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OBDII interface question - newbie alert...

RicardoF1RST

May 28, 2020
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Hi could anyone please advise, I have a car OBDII to USB interface (circuit attached) that needs to connect to another device. Normally a car would provide a 12v power source to pin 16. But ideally I need to use a 5v supply to pin 16. If I do so, am I correct in assuming that the subsequent serial output on either K-Line will be 0v / 5v rather than the normal 0v / 12v high/low? i.e. the output uses the power supply on pin 16 as a reference? Many thanks.
 

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Harald Kapp

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m I correct in assuming that the subsequent serial output on either K-Line will be 0v / 5v rather than the normal 0v / 12v high/low? i.e. the output uses the power supply on pin 16 as a reference?
Your assumption is correct. But:
  1. The output on K-Line(1) will then be 0 V - 5 V only, insufficient for IBDII which requires levels from 0 V to Vbat (12 V)
  2. The treshold for the transition between logic low and logic high are set to 6 V from 1/2 × Vbat (12 V). When using 5 V this threshold will be reduced to 2.5 V and susceptibility to noise will increase.
 

RicardoF1RST

May 28, 2020
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Many thanks for that, it’s what I expected. It does work when connected to a car (using the 5v) which I was surprised at, but it’s a fluke. I will add a circuit to generate the 12v as it’s not available from the car (long story). Thank you very much for the confirmation.
 

narkeleptk

Oct 3, 2019
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Just add a 7805 to your circuit for your 5V. Its old chip but very easy to work with and has always worked well in automotive applications for me.
 

RicardoF1RST

May 28, 2020
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Thanks, but the issue isn’t the quality of the 5v supply, that can come from the USB connector. The issue is the target device expects the logic levels to be 0v / 12v for logic 0 & 1. The interface “kinda works” with 5v power (surprisingly), but the ideal solution is to pull the 5v USB supply up to 12v internally and use that for the comms reference. The currents involved are minimal so it’s straight forward to do. Normally the 12v is provided by the target device but in my case the target only provides a ground plus TX & RX. Baud rates are low too, only 5000 and 7812.5
 

Harald Kapp

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Find a step-up regulator or a boost regulator (same thing, 2 names) on your favorite electronics sales platform. Plenty of these available for a few bucks.
 

RicardoF1RST

May 28, 2020
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How much of an issue do you think it is that the 12v and GND I'll be using is coming from a USB socket, rather than from the same power supply rail that the target device is using? Thinking about it, the OBDII spec is designed so that the chip that does the logic level conversion within the lead provides a logic 1 and 0 voltage supplied by the target device for maximum compatibility. My 12v and GND will be from another source...
 

Harald Kapp

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How much of an issue do you think it is that the 12v and GND I'll be using is coming from a USB socket, rather than from the same power supply
None at all. In fact, you'll have to have a common ground for any setup of this type to work.
Note that your typical USB computer socket as per the standard delivers 100 mA only (without handshake with the load) or max. 500 mA (after handshake with the load). Make sure this is enough for your application or use a USB power supply rateh at more than 500 mA.
 

RicardoF1RST

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I’m thinking 30-40mA so comms USB should be fine. The ECU I’m connecting to provides a GND, TX & RX for the comms. These go to the OBDII K lines and data GND. Then there are the power 12v and chassis GND. I’m tying the GNDs together. Thanks again for your advice.
 
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