Switchable CAN BUS terminating resistor - Page 1
Forum section: electronics / projects

author: zzattack
author_rank: Regular Contributor
timestamp: June 14, 2018, 09:08:51 pm
content: I've encountered some devices (for which I have no schematics) that have a built-in toggle to enable or disable CANBUS termination and I'm wondering how this is usually achieved.I could see them use an analog switch in series with a 120 Ohms resistor but perhaps there's easier/cheaper ways of doing this.

author: JS
author_rank: Frequent Contributor
timestamp: June 14, 2018, 11:05:17 pm
content: As they are digital signals you could just use a 3 state buffer working at the bus voltage and push high or have high-z and connect from there to the bus with a resistor.JSJS

author: max_torque
author_rank: Super Contributor
timestamp: June 15, 2018, 10:03:00 am
content: It depends upon several factors:1) is the CAN bus galvanically isolated from the device power (typically not for 12v passenger car modules, but often for industrial/HD/24v modules (because those can have multiple power sources and voltage levels and offsets etc)2) Is split termination being used for better noise immunity3) What protection level is the CAN bus engineer too (worst case is usually someone accidentally connecting the power supply across the termination resistance!)Depending on those factors, you will see various options for switching in and out the termination, ranging from manual "dip" switches, to photomos relays, mechanical relays (reed and solenoid), to logic gates (buffer), and even just selected population options (ie you have to solder in the termination!)

author: Rerouter
author_rank: Super Contributor
timestamp: June 15, 2018, 09:55:02 pm
content: Generally it is done by an optocoupler/optofet in series with a resistor to trim it.You can go with back to back transistors if you wish, however they will need more protection.My normal approach is to have 2 op amps differentially measure the termination, and if more than 7V is present to disconnect as a self protection for those times someone decides to wire can between battery and ground on a vehicle.

author: Smokey
author_rank: Super Contributor
timestamp: September 05, 2023, 08:54:43 am
content: https://www.ti.com/lit/ug/tiducf3/tiducf3.pdf

author: Siwastaja
author_rank: Super Contributor
timestamp: September 05, 2023, 09:16:45 am
content: Photo-MOSFET is by far the easiest to engineer solution. You just add it in series with the termination resistor, and drive it like you would drive an LED from your microcontroller.

author: Smokey
author_rank: Super Contributor
timestamp: September 05, 2023, 10:14:38 am
content: Quote from: Siwastaja on September 05, 2023, 09:16:45 amI've been adding two of those SSRs to a few designs for switchable split CAN termination recently. It works, but it takes up a lot of board space and it's sort of expensive. I wish there was a smaller cheaper solution, but I guess that's true for just about everything.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: September 07, 2023, 03:04:20 pm
content: You can use a simple JFET as a switch. You only have to figure out how to drive the gate properly in this case.

author: Smokey
author_rank: Super Contributor
timestamp: September 07, 2023, 05:41:06 pm
content: Quote from: Doctorandus_P on September 07, 2023, 03:04:20 pm... Doesn't that describe like 90% of problems in electronics?