Safe max input voltages to MS3X PT4

[walterclark1]

What are the maximum safe input voltages to the MS3X PT4 / Pin 31 input?

I see the input has a bat54swt1g to limit the positive and negative levels. Assuming the input signal voltages and current are below the limits for the Schottky diodes in the bat54swt1g, is this sufficient protection for the processor?

I have a square wave generator that normally outputs a single ended bipolar signal that I want to use to feed a very low frequency test signal into PT4. I can limit both the positive and negative voltage and current levels on this pretty easily to +5V and -.8V with something like a series 1K resistor and 5.1V zener (from the output signal lead to ground). Is that adequate? Thank.

[billr]

My schematic is too fuzzy to read, but doesn't PT4 have a 10K resistor ahead of the BAT diodes? With that, and the diodes, you should be safe without further clamping of the function-generator output.

[dontz125]

It's a 1k resistor, but enough to limit ~5v current to single-digit mA range. If you're using a diode anyway, a single diode pointed at the PT4 pin will take care of any negative voltage, while a 10k series resistor will take care of any high positive voltages.

This is for the OP's positive-pulsing application. If you were to use a grounding switch, flip the diode (band away from PT4) and replace the series resistor with a 1k pull-up to 5v.

[walterclark1]

OK, thanks guys. With the existing input protection of the MS3X and your comments I think the series diode in my signal path and me limiting the output of the generator to +5V should be enough insurance. A 1N914 should do it.

By the way. What I am doing is trying to quantify the delay between triggering an input to the MS3 and seeing a change from it on a Bluetooth (using the EFI Analytics RS232-BT adapter) connected Nexus 10 tablet running ShadowDash. Then compare it to the same setup with an OTG mode USB cable between the MS3 and tablet.

Of course its not clear to me whether some information generated by the MS3 has priority over others or not, or if there is a time penalty when asking for a lot of things versus just a few, but leaving everything else constant and just changing the BT to OTG this test should give me a pretty clear picture of what BT adds in my setup.

[walterclark1]

Well the series diode approach didnt work. Seems that the PT4 input pulls itself up and my input basically needs to pull it down for a low - my series diode prevents that. Rather than spend time coming up with a grounding switch scheme, I ended up just running a +5V /-5V P-P squarewave to the PT4 input which, as suggested earlier, didnt seem to hurt anything. I had PT4 configured to act like an external AC ON/OFF signal for idle-up in the MS3. I set the delays to zero and increase in idle to only 100RPM. This turned on a white (black when off) indicator on the tablet. I made up a light sensor consisting of a clear bodied red LED as the light sensor and a AF signal NPN as an emitter follower to amplify the LEDs small output current. I chose a red LED because their chemistry seems to be the most sensitive of the visible spectrum LEDs (based on published relative light output specs for a given model LED across the available output color ranges red, yellow, green, blue).

I learned that the Bluetooth delays must not be too bad and overall the delay in my setup should be acceptable (for time critical info like tach). I observed around 10-60msec delay between changing the state of the PT4 input and measuring when a virtual indicator on Shadowdash (running on my Nexus 10) came on and 40-100msec when going off. This is with the engine running, and the delay didnt seem to be affected by things I did with the throttle. The difference in delay between on and off times must be due to how the MS3 handles this function. For AC idle up it would be more important to increase the idle ASAP when the compressor comes on than to decrease it quickly when it goes off.