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Topic: GM's ALDL to USB 5V TTL Protection Circuit Needed (Read 4412 times) previous topic - next topic

Hamster7

Hi. I wish to connect my car's ALDL; (Assembly Line Diagnostic Link, sort of OBD) to my pc via usb 5V TTL board(may add arduino later) I looked in the website it said connect USB TTL's Rx/Tx together and then connect to the ALDL data line.

Then someone raised a warning...

Quote
USB TTL drives the output high which could potentially cause the ECU output to blow.

ALDL is half duplex: The ECU output is a 10 kohm resistor to +5V with a switched transistor to 0V. The interface must also have a switched transistor to 0V only. The maximum current that can flow in the ECU output transistor is 5V/10k ~ ½mA.

If the interface is switched via a transistor to 5V in the off state, then when the ECU starts to respond, the maximum current in the ECU transistor is determined by the output capability of the interface and could be 100s of milliamps. You would be advised to put a diode in series (Anode to the interface output and cathode to the ECU). The interface low output will increase by ~0.7V but should still be recognised as a low by the ECU.
So far, I've come up with the circuit shown here. Then someone else said the diode might be the wrong way round.  :smiley-eek-blue:

Can anyone confirm if the protection circuit is correct? Or advise otherwise.


raschemmel

#1
Jun 24, 2016, 08:52 pm Last Edit: Jun 24, 2016, 09:00 pm by raschemmel
I don't know anything about that hardware but your baud rate doesn't sound like anything I've seen on a Hyperterminal settings menu.

Possible baud rates

Have you found anything online about this ? (links ?)
Arduino UNOs, Pro-Minis, ATMega328, ATtiny85, LCDs, MCP4162, keypads,<br />DS18B20s,74c922,nRF24L01, RS232, SD card, RC fixed wing, quadcopter

Hamster7

#2
Jun 24, 2016, 09:16 pm Last Edit: Jun 24, 2016, 09:21 pm by Hamster7
Hi raschemmel,

It's very old car diagnostic link dated back in '90s

https://en.wikipedia.org/wiki/ALDL

raschemmel

Never mind the car. Hoe do you plan to select that baud rate on your PC when it doesn't exist on the terminal program settings menu ?
Arduino UNOs, Pro-Minis, ATMega328, ATtiny85, LCDs, MCP4162, keypads,<br />DS18B20s,74c922,nRF24L01, RS232, SD card, RC fixed wing, quadcopter

Coding Badly

Hoe do you plan to select that baud rate on your PC when it doesn't exist on the terminal program settings menu ?
Get a better terminal program.  For example...
https://sites.google.com/site/terminalbpp/


Hamster7

To answer your question I will tell you more of the background. There was a great guy wrote the program on pc communicating with ALDL via USB(I don't know much about it but would guess he had taken into account of the 8192 baud). Everything works fine for most users (would guess 100+) except few cases of users reports the ECU not communicating. Then someone raised a warning that without a protection circuit the output of TTL could blow the ECU output, which could be the reason why the ECU was not communicating, as per the quote.

Thanks for your time raschemmel, Coding Badly. I really appreciated that.

raschemmel

@ Coding Bafly,
That's not fair.. You didn't give me a chance to hear his answer. to the question. ( peanut gallery violation ha, ha)
Arduino UNOs, Pro-Minis, ATMega328, ATtiny85, LCDs, MCP4162, keypads,<br />DS18B20s,74c922,nRF24L01, RS232, SD card, RC fixed wing, quadcopter

alnath


raschemmel

Quote
A number of people have produced free software of varying complexity and platform support that will work with the above interface. We don't have any 8192 baud software yet, but we do have 160 baud simple and generic software for DOS.
Arduino UNOs, Pro-Minis, ATMega328, ATtiny85, LCDs, MCP4162, keypads,<br />DS18B20s,74c922,nRF24L01, RS232, SD card, RC fixed wing, quadcopter


Paul__B

So far, I've come up with the circuit shown here. Then someone else said the diode might be the wrong way round.
It sure is!


The diode anode would need to go to the connection between RXD and the ALDL data while the TXD line connects to the cathode of the diode.

A 4k7 resistor between the ALDL data and the RXD line might be extra protection.

raschemmel

Would it be silly to ask why Rx and Tx are shorted together on FTDI  side ?

Is that supposed mean ALDL Tx => FTDI Rx ? If so why is drawn that way ?
Arduino UNOs, Pro-Minis, ATMega328, ATtiny85, LCDs, MCP4162, keypads,<br />DS18B20s,74c922,nRF24L01, RS232, SD card, RC fixed wing, quadcopter

alnath

#12
Jun 25, 2016, 12:05 pm Last Edit: Jun 25, 2016, 12:07 pm by alnath
look, here is how they should be wired, they aren't shorted, but
ALDL has only one pin for both Tx and Rx , the other side has to deal with it and I think they should be wired the same way if FTDI is used



Paul__B

Why have you posted an RS-232 interface circuit when the OP referred to "communicating with ALDL via USB"?

alnath

because I think the important thing is the ALDL side and where and where and how to put the diode......
.... I think it should be wired the same way if USB is used, with these little changes :

"T1 IN" becomes Rxd  (USB)
"R1 OUT" becomes Txd (USB)

am I wrong ?

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