage range as -2V to +7V, though in many applications, CAN transceivers maintain a VCC/2 common-mode voltage through split termination.
CAN’s differential signaling differs slightly from RS-485. In a dominant state (logical 0), CAN_H is 3.5V relative to ground, while CAN_L is 1.5V. In a recessive state (logical 1), both bus lines settle around 2.5V due to the split termination and the drivers being in high-Z. CAN is a multi-controller protocol that allows nodes to be added or removed without recalibrating the bus. Arbitration ensures the node sending the first dominant signal wins, meaning the lowest-address node has the highest priority and can transmit to prevent collisions from occurring on the bus.
ISO 11898 supports 1 Mbps over 40 meters, but CAN FD extends this, reaching 5 Mbps while remaining compatible with standard CAN. To accomplish this compatibility, CAN FD mainly operates at the higher protocol level instead of changing the physical layer. Faster speeds reduce cable length, with 2 Mbps supporting 40 meters and 5 Mbps shortening the range further for CAN FD.
Reflections and Takeaways
As we close our field trip, we have learned some of the basics of RS-485 and CAN. Both allow multiple transceivers on the same bus, providing a cost-effective and scalable communication system. By leveraging differential signaling, both protocols decrease power consumption, enhance signal integrity, improve long-range performance, and ensure reliable data exchange at faster speeds, making them extremely valuable for modern applications. Stay tuned for our next field trip to explore why increased fault tolerance is essential in modern applications.
See the blogs in the TranscendingConventionalFieldBus series.