Control Area Network
Summary: Control Area Network (CAN), sometimes called CAN Bus, is the chosen networking method for Layout Command Control.
For this article and to be consistent, the term CAN will be used for Control Area Network.
History of CAN and LCC
CAN found adoption with the automobile industry.
The reason for this is the bus is noise tolerant, an industry standard, and designed for the 12/24V world.
CAN can operate over a wide speed range, with a linear trade off between bus speeds and bus lengths. The OpenLCB engineers selected a 125Kb rate and 1000’ maximum bus length as a compromise for model railroad use. Unlike other Peer to Peer systems, CAN can operate at a 100% data throughput rate with error free collision resolution.
Disadvantages of CAN
The relatively high CAN bus speed does not allow free form network design. A CAN network segment requires a linear bus with terminations at each end. Timing and other electrical limitations mean that a single CAN segment is limited to 40 or fewer physical nodes. There are solutions to expand a CAN network into multiple segments.
CAN supports several different cabling and connector standards. Some require large (and costly) connectors. Often CAN will use the DB9 connectors found on RS232 serial cables. Another CAN connector option uses RJ45 connectors and cables, as wired Ethernet does.
The OpenLCB engineers opted for RJ45 connectors because of relative low cost and availability world wide. The 4 wire pairs of an Ethernet cable also allow for additional options such as power and other signals in addition to the CAN signal pair itself.
Layout Command Control chose to use CAN for a number of reasons.
As mentioned above, the 125 kHz signalling and a maximum run of 1000 feet or 305 metres was determined to be an ideal situation for model railroading. These parameters are directly impact the cabling, which is determined by the propagation time and pulse widths.
For this to work reliably, the ethernet cabling should be made of at least 24AWG conductors. Lighter gauge wiring could impact the bus operation. The driver at the beginning of a network segment must be able to reliably drive a terminator at the other end, without going out of spec. A termination of approximately 120Ω is required to prevent reflections distorting the signals on the cable. The impedance of 24AWG wiring is such that it is possible to have a 1000' run of CAT5 wiring which will work reliably with a 120Ω termination.
Using lighter gauge CAT5 cables will work, but it will impact speed and maximum bus distance