I'm looking to incorporate a switchable 120Ω termination resistor (using a photo MOSFET) into a CAN design. Are there any issues with locating the 120Ω resistor at the CAN transceiver versus at the CAN_H/CAN_L connector? The CAN_H/CAN_L traces are approximately 100mm long (i.e. the CAN transceiver is located approximately 100mm from the physical CAN connector).
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\$\begingroup\$ The main issue here is rather that photo electronics are very slow at toggling. You might place the bus in bad state for some 20ms, which could be enough to kill the whole bus (put every node in error passive) depending on baudrate. Consider letting the MOSFET toggle an analog switch instead, to minimize downtime. \$\endgroup\$– LundinCommented Dec 7, 2020 at 7:24
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\$\begingroup\$ That's a fair point. The photo MOSFET approach was based on this Texas Instruments document: ti.com/lit/pdf/tiducf3 \$\endgroup\$– jars121Commented Dec 7, 2020 at 8:06
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\$\begingroup\$ The TLP175A photocoupler in that app note has a max switch time of 5ms. Not horrible, but not good either. If the only purpose is to achieve galvanic isolation, I'd recommend to use something like digital isolators instead. Faster and more reliable. The n channel MOSFETs in their design are fine to use. \$\endgroup\$– LundinCommented Dec 7, 2020 at 9:44
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\$\begingroup\$ Thanks, I'll also give that a good read. How would the analog switch option fair in comparison? The MAX14759 (for example) datasheet specifies switchable CAN termination as a potential application (see Figure 9 in the datasheet). \$\endgroup\$– jars121Commented Dec 7, 2020 at 10:09
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\$\begingroup\$ Well, do you need galvanic isolation or not? Analog switches are the best choice since they are fast and don't mess up the impedance as much, but they aren't isolated. \$\endgroup\$– LundinCommented Dec 7, 2020 at 10:12
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1 Answer
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CAN transceivers have slewrate limiting, to reduce interference and to not be bothered by moderate termination errors in LONG cables.
At 100mm (4"), you can place the resistors wherever you wish.
answered Dec 7, 2020 at 6:53
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\$\begingroup\$ That was my understanding, thank you for confirming! \$\endgroup\$– jars121Commented Dec 7, 2020 at 7:21