Speedo pull-up question

This forum is for general questions you have on assembling and troubleshooting your GPIO board. You can purchase a GPIO board at DIYAutoTune now!
Post Reply
Rukavina
Posts: 47
Joined: Fri Aug 15, 2014 7:58 pm

Speedo pull-up question

Post by Rukavina » Mon Nov 10, 2014 9:40 pm

Gpio speedo pull-up circuit doesn't work with Abbott cable x mechanical converter. The abbott does work and is wired to vas currently. Question is what direction do I go with resistor. It's a 1k now per spec. Do I drop down to 330 or go other way. And how far can I go to no resistance before getting close to damaging transistor

Bernard Fife
Posts: 1696
Joined: Fri Apr 04, 2008 1:28 pm

Re: Speedo pull-up question

Post by Bernard Fife » Wed Nov 12, 2014 12:57 pm

Rukavina,

This is the speedo output signal (not the VSS) I assume. The pull-up resistor limits the current to the external device (i.e. Abbott cable + convertor in your case).

Are you using a 5 Volt or 12 Volt pull-up?

With a 5 Volt pull-up, you need to limit the current to the device, but you also need to limit the current drawn from the GPIO's regulator to less than about 50 milliAmps. So I would use no less than R = V/I = 5/0.050 = 100 Ohms.

If you are using a 12 Volt pullup, you probably don't want more than about 200 milliAmps (to avoid stressing the power supply on the board), so the minimum resistor would be R = 12/0.200 = 60 Ohms.

You might want to put a weak pull-down on the signal as well (to pull it to zero volts when the signal is off). A 10K Ohm to 100K Ohm resistor on the signal to ground (this can be done on the board or externally) should work.

You might also want to experiment with the duty cycle on the speedo output, as some external devices can be quite sensitive to this parameter. See: http://www.msgpio.com/manuals/mshift/V4tune.html#su

Lance.
"Never wrestle with pigs. You both get dirty and the pig likes it." - George Bernard Shaw

Rukavina
Posts: 47
Joined: Fri Aug 15, 2014 7:58 pm

Re: Speedo pull-up question

Post by Rukavina » Wed Nov 12, 2014 9:03 pm

Yes it's a 12 volt signal. It would not work on gpio speed output. Hooked it straight to vss sensor temporarily so I can get the speedometer/odometer working for registration requirements. I have not put my scope on it yet. I will step resistors down / pull down etc and post findings. Thanks for the help!

Post Reply