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CAN architecture

I am using the TI SN65HVD230DR CAN transceiver on two separate CAN buses between three microcontrollers (MCUs) -- two are on the same board, and one is off-board. This transceiver includes a Vref pin:

The Vref pin (pin 5) on the SN65HVD230 and SN65HVD231 is available as a VCC/2 voltage reference. This pin can be connected to the common mode point of a split termination to help further stabilize the common mode voltage of the bus. If the Vref pin is not used it may be left floating.

Since the datasheet is terse with Vref details (How is it generated? Can two transceivers have Vref connected simultaneously?) I am not certain how to connect this pin. My intuition is to only have one transceiver connect Vref on the on-board CAN bus (CAN2 above) since the MCUs/transceivers are only a few cm apart.

For CAN1 between MCU1 and MCU3 (off-board) there will be 1~2 m of cable between them, so with the increased impedance my intuition is to connect Vref on both transceivers. Is this a fair assumption? Is there a better way?

EDIT: Forgot to mention I am using the split termination scheme at all transceivers.

Split termination

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The bottom of pg 33 says use it with split termination scheme which is shown in Figure 39. From figure 39, you can see that you could have a Vref+split termination at each end with no harm due to the presence of the termination resistors taking up any imbalance in Vref.

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  • \$\begingroup\$ Ah yes, just edited my question to note that I have split termination at all transceivers. \$\endgroup\$ Jun 19 '19 at 14:33
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Yes. In your case, you only have 2 transcievers on each bus and each end of each bus must have a termination. Since you are having the split termination at each end, you can connect the Vref to the midpoint of the split termination at each end. You're right that the manual doesnt say much about the current capabilities of Vref except to show a V-I chart that has about 50uA. Note that if you had more than 2 devices on the bus, you'd only have the end controllers terminated.

Or, for your needs, it will probably be fine to just use 120 ohm terminations and leave the Vref alone.

Are you going to power these chips all from the same power supply or will they have their own supplies (from a common 12v perhaps)?

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