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In the deutsch 9 pin obd interface for heavy trucks, some manufacturers use the pin F,G for J1939 protocol and some use it for J1708 protocol. I am trying to use a NLAS4684 analog switch with dual SPDT configuration to get the desired output based on the pins usage by manufacturers.

I am worried about connecting an Analog switch next to the vehicle BUS pin. Is it safe to connect this switch to the diagnostic port to route signals? The Rd_on for this switch is low and the current per channel is around 300mA.

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  • \$\begingroup\$ if you have 1 picoFarad of coupling, would that upset the signals? what model of Rs and Cs and Ls, and controlled-rise-time pulses would you use to define that coupling? \$\endgroup\$ – analogsystemsrf Oct 2 '19 at 12:08
  • \$\begingroup\$ Can this switch handle the high common mode voltage of CAN bus? \$\endgroup\$ – Jeroen3 Oct 2 '19 at 12:33
  • \$\begingroup\$ @Jeroen3 What is the common mode voltage range of CAN bus generally? \$\endgroup\$ – adnan Oct 4 '19 at 7:42
  • \$\begingroup\$ @adnan It can be up to 40V. \$\endgroup\$ – Jeroen3 Oct 4 '19 at 13:22
  • \$\begingroup\$ @Jeroen3 Most of the transceivers that I have seen are not rated for that high voltage range. \$\endgroup\$ – adnan Oct 7 '19 at 0:38
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I have never used J1708 but according to wikipedia, J1708 is not a CAN bus. It is apparently a UART-based bus using RS-485 transceivers. Meaning you can't connect it to a CAN transceiver in the first place.

J1939 on the other hand is a standardized CAN-bus protocol, using CAN.

So, no you cannot use analog switches or dip-switches etc because you don't have two CAN buses. Instead connect the J1708 through a RS-485 to the UART peripheral of the MCU, completely separated from CAN.

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  • \$\begingroup\$ If I understand the question correctly they don't want to connect J1708 to a CAN transceiver, they want to connect one pair of pins to either a CAN transceiver or an RS-485 transceiver depending on the vehicle's connector wiring. \$\endgroup\$ – nekomatic Oct 7 '19 at 15:10
  • \$\begingroup\$ @nekomatic Nope, it started here electronics.stackexchange.com/questions/460489/…. Where the OP said "Lets say that one is CAN classic and other is J1939(same physical layer as CAN)". After which I told them they weren't making any sense. \$\endgroup\$ – Lundin Oct 7 '19 at 15:14
  • \$\begingroup\$ @nekomatic you are right. \$\endgroup\$ – adnan Oct 8 '19 at 0:02
  • \$\begingroup\$ @Lundin That was a different question. As nekomatic explained in the above comment, I will route the pin using an analog switch to either a CAN transceiver if the protocol is J1939 or else to a Rs-485 transceiver if the protocol is J1708. \$\endgroup\$ – adnan Oct 8 '19 at 0:04
  • \$\begingroup\$ @adnan CAN and RS485 use different voltage levels though. \$\endgroup\$ – Lundin Oct 8 '19 at 6:41
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The datasheet you linked to shows the permitted voltage range on the analogue pins of this switch IC is from 0.5 V below GND to 0.5 V above Vcc, and the maximum recommended Vcc is 5.5 V. This does not look adequate for the expected common mode voltage range of a CAN bus.

If you want to be on the safe side the most obvious suggestion would be to use relays, since your application doesn't need frequent switching. This answer points out some issues with using relays to switch a live CAN bus but that doesn't sound like a problem for your application where you want to set up the configuration of a test system (presumably) before you connect it to the vehicle.

If you really want the switching to be solid-state you'll need to find out the maximum expected voltages on both bus types and find an appropriately specified part. I suggest you get this information from the standards for each bus type and/or from the datasheets for transceivers for each bus type.

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So... you are willing to risk signal degradation and a chance for voltage mismatch if switch is in the wrong position (with damaged electronics as a result), all for dubious convenience of having one connector and a switch instead of two connectors?

Also, the diagnostic equipment usually has heavy TVS protection on inputs. The components for this are different for two protocols, so you cannot put them before the switch. And after the switch they don't make much sense.

My advice would be to use two clearly marked connectors, each routed to its own transceiver and equipped with suitable protection. If nothing else, it would make PCB routing easier and traces shorter.

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