CAN -- Need the ground pin?

[valemike]

The way I understand it, a CAN connection only requires two pins: CANTX and CANRX.

However, in the CCS demo kit, they use a 3-pin connector: CANTX, CANRX, and Ground.

Is it really necessary to connect every device's Ground to each other The way i thought it was is that all you needed was CANTX and CANRX pins and they are differentials. Oh well, i'm confused.

-Mike

[rnielsen]

As with every circuit, you need a supply line and a return line. If you try to push current into a pin, that current needs to return to the supply or else it's like trying to stuff it into a dead end. Tieing the grounds together enables this to make a complete loop. If you need to keep certain equipment isolated from each other then you need to use opto-isolators or some other type of isolation protocol. Just like when you use RS232, you need TXD, RXD and GND as the bare minimum signals.

Ronald

[SherpaDoug]

I don't know CAN bus, but CANTX and CANRX don't sound like differential signals. Are there two pins each for CANTX and CANRX? If you had a differential signal such as RS485's AB pair then you may not need a ground. But if you have one pin to transmit and one pin to receive they are not differential signals and you need ground.
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[valemike]

Thanks Sherp and Rnielsen for the feedback.

I sent an email to tech.support@microchip.com and am awaiting the response.

What has me confused is that Microchip's App. Note AN1215 has a schematic where the CAN connector to the outside world uses only the two lines CANH and CANL of the MCP2551 transceiver.

-Mike

[dyeatman]

CAN Networks are differential and the ground DOES NOT need to be connected, however, the CAN specification recommends that you do.

Proper bus termination is also very improtant in CAN just like it is in RS485 diferential networks.

Here is a design document that mibght be helpful:
http://www.interfacebus.com/Design_Connector_CAN.html

[Yash]

[valemike]

Here is CCS's reply for me to share:

[SherpaDoug]

7V is the ground difference spec from RS485. CAN probably just copied the 485 spec on that point.
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