When implementing an RS-485 interface, I tend to follow TI's recommendation in the SN65HVD75 datasheet to include \$10 \Omega\$ series resistors on the A and B lines. The TVS diodes I'm using clamp at 15 V maximum, and the transceiver can handle up to 16.5 V, but I prefer some design margin. The comms are enclosed in a large, metal enclosure, and do not interface outside of it.

SN65HVD75 datasheet

As a carry-over, I have been including series resistors on CAN bus interfaces, right at the driver/receiver pins. This bus also resides in the enclosure and does not interface outside of it. The TVS diodes clamp at 8 V maximum and the transceiver I'm using can tolerate up to 16 V on the H/L lines.

Given this much higher margin it would seem that series resistors are unnecessary, but I am still hesitant to remove them from the design. Being only \$10 \Omega\$, they won't drop too much voltage (~250 mV of 3.3 V; high/low input thresholds are 2 V, 0.8 V, respectively). Yet, out of four sample CAN transceivers from different manufacturers none of them recommend series protection resistors, either. Should I just leave them down or is there a strong reason to remove them?

EDIT: The CAN bus also interfaces with a user interface board, which drives an LCD screen on the outside of the enclosure.

  • \$\begingroup\$ You should first identify the existing dangers before applying precautions: The comms are enclosed in a large, metal enclosure, and do not interface outside of it. So, what can happen? If transmitter and receiver are really inside a metal enclosure, you don't need to worry about ESD (provided that during production, measures against ESD are taken), the enclose may act as cage of Faraday, such that you neither need to worry about surge transients. \$\endgroup\$ – Huisman Apr 4 at 6:47
  • \$\begingroup\$ @Huisman True, but I neglected to mention the CAN bus extends to a user interface board that drives an LCD, which does get tested for ESD events. Also, in production and service these boards are handled by humans possibly without proper ESD mitigation. \$\endgroup\$ – calcium3000 Apr 4 at 13:40

I never saw series resistors in CAN Networks but the usage of a common mode choke instead (e.g. DLW43SH101XK2#) together with a split termination of 30R + 30R + 4.7nF. (e.g. TJA1057 Datasheet Fig 8 on Page 14). The common mode choke is for EMI compliance at high baudrates. Sometimes I see additionally protection by a Varistor (e.g. B72500E2170S160) to GND on CAN_H and CAN_L.


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