Any EEs in the house?

NSFW · 01-20-2009, 04:11 AM

I want to read a TTL serial signal via USB. I think this should be pretty straightforward, but I'm hoping that someone better versed in electronics than myself can give me a sanity check first.

I've got a wideband O2 sensor in my car, made by PLX Devices, which puts out a TTL serial signal on a 1/8 headphone-style jack. And I've got a CarPC with a spare USB input. To bridge the gap, I've got an FTDI serial level converter, conveniently packaged like this.

I plan to connect the grounds from the WBO2's serial output and the converter's input, and connect the WBO2's serial output to the converter's serial input, and I'm hoping that this won't let the smoke out of any of the aforementioned components.

Some other folks have done essentially the same thing using an old-school 9-pin serial adaptor, as seen here.

My only worry is that the various power supplies (in the WBO2 controller, the CarPC, the USB hub's power supply) won't all agree, and I'll have current flowing through the ground connection or possibly the serial signal line. I plan to test those paths with a voltmeter - if it reads zero, I assume I'm safe. Am I right? Are there any precautionary steps I can/should take to keep this safe?

Thanks in advance.

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Qcanfixit · 01-20-2009, 04:36 AM

people want to help... but with no flesh offering... especially with a handle like "NSFW"

it's soo hard...

NSFW · 01-20-2009, 12:47 PM

Oh, right. How could I forget?

I'll post more and better if y'all make it worth my while.

hooziewhatsit · 01-20-2009, 01:07 PM

The pinout on the 1/8" headphone jack is probably RX/TX/Ground, correct?

If so, you shouldn't have any issues with power going through the system. The FTDI chip will be powered from the laptop, and the O2 sensor from its supply. You must connect the grounds.

Also make sure TX from the O2 sensor is connected to RX into the laptop. If it doesn't work, swap them

Sounds to me like it should work just fine.

-Dave (embedded hardware & software engineer)

NSFW · 01-21-2009, 03:59 AM

Quote:

Originally Posted by hooziewhatsit

The pinout on the 1/8" headphone jack is probably RX/TX/Ground, correct?

If so, you shouldn't have any issues with power going through the system. The FTDI chip will be powered from the laptop, and the O2 sensor from its supply. You must connect the grounds.

Also make sure TX from the O2 sensor is connected to RX into the laptop. If it doesn't work, swap them

Sounds to me like it should work just fine.

-Dave (embedded hardware & software engineer)

Cool, I was hoping it would be that simple. Just afraid it might not be.

The headphone jack is TX/not-connected/ground, actually. It doesn't listen, only transmits. Thanks very much.

NSFW · 01-31-2009, 02:39 AM

Just got around to trying this, and it worked as advertised - I got a stream of serial data that looks like what's described in PLX's documentation. Now I gotta write some code to turn that into AFR numbers, and then see if I can get it incorporated into RomRaider, etc.

Thanks again.

01-20-2009, 04:11 AM	#1
NSFW Scooby Specialist Member#: 140444 Join Date: Feb 2007 Chapter/Region: NWIC Location: Near Seattle, WA Vehicle: 05 Stage Free LGT ATP 3076 + AVO FMIC	Any EEs in the house? I want to read a TTL serial signal via USB. I think this should be pretty straightforward, but I'm hoping that someone better versed in electronics than myself can give me a sanity check first. I've got a wideband O2 sensor in my car, made by PLX Devices, which puts out a TTL serial signal on a 1/8 headphone-style jack. And I've got a CarPC with a spare USB input. To bridge the gap, I've got an FTDI serial level converter, conveniently packaged like this. I plan to connect the grounds from the WBO2's serial output and the converter's input, and connect the WBO2's serial output to the converter's serial input, and I'm hoping that this won't let the smoke out of any of the aforementioned components. Some other folks have done essentially the same thing using an old-school 9-pin serial adaptor, as seen here. My only worry is that the various power supplies (in the WBO2 controller, the CarPC, the USB hub's power supply) won't all agree, and I'll have current flowing through the ground connection or possibly the serial signal line. I plan to test those paths with a voltmeter - if it reads zero, I assume I'm safe. Am I right? Are there any precautionary steps I can/should take to keep this safe? Thanks in advance. * Registered users of the site do not see these ads.

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01-20-2009, 04:36 AM	#2
Qcanfixit Scooby Specialist Member#: 135762 Join Date: Dec 2006 Chapter/Region: Tri-State Location: Cool Story, Brogue Vehicle: 4002 amixam Q Wheel Drive	people want to help... but with no flesh offering... especially with a handle like "NSFW" it's soo hard...

01-20-2009, 12:47 PM	#3
NSFW Scooby Specialist Member#: 140444 Join Date: Feb 2007 Chapter/Region: NWIC Location: Near Seattle, WA Vehicle: 05 Stage Free LGT ATP 3076 + AVO FMIC	Oh, right. How could I forget? I'll post more and better if y'all make it worth my while.

01-20-2009, 01:07 PM	#4
hooziewhatsit Scooby Newbie Member#: 143938 Join Date: Mar 2007 Chapter/Region: NWIC Location: State of Jefferson	The pinout on the 1/8" headphone jack is probably RX/TX/Ground, correct? If so, you shouldn't have any issues with power going through the system. The FTDI chip will be powered from the laptop, and the O2 sensor from its supply. You must connect the grounds. Also make sure TX from the O2 sensor is connected to RX into the laptop. If it doesn't work, swap them Sounds to me like it should work just fine. -Dave (embedded hardware & software engineer)

01-31-2009, 02:39 AM	#6
NSFW Scooby Specialist Member#: 140444 Join Date: Feb 2007 Chapter/Region: NWIC Location: Near Seattle, WA Vehicle: 05 Stage Free LGT ATP 3076 + AVO FMIC	Just got around to trying this, and it worked as advertised - I got a stream of serial data that looks like what's described in PLX's documentation. Now I gotta write some code to turn that into AFR numbers, and then see if I can get it incorporated into RomRaider, etc. Thanks again.