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Physical layer
uCAN uses RJ45 connectors and CAT5 or CAT6 UTP cabling, standardised as popular for Ethernet and for building cabling applications. Both power and data are transported over a single RJ45-terminated cable.
Pinouts are designed to minimise the consequences of accidentally confusing uCAN and Ethernet sockets.
| Pin | Assignment |
|---|---|
| 1 | CAN_H |
| 2 | CAN_L |
| 3 | Reserved |
| 4 | VCC |
| 5 | VCC |
| 6 | Reserved |
| 7 | GND |
| 8 | GND |
Pins 1 and 2 are the standard CAN BUS high and low pins.
Termination is accomplished using split-termination: each terminating device has two 60 ohm 1% resistors in series, between the CAN_H and CAN_L lines. A 4.7nF capacitor is connected between the two resistors to GND.
Devices may either have mandatory or optional termination. Mandatory termination devices have only one RJ45 port, and include a permanent split termination network. Examples of this include a CAN bus hub or a power injector. Optional termination devices have two RJ45 ports, connected in parallel, with a split termination network that can be enabled or disabled by a user through the use of jumpers or a switch. For maximum flexibility, device designers should use optional termination unless there is a good reason to expect the device will only be configured as an end node.
Reserved pins (3 and 6) should be forwarded from one jack to matching pins on the other jack.
Power is supplied to attached uCAN devices through pins 4 & 5 (VCC) and 7 & 8 (GND). Each pair of pins must be connected together. These pairs of pins each terminate a twisted pair in standard ethernet (T568) cabling, so accidental attachment to a port with magnetics such as Ethernet will not cause a short. Power is not optional; all uCAN devices can expect to have power supplied over the same connector as the network.
A uCAN power injector supplies the power to attached devices. A power injector is always a mandatory termination device. Power must be supplied as DC, between 12 and 36 volts; all uCAN devices should function correctly within this entire range, though they may opt to provide the line supply in a user-accessible fashion, in which case it is up to the user to ensure the correct voltage is used.
uCAN devices should incorporate reverse power protection in case a crossover cable is accidentally attached. They need not operate in this situation, but they should avoid serious issues such as short circuiting the power supply.
Cabling limitations mean that no more than approximately 1 amp should be supplied over the two pairs used for power; this means that at 36 volts, a little under 36 watts (after line losses) is available to all uCAN devices on a connection. The use of a CAN hub, which can provide power separately to each connection, can increase this to a per-branch limit rather than a network-wide limit.
If a uCAN device requires more power than is available, it can take its power from an external supply.