Relation between speed and cable length of CAN bus - Page 1
Forum section: electronics / general

author: eeguy
author_rank: Regular Contributor
timestamp: March 05, 2019, 02:37:19 am
content: Hello, as the speed increases, the max cable length decreses. What is the reason for this?

author: timelessbeing
author_rank: Frequent Contributor
timestamp: March 05, 2019, 03:14:52 am
content: https://en.wikipedia.org/wiki/Transmission_line

author: IDEngineer
author_rank: Super Contributor
timestamp: March 05, 2019, 04:36:10 pm
content: CAN bus data rates aren't fast enough to encounter transmission line effects. The fastest standard CAN data rate is 1Mbps, whose wavelength is ~300 meters... probably not too many 1Mbps 1000 foot CAN networks out there!Instead, the effects leading to the inverse relationship between bus length and max reliable data rate include:* Cable capacitance, which increases with length and affects edge timing* Delay, which increases with length and affects inter-device ACK signalingThere are many available discussions of this topic, specifically WRT the CAN standard. Here's one from TI:A signaling rate decreases as transmission distance increases. While steady state losses may become a factor at the longest transmission distances, the major factors limiting signaling rate as distance is increased are time varying. Cable bandwidth limitations, which degrade the signal transition time and introduce inter-symbol interference(ISI), are primary factors reducing the achievable signaling rate when transmission distance is increased. For a CANbus, the signaling rate is also determined from the total system delay – down and back between the two most distant nodes of a system and the sum of the delays into and out of the nodes on a bus with the typical 5ns/m prop delay of a twisted-pair cable. Also, consideration must be given the signal amplitude loss due to resistance of the cable and the input resistance of the transceivers. Under strict analysis, skin effects, proximity to other circuitry, dielectric loss,and radiation loss effects all act to influence the primary line parameters and degrade the signal.EDIT: Also read page three of this article on EETimes regarding propagation delay:https://www.eetimes.com/document.asp?doc_id=1274178&page_number=3

author: David Hess
author_rank: Super Contributor
timestamp: March 05, 2019, 05:32:25 pm
content: I had to study this for an answer because with the proper design, the slow specified speed is not limited by the physical layer.The CAN bus arbitration scheme relies on complete propagation and detection of every bit of the identifier so the maximum data rate is inversely proportional to the reflected wave (round trip) propagation time between the furthest two CAN transceivers. When two transceivers send an identification bit, only the one which has not asserted the open collector/drain bus can see the collision and only that transceiver backs off. Doubling the round trip propagation delay is roughly the minimum bit time. It looks like they actually multiplied it by 2.5 but this takes into account other factors like transceiver propagation. (1)40 Meters = 400ns round trip 1 Mbps = 1000 ns bit time1000 Meters = 10us round trip 40 kbps 25 us bit timeThere is some margin to increase the speed with a better physical interface but it could only be doubled in the best case.Note that Ethernet shared media detects collisions in a completely different way where both transceivers back off showing that the data rate is not limited by the transmission line itself but by the maximum propagation delay between nodes even under ideal conditions.(1) TI's application noteheregives the timing information on page 10. For long transmission lines, the propagation time of the transmission line dominates.

author: timelessbeing
author_rank: Frequent Contributor
timestamp: March 06, 2019, 02:02:46 am
content: Quote from: IDEngineer on March 05, 2019, 04:36:10 pmI'm not an RF engineer, but the first few sentences I read in the article I linked say that transmission lines are used above 30kHz.https://en.wikipedia.org/wiki/Transmission_line#Overview

author: IDEngineer
author_rank: Super Contributor
timestamp: March 06, 2019, 02:28:18 am
content: Quote from: timelessbeing on March 06, 2019, 02:02:46 amKeep reading further into that article you cited and you'll find this:When the voltage changes in a time interval comparable to the time it takes for the signal to travel down the wire, the length becomes important and the wire must be treated as a transmission line. Stated another way, the length of the wire is important when the signal includes frequency components with corresponding wavelengths comparable to or less than the length of the wire. A common rule of thumb is thatthe cable or wire should be treated as a transmission line if the length is greater than 1/10 of the wavelength. At this length the phase delay and the interference of any reflections on the line become important and can lead to unpredictable behaviour in systems which have not been carefully designed using transmission line theory.[emphasis mine]30KHz isn't some magic threshold. You must relate spectral content to the line length. In the specific case of CAN networks which are the subject of this thread, and as I and David Hess both pointed out above, one of the primary limiting factors is the end-to-end propagation delay and how that affects ACK's (really, bus contention) from node to node.