CAN comms between two wire and three wire devices

[Schmoleskin]

We are connecting two generator controllers (Woodward EasyGen 3400s) to a utility breaker controller (Woodward LS-511-5/P1). We can get the two generator controllers to recognize one another over the CAN network, but neither can see the LS-5. One difference between these devices is that the LS-5 has only a two-wire CAN interface (screw terminals), and the 2 EasyGens have a three-wire interface on a DB-9. The DB-9 is wired: 2-CANL, 3-Ground, 7-CANH, which matches what the manual says, and all other references I've found online. We made all possible Grounding connections to the devices (Case and Engine ground on EasyGens) and a case ground on the LS-5 (It does not have any other signal ground connection).

With the units all powered down, I notice that the ground wire (Pin 3 of the 2 EasyGen controllers) is not solidly grounded. Is it possible that the LS-5 needs the other two controllers' CAN grounds to be referenced to ground? Can a two wire connection function without a ground reference on CAN ground? Should I ground the Pin 3 CAN ground, or let it float?

Thanks, DaveO

 

[IRstuff]

Theoretically, a differential system only needs the two lines, but that assumes infinite common mode rejection, which is not physically possible. So, it's possible that the outputs are wiggling correctly, but the inputs are railed because of common mode. Having a ground reference helps the common mode issue, and helps on a related issue; if the differential inputs are TTL-based, the emitters can get zapped by the ground transients during power up, particularly if the two ends of the line are powered through different power supplies that turn on at different times. It can take as few as three power cycles to completely fry a differential input if the transient common mode voltages are very high (BTDT). It's not inconceivable that the inputs are already damaged. A typical TTL input will have less than 100 uA input current with the input shorted to ground. If it's larger than that, the input could be damaged. This only applies to RS-422 type inputs, as other types might have pullups, etc. that mask the input.

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