USB-8506 LIN bus power

[Fredrabine]

Hi!

I've just bought the "bus powered" LIN adapter USB-8506. When trying it I can't make it work without supplyind 8-18V to a Vbat pin on the D-sub9 connector. In the specifications (which I read afterwards), it says Vbat is input. I'm very very surprised that, unlike another adapter from Vector, I need to supply a voltage. Morever, it's inconevnient  to supply it on the D-sub, there could have been a wire terminal. To me the adapter is not completey bus powered then. Unless there is a software settings to supply the LIN bus ? Can anyone confirm I need a power supply to my setup  ? Thanks a lot (I find NI documentation quite confusing and spread out sometimes)

[Hooovahh]

Yeah I agree the documentation isn't always clear.  I opened the 8506 specification PDF and figured I'd find a pinout of the DB9 saying what pins are what and seeing power labeled.  Nope.  I did find the power input you mentioned and it does say it is required.  I'm guessing the reason is that the NI device can draw more than the USB port can provide. 

Some C-series XNet devices also need external power, and then the transceiver itself also needs power, this is in addition to the chassis which needs power.  All 3 can be tied together and it works just fine, but I asked this question and while no one from NI responded, a plausible answer was given with power requirements.  This also allows for wiring up system to not share a common ground if that is desired.  Still it is an odd choice for a USB device.

[ZY_Ong · Accepted solution]

The external power supply requirement for NI-XNET devices are summarized in NI CAN/LIN/FlexRay/Automotive Ethernet Device Power Supply Requirements. NI offers NI CAN SINGLE TERMINATION CABLE W/POWER TERMINALS, HIGH-SPEED/FD, 2M (PN: 782579-02) that exposes the Vbat pins to power terminals.

As for the reason why LIN interfaces are not internally powered, here is my finding (not the official explanation from R&D):

1. From Physical Layer Requirements: Typical voltage levels for the TXD and RXD are typical of most microcontroller levels: 3.3 V and 5 V. The LIN bus, and LIN transceivers, typically operate at voltages ranging from 9 V to 18 V, but some go up to 30 V, depending on the application; a typical vehicle is a 12-V battery system, but some larger vehicles go up to 24 V.
2. The above is aligned with the TJA1028 used in USB-8506. This might explain why USB-8506 does not have a built-in Vbat. It largely depends on the DUT.
3. As for the Vector device, Bus-side voltage supply section states that: Since the recessive level on the bus depends on the supply voltage of the master, it is advisable to operate the LINcab/LINpiggy with an external supply voltage that is also used by the other bus nodes. This prevents cross currents between the individual nodes on the LIN bus. My best guess is that the device has a fixed internal voltage for loopback testing purpose. To communicate with actual ECU, external supply voltage should be considered.

[Fredrabine]

Thank you! Your first link provides the information indeed. I never found it. Even the pinout (with pin numbers) is hard to find, I found it on a chinese forum!