It's been a while since I gobbled together a 3.2 DME bench tester. Back then I used a lame counter to generate a reference pulse from a speed signal that itself was an output from a simple VCO. not very elegant and not suitable to drive a later DME.
However, lately I get more and more 964 and 993 units and testing in the car isn't always practical. So I want to build a test stand for those as well.
First task: Come up with a flywheel simulator.. There are some options out there but none appealed to me. So here is what I came up with: a Freescale MC9S12 MCU off eBay for cheap and an LCD display I had floating around. With a some C programming I finally got a working prototype that accepts commands via one of its serial ports from the PC to adjust RPM and O2 sensor duty cycle. It puts out text with RPM and O2 status on the LCD display. C is an alien concept to say the least..... It was quite a struggle to understand its concepts and embedded programming in general. Fortunately Freescale offers a pretty powerful IDE for free.
The next shot shows 4 of the outputs when simulating a 964 running at 2000 RPM: from top to bottom,:
- The yellow trace is the flywheel signal with 60-2 pattern.
- The cyan trace is the Hall sensor going off ever other turn at 720 degrees
- The purple is the RPM signal
- The green trace is the TDC signal that can be used for triggering later
Next I need to build some driver and interface circuits to buffer the signals before hooking them up to a DME for testing.
Ingo