MAX14883E bus load when unpowered

Hi,

Your schematic screenshot only shows a 120ohm termination resistor with a 2-pin heater PN1 across the CAN bus. When the note is not powered, is the 120ohm sill connecting to the bus line? The CAN bus should be terminated properly with two 120ohm resistors at the further end notes. To better understand your system, please provide the following information.

• How many notes are connected on the bus?
• What is the distance between the nots?
• Are all notes using the same transceiver with the same configuration? Both 3V and 5V transceiver are on the bus?
• What is the Vcm in the system? Both screenshot pictures showed the CANL shifted down and having an offset around 0V.
• What is the total load resistance on the bus?

Best,

Jane

Yes, the terminator is still inserted, the only thing that changes it the supply to the board; there is another terminator at the other side. There are other two nodes on the bus and there are about 5 m of cable (one at about 1 m the other one at the end).

I don't know all the transceivers included, the other nodes are commercial components (a PCAN probe and a battery charger).

I agree that CANL is a little low (1.25 V instead of the nominal 1.5 V) while the CANH is essentially perfect. The really strange thing is that the unpowered node pulls down both the lines to 2 V (still with a good recessive level)

Hi,

Thank you for pointing out the common mode voltage is around 2V instead of 0V. The side mark and the ground reference from the screenshot are not clear. When the part is powered down, we cannot guarantee the CANH and CANL pins are high impedance. That means there are leakage currents from the bus into the part, and the part acts like a load. The part also provides a reference volage (2.5V) while it is on. Therefore, when the part is powered down, the recessive voltage level on the bus will drop. If there is an extra supply voltage (usually from split pin), the bus lines should be able to hold the recessive voltage level around 2.5V. If an extra supply volage is not available, a split termination (recommended by the Typical Application Circuits from MAX14883 datasheet) would also help to hold the recessive voltage level.

Best,

Jane