Electronic interfaces/Controller Area Network (CAN)

From LabAutopedia

1 Controller Area Network (CAN)
- 1.1 Cable and connectors
- 1.2 Laboratory usage
2 References

Controller Area Network (CAN)

The controller-area network (CAN or CAN-bus) is a computer network protocol and bus standard designed to allow microcontrollers and devices to communicate with each other without a host computer. It was designed specifically for automotive applications but is now also used in other embedded control applications . Development of the CAN-bus started originally in 1983 at Robert Bosch GmbH.^[1] The protocol was officially released in 1986 at the SAE (Society of Automotive Engineers) congress in Detroit. The first CAN controller chips, produced by Intel and Philips, came on the market in 1987. Bosch published the CAN 2.0 specification in 1991.

CAN is a broadcast, differential serial bus standard for connecting electronic control units (ECUs). Each node is able to send and receive messages, but not simultaneously: a message (consisting primarily of an ID — usually chosen to identify the message-type/sender — and up to eight message bytes) is transmitted serially onto the bus, one bit after another — this signal-pattern codes the message (in NRZ) and is sensed by all nodes.

The devices that are connected by a CAN network are typically sensors, actuators and control devices. A CAN message never reaches these devices directly, but instead a host-processor and a CAN Controller is needed between these devices and the bus.

Details of the CAN operation:

Base-band transmission. Unlike wide-band transmission technique which encode the information before transmission, base-band transmission technique transmit the information in their original format. This technique is simple and relatively cost effective. The draw back is that it cannot handle large amount of data with high speed. Therefore it is only suitable for low-level transmission. For low cost reason, CAN uses this technique.
Twisted Pair Wire. CAN uses twisted pair wire, namely CAN-High and CAN-Low, for transmitting signal. The use of this is to filter out the interference that may simultaneously acting on the bus. It is a proven technique for short distance transmission. However it is not a good technique in long range transmission due to the losses.
CSMA/CD NDBA. Carrier Sense Multiple Access with Collision Detect. Unlike conventional serial transmission technique (such RS-232) a CAN node will always sense the carrier on the network bus and transmit only when the bus is idle. In this way, it allows more than one node to be hooked-up and communicate on the bus. In some circumstances, two or more nodes may be transmitting at exactly the same time. In this case the nodes are able to detect the collision and decide the next step. Non-Destructive Bit Arbitration technique is used in CAN in which nodes with the lowest identifier will continue transmitting without error and others will pull back to listen again until the next possible transmission time.

Cable and connectors

J1939, CAN-CIA standard connector configuration
RJ45	9 Pin(male)D-Sub	Name	Description
	1	Reserved	Upgrade Path
2	2	CAN_L	Dominant Low
3	3	CAN_GND	Ground
4	4	Reserved	Upgrade Path
6	5	CAN_SHLD	Shield, Optional
7	6	GND	Ground, Optional
1	7	CAN_H	Dominant High
5	8	Reserved	Upgrade Path
8	9	CAN_V+	Power, Optional

Signaling rate decreases as transmission distance increases. Bit rates up to 1 Mbit/s are possible at network lengths below 40 m. Decreasing the bit rate allows longer network distances (e.g. 125 kbit/s at 500 m). A conservative rule of thumb for bus lengths over 100 m is derived from the product of the signaling rate in Mbps and the bus length in meters, which should be less than or equal to 50. Signaling Rate (Mbps) x Bus Length (m) <= 50. Cables consist of shielded 120-Ohm twisted pair. CAN will operate in extremely harsh environments and the extensive error checking mechanisms ensure that any transmission errors are detected. Flat pair wire can also be used, but but generates more noise itself, and may be more susceptible to external sources of noise. The ISO11898 standard "Recommends" that bus interface chips be designed so that communication can still continue (but with reduced signal to noise ratio) even if either of the two wires in the bus is broken, either wire is shorted to power, or either wire is shorted to ground.

Laboratory usage

CAN technology is used in automobiles, industrial control applications, and in some medical and laboratory instrumentation (Agilent^[2], Beckman-Coulter^[3]). It is more often used as embedded control approachnot vs. a user-accessible general purpose interface methodology, such as RS-232 or USB.

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