My Take On...
The Myth of CAN Bus Length Extension per Repeater

I had just received the September issue of the CAN Newsletter in November - a quarterly publication by the gatekeeper of the CAN and CANopen standard, CAN-in-Automation (CiA). It seems, delivery takes a little longer to recipients within the United States. The CAN Newsletter is nevertheless a publication worth-while to be read by everybody who is involved with Controller Area Network (CAN), may it be for the articles or just the advertisement that shows what kind of CAN products are available in the North American market.

One article, "Single chip repeaters boost CAN applications", caught my attention due to a blatantly misleading statement regarding CAN repeaters and their ability to overcome the restricted CAN bus length. Let me quote: "One of the original drawbacks of CAN was the limited physical distance over which it could operate effectively; CAN repeaters, as the newly released Amis-42700 helped overcome this issue and also opened the door to many non-automotive applications such as elevators, industrial control systems and security systems where the physical distances involved were previously prohibitive to the use of the technology."

Not am I only flabbergasted by the guts of this statement, but also by the mere fact that such a statement was published with the support of CAN-in-Automation.

As a matter of fact, repeaters do not and can not extend the length of a CAN network - unless they have managed to overcome the limits of the speed of light, in which case they should offer their invention to NASA. The physical length of a CAN bus is limited due to one important CAN feature, the bit monitoring. Each transmitting CAN node does monitor the bus level and it does that after twice the signal propagation time. Since the signal propagation speed is limited - again, to the speed of light - it takes a little bit for the signal to spread into the bus and then to return to the transmitting node. The time that it takes depends on the bus length. At a baud rate of 1 MBit/sec the maximum distance - according to CiA DS-102 - is about 25 meters (roughly 75 feet) and if you extend the bus beyond that distance you will encounter bit monitoring errors, because the signal is on the road for much longer than one bit time.

The only way to extend the bus length is to lower the baud rate, which opens the door to applications such as elevators, etc. Repeaters have nothing to do with that. As a matter of fact, repeaters - due to their internal delay time - do shorten the bus length even further, even up to several meters.

Repeaters are nevertheless not useless - there are other benefits of using them - and there are other ways to extend the reach of a CAN application under certain conditions - even from here to China.

For more technical background information read my article:
Extending the reach of your CAN application